Forensic Analysis of Time of Death

Hey everyone, in this video we're going to be talking about time of death calculations, what they are, and a little bit of their limitations. So this very first portion, we're going to talk about the postmortem interval. So this is going to be done by forensic pathologists and done in the ME's office. You may remember when we talked about forensic pathology, this idea of algor mortis, and this is that body temperature dropping to whatever the room temperature or air temperature is surrounding the body. Now, I told you this happens at a rate about one to one and a half degrees Celsius per hour, but there is a rate of change. that is not necessarily linear. So we can't always go based off of that. So we do have a calculation specifically for Algermortis. Now this calculation is very simple. All it has is 37 C. So you may realize that is the average human body temperature. And then you subtract that by the current body temperature. So you have to take the decedent's temperature at the time that you discover it. And then you divide this by a constant variable. And this constant variable is called T. And so T just changes based on whatever the ambient temperature is. So if the temperature is less than 32 degrees Fahrenheit, which is 0 degrees Celsius, so that's freezing, right? Then T is 1.5. And if it is above freezing or equal to freezing. it's actually 0.75. So what this gives you is how many hours since death. This will not ever be days because this is going to happen fairly quickly. Now rigor mortis is a fairly common assessment of time of death as well. It's not as clear-cut as algor mortis but if you recall, rigor mortis is just that stiffening of the muscles so the body becomes very tense and then over time, like I told you, it's going to become relaxed again. So we can actually look at the body, where different parts of the body are undergoing rigor, and establish an approximate timeline. So within one to four hours you expect just the jaw and the neck to undergo rigor and then after eight the full body being under rigor. Now when rigor stops it is more or less in the opposite order in which it initiated so after 24 hours the jaw will start to go limp but after 36 hours we can't use rigor mortis anymore because decomposition has fully underwent. undergone. So if the body is completely limp and there's evidence of decomp, we are not early stage decomp where rigor might just not have set in yet or not recognizing where. So after that happens, we know decomp happens and rigor or post rigor, we know that the individual likely has been dead for at least 36 hours. So I do want to just kind of remind you that during these early hours of decomposition there's still a lot of changes happening. So there is that blackening of the eye called tosh noir, but that isn't going to help us really establish the time of death. So we're going to actually have to pull out fluids from inside the eye to be able to calculate time of death using eyes. Now what we're looking for is what's called the vitreous humor, and inside it we're looking for potassium concentrations. So you may, if you're in intramolecules and cells, be talking about potassium pumps, and so using active transport of potassium. Now if you are dead, your potassium ion pumps are not... filtering potassium out of the center of your eye and creating that equilibrium. So what ends up happening is the potassium starts to build up in the vitreous humor. So the more potassium that we have in that vitreous humor, the longer the person has been dead. So that's the theory behind this. So we have these two calculations. So you can either do hours since death or days since death using this calculation. you'll realize that the only difference is dividing by 24 if you're looking for days. Okay, so what you want is this 5.26 is a constant. So this number has been calculated based on a standard, so we are just going to roll with this. So you'll actually, if you realize, this is a linear equation, right? Like y equals mx plus b, we're just, it's a sloping downward so it's a negative right so this number comes from the slope right so this is how steep it is so all you're going to do is take 5.26 and multiply it by whatever the potassium concentration it is so you'll remember from the forensic pathology lab we used a syringe to attempt to pull the vitreous humor from our rats right but there was that fluid some of y'all were able to see at least coming out of that eye, we take that and then we send it to toxicology and they tell us that concentration. And then you subtract it by 30.9. Okay. And then that will tell you how many hours it has been since the individual has died. And again, if you want to do it by days, you just divide that by 24. And it's really important. Remember, your order of operations, multiply these two things together first before you subtract 30.9. And that's it for pathology. So like I said, it's fairly simple mathematical equations. Now we're going to move into time of colonization estimations. These are a little bit trickier, not because the math is harder. There's just multiple steps to get to one calculation. through this step by step in order to help establish what and how we do these things. Okay, before we get into that, I want to talk about the difference between time of colonization and postmortem interval, because it drives me nuts when people say I do postmortem interval, I physically cannot do postmortem interval, and we're going to talk about why. So in this, we have our last known alive date for this person. we have the date of discovery. So as the investigators, they know that somewhere between here and here, they have died, right? So what we're trying to establish is the time in which this individual had died, right? So time of death. Okay, so when we look at the physiological process of decomposition, right? We just talked about vitreous tumor concentrations, rigor mortis, alga mortis. Those are physiology of the person, right? We're looking at decomposition and using calculations of decomposition of the person to establish their time of death. That is what I'm defining as the postmortem interval, right? So this is going to tell you from the date of discovery all the way from the time that they died, because these things happen immediately, right? Now, there is going to be an elapsed period of time between when the person dies. and when insects locate the body and lay eggs on it. Okay, this could be minutes, it could be hours, it could be days, it could be weeks. This time is called the pre-colonization interval, right? There's no insects laid on the body. We don't know enough to establish this, right? So when we're doing the time of colonization, we're really looking at what's called the post-colonization interval. Okay, so this is when individuals have colonize the body we're looking at immatures right the eggs maggots and things like that we're going to be using the post colonization interval to go from the time of discovery to when these insects laid right so this is why we call this the time of colonization until we can estimate this pre-colonization interval and add that to the time of colonization we can't say we're establishing the exact time of death because we don't know this right we can't say with certainty and we also can't say because we can't say it's time of death we can't call it the postmortem interval all right so what we're looking for in research right is how to establish the entire period of insect activity so again even if we could calculate the pre-colonization interval and establish it on top of the post-colonization interval we would call it the period of insect activity right because there's always going to be scenarios Where there might be a barrier that we have no clue about. If someone murders someone and then puts them in a freezer. for three years no insects are going to be on that body until they take them out right and we can't establish what that context is unless we have all the information and so we're going off the assumption we don't have all of the information so we would always change time of colonization would just be the period of insect activity not post-mortem interval all right but i digress right so how do we calculate this post or this time of colonization right so we need a few different things like i said it's not the calculations hard there's just a few steps okay and we're going to walk through all of these different steps the first thing that we need is to understand what insects we have what species they are and their age so remember i just told you in the post colonization interval we're working with immatures right? So these are going to be the eggs, larvae, pupae, right? Not the flying adults, unless there's evidence that they've completed their life cycle. We had larvae at one point and now they're all adults, then we would use the adults. Otherwise we're really just going to be working with immatures. We need to also have development data for that species. It needs to be specific for the species and ideally from the same area. We also need something called base temperature. We'll talk about what that is. And then environmental data from the actual scene. So we need two different types of data sets here. So the first thing we do after we get all this data is calculating the accumulated degree days. for insect development, so for this development data set, and then calculating it for the scene, and then we're going to compare those. That probably sounds very confusing to you, do not panic. Like I said, we're going to walk through every single one of these. First, I want to go over what an accumulated degree day is. Okay, so a degree day is just a measurement of time in relation to temperature. It is a weighted way to look at time. So the accumulated degree days is just a single degree day accumulated on top of each other. So I like to think of it this way. A day is to week as a degree day is to accumulated degree day, right? You have seven days in a week. So you can have a hundred degree days in an accumulated degree day. So hopefully that makes a little bit more sense. Where basically the degree day is just a singular day and the accumulated degree days is that entirety of the time that we're looking at. Okay, again if that still confuses you, it might help to see it, right? So the next thing I wanted to talk about is this idea of base temperature. So base temperature is just the temperature in which the insects will stop developing. So I said in class that insects rely on temperature as a way to continue development. They're not like us. Right. You can't just put a timer on and say, OK, well, we'll wait two years and then it'll be a toddler. Right. It needs to have an appropriate temperature for it to continue developing. So what we have here is what's called activity rate. This is a thermal model. This is general for any organism. So we can establish this with plants, with microbes, with insects, the list goes on. All organisms that rely on temperature as a function of their development have a base temperature. You have what's called a critical thermal maximum. This is the temperature in which someone will die. And then you have what's called the critical thermal minimum. This is when an individual will die when it's too cold, right? So we have this is zero degrees, this is like 300 we'll say. It'll probably not be that high. Usually organisms, especially insects, will die around like 40 degrees celsius, so above human body temperature. What you can see here is you have your critical thermal minimum and maximum, and then there's a flat line here. So what is actually happening during this flat line is no activity. We call this knockdown. This just means that at this temperature, right, this could be let's say this is five is our critical thermal minimum right but between five degrees celsius and ten degrees celsius they're still alive they're just not awake right so they are in a comatose state um this temperature right this lowest temperature is called your base temperature right this is the lowest temperature in which development stops so this is just talking about activity rate we can replace this right and just do development here so once development stops right and they're not dead they're just stopped developing that is your base temperature so right here and then um there is a hot equivalent to this we do not worry about high temperatures in our calculations this is a source of error within forensic entomology and so there's a lot of people working on how to incorporate this into calculations especially when you have cases in um arizona and texas and nevada and all these places that get very very hot we for purposes of this class base temperature will only be at this minimum threshold here um what's important to note right is that Not all species will have the same temperature. The temperature that is considered base temperature is going to be species-specific, but also population-specific. So, for example, you have a species that exists in both Mexico and northern Maine. Their geographic range spans the entirety of the U.S. and Mexico. The population that has lived their entire lives in Mexico and all of their generations before them have come out of Mexico is going to have a higher base temperature than those that have been living in northern Maine because the ones that have been living in northern Maine have been subjected to very very cold temperatures so they become more cold tolerant as generations go on. So if you take an intro to organisms we've talked about natural selection and how populations change over time based on environmental pressures. That is why. So this is why we have to be very specific when we're looking at the species that we're looking at. That's why step one, identify who you have, and then we can start identifying this biological information. Okay, now as far as base temperature, you don't have to know anything other than what identifying that temperature is and I'm going to talk to you about what that is. Okay, so just know there's a temperature that lower temp that temperature just means that is when temperature stops or development stops. Okay, so now we can start talking about how we establish this time of colonization. Right, so you are going to have a maggot that's collected from the scene. And your job is to establish when eggs were laid, right? So every time we do this, it doesn't matter if, you know, the whole life cycle has been completed and we're looking at adults, it would kick all the way back around to establish when eggs were laid. Remember, we're always working with time of colonization, not postmortem interval when we're looking at insects. Okay, so how do we actually know the age of the larvae? It's kind of easy. So this is a maggot right here. So right here are the mouth hooks. So this is the mouth and this is the posterior end. Right. So this is the back end. Now, it's might be easy to see. Maybe it's a little hard. These two little black dots. Right. These are called spiracles. These spiracles, I like to akin them to nostrils. There are they are breathing holes. OK, so these maggots breathe in part out the. back end of their body. This allows them to continually feed or exist in the maggot mass, right, and still be able to breathe. So when you're looking at the maggot, all you need to do is look at the posterior end, look at those slits or the the spiracles, and what you're looking for is the number of slits. Okay, so you have three instars, right, for your your maggots. We're not talking about beetles right now, just by the way. This is strictly for our flies. And you'll look at the nostrils. So this is one nostril. So this is actually the left nostril. This little button is actually called a button is towards the midline. So towards the center of the maggot. And if you see a singular use shape slit, you're looking at a first instar larvae. So I always tell individuals, if there's one slit, first instar, right? So one. If there are two slits, super challenging, it's a second instar, right? So two slits equals second instar. And if you have three slits, then you're looking at a third instar. So all you're looking for is these number of slits right here. And they actually are called slits. It's not something super complicated. So if you have a maggot, in class, I will show you a picture of the posterior end of a fly. I want you to count the slits. If it is super hard to tell because these slits are so close together, um if it's one or if it's two it's likely one right because sometimes they fuse but look how much space there is and i know this is a drawing but it's fairly accurate um it's also important to note that these slits do not always look the same so sometimes you have um different so there's housefly larvae that have an s-shaped slit so we call that sinuous That's not something that you have to worry about. The ones that you'll be working with in this class are going to be flat. But regardless, the rule is the number of slits equals the number of instars it is. OK, so calculating time of colonization. Again, we are looking at the time of discovery to when the adult fly had laid the eggs. OK, so first. we're going to take the species and the age, then we're going to look at temperature, the environmental temperature in which the remains or the larvae are found, and then we're going to go back in time to this point, right? So we need now temperature. This temperature goes two different ways, right? So you're going to get something called development data. This is something that is done specifically with a species. So you'll see here the species that we're working with is Chrysomia megacephala. And the temperature that they're using is 23.13 degrees Celsius. This data set is generated inside a lab. So they had taken Chrysomia megacephala eggs. They put them in an incubator at this temperature. And they counted, they went in and they checked every two hours what life stage was present over and over and over and over again until the cows come home. Okay. And then every single time the stage ended and the new stage started, they determined what that time was. Okay. So what you see here is the different life stages. So it's just written out. Right. And so remember, you have your third instar larvae and your pre-pupae. This is just the wandering third instar. So remember I told you there's a time where third instars stop feeding and they leave the body, but they're still maggots, right? They're just clearing out their crop, so their stomach, and then they're going to go pupate, okay? What's really important too is development tables will tell you what the calculation is. So this is hours to complete the life stage, right? So this is 15 hours. between the time the study started until the time it finished the egg stage. So when you started seeing first instars, it was 15 hours. All right, and what I also want you to realize, because we're going to do accumulations, this 18 is not 18 to the start time of the study. This is 18 hours from the time you saw first instar to when you no longer saw first instars. Okay. And this was 45 hours from the time that you stopped seeing first instars to the time that you started seeing third instars. Right. So this is strictly for the second instar stage. It is not to the start of this study. OK, it is very rare that that happens. What I also want you to note is there's always going to be a base temperature that is given to you in this class. In reality, they don't always give a base temp. Okay. It's really annoying. Um, but in this case, I will always give you the base temperature. So it's not up for questioning or debate or, and I won't make you calculate it. it's fine. Just always look for that base temperature. So the first thing we need to do, because we operate not in hours when we're working with time of colonization, we work with days. And the reason being is because, especially since we don't have development data for every single location in the world, you have to operate at bigger time intervals to be more conservative, right? We don't want to cut off. a certain part of a day just because the calculation said it didn't go into that 30 minute mark, right? So to be more conservative, we say it could have been any time during that entire day. So we'll talk about what that means. All I want you to know right now is you have to convert those hours to days because we're going to be using days in our calculations. You'll also thank me later because we'll have to convert environmental data. and if you had to do hours that is so much calculations so convert to days all right and what you'll get are these decimals right so just to show you the calculation right we had 15 hours in our egg stage divide that by 24 we get 0.63 okay the next thing we have to do is calculate what is called the degree days remember our degree days is taking account temperature into time, right? So it's temperature and time. Because we're operating in days, we're going to be doing the number of days, which will be what we just calculated, right? And the temperature is going to be the temperature, the ambient temperature minus the base temperature. So what this looks like, right? So we have our temperature that the study was at. minus base temperature, and then multiply that by the time interval. So when we do this first column, right, when we do this for the eggs, we get 8.21, all right? And then you can do this, like you can double check my math on all these individual ones, right? But the more important thing is you have to go line by line in this. Do not do them all together. It'll be a really long calculation. It should not. We're doing things truncated right now. Okay, so now we have to accumulate our degree days. This is where students start getting a little confused. Okay, so we're going to walk through this fairly slowly. So what you need to do is take your previously accumulated degree days. So that is in purple here. If we were going to do the accumulated degree days for the first Instar, then we have the degree days that we previously calculated for our first instar. We have to add that to the 8.21, okay? And then we get our accumulated degree days. What is really important is you'll note that we just shifted this 8.21 over. So in that first stage, it'll always just shift over because there's nothing accumulated before it, right? It's just zero. So if you wanted to, you could just 8.21 plus zero. but you can just shift it over right and so when we add these two things together we get 18.38 right and so you have to do this down again right so now you take that 18.38 you add it by 24.64 and then you get this 43.21 and then you can do this all the way down right so again still moving line by line but taking your previous calculation and adding it to the degree days you calculated in the previous step All right, and that's it. So remember the degree days is a day as accumulated degree days is a week, right? So now we've accumulated all this time. So if you had a pupae, right, the amount of accumulated degree days it took for a pupae to develop at this temperature was 266.97 days. OK, that's a long time. right? But it is weighted by temperature. So this isn't actual days, right? It's degree days. Okay. Now let's talk about environmental data. Okay. So first step, do the development data, set that to the side. Okay. We don't have to worry about that until the very end. So we do these steps independent of one another until it's time to compare them all together. All right. So don't worry about the development data for now. we're going to move on to environmental data. Okay, the important thing, right, we always get environmental data from what's called NOAA. So this is the National Oceanic Administration. So this is a government agency, right, that is one of their jobs is calculating the temperature for regions, right? So there's a bunch of different stations that are located all over the United States. Many of them are at airports. So I usually pull a lot of my data from the airport. You don't have to worry about pulling this. I'm going to give you environmental data. What is really important is we work in the United States, right? And so a lot of this data is going to either be solely in Fahrenheit or is going to be given to you inherently in Fahrenheit. Now, I usually pull it with Celsius because I have a way to do that. But for practice, I want you to get in this established idea, right, that you're going to have. Data that is in Fahrenheit and you have to convert it. Okay, so You can convert Fahrenheit to Celsius using Google. There's also Excel that you can use there is a if you google how to convert Fahrenheit to Celsius. It's a very simple Equation that you can put into Excel and in that particular cell highlight what you want to convert and it will give you the answer Or you can use this calculation, right? So you have your temperature in Fahrenheit minus 32 multiplied by 0.5556. Now, how you convert is going to change the ultimate answer, right? But because we're working in days, changes in decimals should not change the calendar days we are giving investigators. So that's also why we work in days, right? Okay, what you need to do regardless, right? So find your degree Celsius. We're going to have to find the average temperatures for our environmental data. So usually you're going to be given two temperatures for every day, right? So you need to figure out that daily max temperature, the daily minimum temperature, divide it by two, and then you'll have your average temperature for that day. Okay, I'm going to show you what this looks like. But you have to do this because we're not going to be working. It's not going to be the hottest temperature for the entirety of the day. Right. And it's not going to be the lowest temperature for the entirety of the day. So we do need to find that average. OK, so what this looks like. Right. When you get your development data set, you're going to have that average or the maximum. You'll have to calculate this average. OK. And again, you're doing this for every single individual day. Now, to be able to use this environmental data for time of colonization, you might have already started reading, we have to flip this. Now, you might be like, why would we go backwards on a calendar, right? So why this is asterisked is because this is the date of discovery, right? So remember that when we're working with time of colonization, we are going backwards in time, okay? So what we need to do is start the first row should always be your date of discovery and you should be operating backwards. So you could see 3 uh 3-7 3-7 15.83 15.83 if you double checked that all of these would be the reverse order if that's what what is over here. Okay that is step one. If you miss this step you're going to be wrong um every single time. right so this is a really really important step that you cannot miss and students oftentimes forget all right the next thing that we need to do is determine how many hours the insects were on the body that day now if it's not the date of discovery it will always be 24 because we're assuming the entirety of that day possibly had insects on them right because we weren't there We don't know all the contextual information. So if the body was exposed, right, and insects had access on 3-8, we don't know when. So we're just going to give the full 24 hours, okay? And we'll talk about why that works in a little bit. But this date of discovery, we know that they weren't there the full 24 hours, right? Unless it was at 1159 that night, right? So what we need to do is count how many hours it has been into the 21st day of this month, right? So how you do this, right, because we also work with police officers who, you know, grant us the opportunity to work in military time. So you might already been establishing that this is 1800 hours. Right. So 1800 hours, 18 hours have accumulated in the day. OK, so this would be six o'clock at night. So if it's six o'clock, you have to convert it. Right. You have to figure out how many. hours it's been since midnight, right? Because that's when the day starts. All right, moving on. Now, we have to have a column for base temperature. Remember, we just worked with our development set and we had to incorporate base temperature into our development set. We have to do that with our environmental data as well. Now, base temperature is going to be dictated based on your development data, okay? So your development data, if it says the base temp is five, your environmental data better have five in the base temp column. In our example, it was 10. So we're going to put 10 in our column. All right, now we're going to do degree days just like we did for all of our life stages, just for every single day, right? So remember based degree days is temperature minus the base temperature. multiplied by the days. So we're taking this average temperature that we calculated from our environmental data. Then we are subtracting this base temperature, and then we're multiplying it by the number of days. Now, the trick is, right, that we have hours here, not days. So don't multiply it by 24, right? Because that's going to be 24 days. Don't do that. So what we what I tend to do instead, because you will never forget this first step of figuring out how many hours was actually accumulated instead of just slapping a 24 on there. Take the number of hours and divide it by 24. Right. That will give you days. If it was a day that we are assuming the full day. Right. It's just 24 divided by 24. That's one. Right. But if it's the 18 divided by 24. Right. That will give you a different calculation. All right. So in this section here, it actually looks like this. Right. Instead of just slapping a one down. If you don't want to do it this way, you can just put a one here. You just have to remember to put the decimal here on that first the date of discovery. Right. OK. So then all we need to do is. accumulate those degree days right so we push this 1.88 over and then we add 1.88 to 7.5 and we get 9.38 then we take that 9.38 add it to 13.06 and then we get 22.43 so on so on and so forth right so we do that over and over and over and over again now if you're clever right you just do this in excel right excel you can make these equations right and it just spits it out for you I can give you a time of colonization estimate in less than five minutes because I have Excel sheets that do the thing for me. At this stage, it'll be easier for you to do it by hand because you have a little bit more control. If you mess up on an Excel document and you don't know exactly what you're doing, your calculation will be wrong, but you won't know why. Okay, now, we have... both our development data that we just used, we just calculated all the degree days, right, and then two seconds ago we developed the environmental data, right? Now we need to look at our data here and compare these two things. All right, so up here is an actual picture of a spherical. So I said that picture was pretty accurate. This is a actual spherical that you'd be looking for. So hopefully you've counted the number of slits already. Right. So then we have one, two, three. Hopefully you've realized that this is our third instar. Right. We are going to assume that that crop is full. Right. I would tell you if it was. Right. So what this will look like in class. I will give you a picture of a fly. I will tell you that species of fly. Remember, you have to have the correct development data for that species and you'll have to age it. Right. So hopefully you've already aged it. We have chrysomia megacephala. We already worked with the chrysomia megacephala development set. We're good to go. right? And so now this is what the tricky part is. We know we have a third instar, okay? And we're saying this is a third instar that's feeding, okay? You have to remember that this data set was established to complete the life stage, okay? So what I was telling you before, right, that zero is the start of the study and that 15 is the end of that life stage, right? So anywhere between 0 and 15 hours. So if we went in at two hours, it would still be an egg, right? So we need to get that entirety of our sample, right? So what this means is we have to get this accumulated degree day for the end of the third instar and the end of the second. Because remember, the end of the second instar also means it's the start of the third instar, okay? So always what I always remember, right, you take your known age and include the previous one, right? So for the purposes of this class, always operate with that mentality, right? So we have third and star, so grab third and star and grab that previous one for this reason, right? Because we want that beginning and end of that third and star. Okay, so let's just pull this over here. And now we're going to look at our environmental data set. At this point, there is no more math. We are just comparing our development data set. So we know that at the temperature in which we had the data set, that it will take 78.90 accumulated degree hours to end the third instar and 43.02 to start it. So this is our range of accumulated degree days. So what I want you to do is pop over here and look to see if you can find the upper bound of the 78.90, right, of when our third instar will end. OK, so take a second and look at that. And then I want you to find the lower bound. All right. Do you think you have your guess? So for our lower bound, right, we have 51.32 for our lower bound. Now, why would that be? So we have 37.15. Why would we not include that, right? So just like our development set, right, this time is from the date of discovery all the way to midnight. And this is midnight to midnight, midnight to midnight. So this day ends at 37.15. So this 43 that we're looking for is not included on the 18th, but it is included sometime on the 17th. Do you see what I mean there? So now if we keep going forward, right, hopefully you can estimate that we're also going to go with. the 14th, right? Because this has 84.38, all right? So now we don't just give these times, right? Because it could be any time between this day and this day. So we fill in that timeline, so it is a full range. It's not just saying it's on day seven and then day 14. It's all the days in between as well. And then what we can do is just kick across and see what calendar days these dates are or these accumulated degree days are okay so how this ends up being written out is time of colonization likely likely occurred sometime between the 14th of march and the 17th of march in the year 2021 okay now i say likely right because we could have missed an insect that had already finished developing Right. We could be biased in the way that whoever sampled the larvae did. Right. There's a lot of things that we can't control. So we leave it up to interpretation a little bit. Right. But based on the information we do have, this is our best estimate. OK. Usually it is a range like this. Sometimes there has been only one case that I had it within a 24 hour period. Like it was one day. There was no way the entire life stage. was accumulated in a single day. That only happened once, and it was with eggs. So it was a very early... stage of development, the later that you get in stages of development, you'll notice, you may have noticed, right, the development, the time, those hours get longer and longer. Okay, that's a general rule. So sometimes I'll get these ranges. Sometimes I'll have ranges that are multiple weeks, especially if we're working with beetles, not so much with flies. Flies, it's usually within a couple days or a few days. So this range is not unheard of. Okay. So that's it. It's fairly simple. Once you get past all the noise, right, if you can keep your system and your process pretty straight lined and focus on one thing at a time, it does become fairly simple. If you try thinking about everything all at once, it becomes super jumbled and mess in your brain. It took me a very long time when I was actually first starting to really take the time and separate those two things because I was trying to figure it all out at once. Don't do that. It is far too complicated. But once you separate it out, it is a very, very simple calculation. Just take it step by step by step. All right. At this point, you should be able to do your assignment that's affiliated with this lecture. You are able to answer that as many times as you need to get the correct answer. OK, so I want you to use this video. a way to practice. Hopefully you've been taking notes so that way it is easier for you to remember the steps later on. But if you have any questions please make sure to reach out to me for any help.

Now, I told you this happens at a rate about one to one and a half degrees Celsius per hour, but there is a rate of change. that is not necessarily linear. So we can't always go based off of that.

So we do have a calculation specifically for Algermortis. Now this calculation is very simple. All it has is 37 C. So you may realize that is the average human body temperature. And then you subtract that by the current body temperature.

So you have to take the decedent's temperature at the time that you discover it. And then you divide this by a constant variable. And this constant variable is called T.

And so T just changes based on whatever the ambient temperature is. So if the temperature is less than 32 degrees Fahrenheit, which is 0 degrees Celsius, so that's freezing, right? Then T is 1.5.

And if it is above freezing or equal to freezing. it's actually 0.75. So what this gives you is how many hours since death.

This will not ever be days because this is going to happen fairly quickly. Now rigor mortis is a fairly common assessment of time of death as well. It's not as clear-cut as algor mortis but if you recall, rigor mortis is just that stiffening of the muscles so the body becomes very tense and then over time, like I told you, it's going to become relaxed again.

So we can actually look at the body, where different parts of the body are undergoing rigor, and establish an approximate timeline. So within one to four hours you expect just the jaw and the neck to undergo rigor and then after eight the full body being under rigor. Now when rigor stops it is more or less in the opposite order in which it initiated so after 24 hours the jaw will start to go limp but after 36 hours we can't use rigor mortis anymore because decomposition has fully underwent. undergone.

So if the body is completely limp and there's evidence of decomp, we are not early stage decomp where rigor might just not have set in yet or not recognizing where. So after that happens, we know decomp happens and rigor or post rigor, we know that the individual likely has been dead for at least 36 hours. So I do want to just kind of remind you that during these early hours of decomposition there's still a lot of changes happening. So there is that blackening of the eye called tosh noir, but that isn't going to help us really establish the time of death.

So we're going to actually have to pull out fluids from inside the eye to be able to calculate time of death using eyes. Now what we're looking for is what's called the vitreous humor, and inside it we're looking for potassium concentrations. So you may, if you're in intramolecules and cells, be talking about potassium pumps, and so using active transport of potassium. Now if you are dead, your potassium ion pumps are not...

filtering potassium out of the center of your eye and creating that equilibrium. So what ends up happening is the potassium starts to build up in the vitreous humor. So the more potassium that we have in that vitreous humor, the longer the person has been dead.

So that's the theory behind this. So we have these two calculations. So you can either do hours since death or days since death using this calculation. you'll realize that the only difference is dividing by 24 if you're looking for days. Okay, so what you want is this 5.26 is a constant.

So this number has been calculated based on a standard, so we are just going to roll with this. So you'll actually, if you realize, this is a linear equation, right? Like y equals mx plus b, we're just, it's a sloping downward so it's a negative right so this number comes from the slope right so this is how steep it is so all you're going to do is take 5.26 and multiply it by whatever the potassium concentration it is so you'll remember from the forensic pathology lab we used a syringe to attempt to pull the vitreous humor from our rats right but there was that fluid some of y'all were able to see at least coming out of that eye, we take that and then we send it to toxicology and they tell us that concentration.

And then you subtract it by 30.9. Okay. And then that will tell you how many hours it has been since the individual has died.

And again, if you want to do it by days, you just divide that by 24. And it's really important. Remember, your order of operations, multiply these two things together first before you subtract 30.9. And that's it for pathology. So like I said, it's fairly simple mathematical equations. Now we're going to move into time of colonization estimations.

These are a little bit trickier, not because the math is harder. There's just multiple steps to get to one calculation. through this step by step in order to help establish what and how we do these things. Okay, before we get into that, I want to talk about the difference between time of colonization and postmortem interval, because it drives me nuts when people say I do postmortem interval, I physically cannot do postmortem interval, and we're going to talk about why. So in this, we have our last known alive date for this person.

we have the date of discovery. So as the investigators, they know that somewhere between here and here, they have died, right? So what we're trying to establish is the time in which this individual had died, right?

So time of death. Okay, so when we look at the physiological process of decomposition, right? We just talked about vitreous tumor concentrations, rigor mortis, alga mortis. Those are physiology of the person, right? We're looking at decomposition and using calculations of decomposition of the person to establish their time of death.

That is what I'm defining as the postmortem interval, right? So this is going to tell you from the date of discovery all the way from the time that they died, because these things happen immediately, right? Now, there is going to be an elapsed period of time between when the person dies. and when insects locate the body and lay eggs on it. Okay, this could be minutes, it could be hours, it could be days, it could be weeks.

This time is called the pre-colonization interval, right? There's no insects laid on the body. We don't know enough to establish this, right?

So when we're doing the time of colonization, we're really looking at what's called the post-colonization interval. Okay, so this is when individuals have colonize the body we're looking at immatures right the eggs maggots and things like that we're going to be using the post colonization interval to go from the time of discovery to when these insects laid right so this is why we call this the time of colonization until we can estimate this pre-colonization interval and add that to the time of colonization we can't say we're establishing the exact time of death because we don't know this right we can't say with certainty and we also can't say because we can't say it's time of death we can't call it the postmortem interval all right so what we're looking for in research right is how to establish the entire period of insect activity so again even if we could calculate the pre-colonization interval and establish it on top of the post-colonization interval we would call it the period of insect activity right because there's always going to be scenarios Where there might be a barrier that we have no clue about. If someone murders someone and then puts them in a freezer. for three years no insects are going to be on that body until they take them out right and we can't establish what that context is unless we have all the information and so we're going off the assumption we don't have all of the information so we would always change time of colonization would just be the period of insect activity not post-mortem interval all right but i digress right so how do we calculate this post or this time of colonization right so we need a few different things like i said it's not the calculations hard there's just a few steps okay and we're going to walk through all of these different steps the first thing that we need is to understand what insects we have what species they are and their age so remember i just told you in the post colonization interval we're working with immatures right?

So these are going to be the eggs, larvae, pupae, right? Not the flying adults, unless there's evidence that they've completed their life cycle. We had larvae at one point and now they're all adults, then we would use the adults.

Otherwise we're really just going to be working with immatures. We need to also have development data for that species. It needs to be specific for the species and ideally from the same area.

We also need something called base temperature. We'll talk about what that is. And then environmental data from the actual scene. So we need two different types of data sets here.

So the first thing we do after we get all this data is calculating the accumulated degree days. for insect development, so for this development data set, and then calculating it for the scene, and then we're going to compare those. That probably sounds very confusing to you, do not panic.

Like I said, we're going to walk through every single one of these. First, I want to go over what an accumulated degree day is. Okay, so a degree day is just a measurement of time in relation to temperature. It is a weighted way to look at time.

So the accumulated degree days is just a single degree day accumulated on top of each other. So I like to think of it this way. A day is to week as a degree day is to accumulated degree day, right?

You have seven days in a week. So you can have a hundred degree days in an accumulated degree day. So hopefully that makes a little bit more sense. Where basically the degree day is just a singular day and the accumulated degree days is that entirety of the time that we're looking at. Okay, again if that still confuses you, it might help to see it, right?

So the next thing I wanted to talk about is this idea of base temperature. So base temperature is just the temperature in which the insects will stop developing. So I said in class that insects rely on temperature as a way to continue development. They're not like us. Right.

You can't just put a timer on and say, OK, well, we'll wait two years and then it'll be a toddler. Right. It needs to have an appropriate temperature for it to continue developing. So what we have here is what's called activity rate. This is a thermal model.

This is general for any organism. So we can establish this with plants, with microbes, with insects, the list goes on. All organisms that rely on temperature as a function of their development have a base temperature. You have what's called a critical thermal maximum. This is the temperature in which someone will die.

And then you have what's called the critical thermal minimum. This is when an individual will die when it's too cold, right? So we have this is zero degrees, this is like 300 we'll say. It'll probably not be that high. Usually organisms, especially insects, will die around like 40 degrees celsius, so above human body temperature.

What you can see here is you have your critical thermal minimum and maximum, and then there's a flat line here. So what is actually happening during this flat line is no activity. We call this knockdown.

This just means that at this temperature, right, this could be let's say this is five is our critical thermal minimum right but between five degrees celsius and ten degrees celsius they're still alive they're just not awake right so they are in a comatose state um this temperature right this lowest temperature is called your base temperature right this is the lowest temperature in which development stops so this is just talking about activity rate we can replace this right and just do development here so once development stops right and they're not dead they're just stopped developing that is your base temperature so right here and then um there is a hot equivalent to this we do not worry about high temperatures in our calculations this is a source of error within forensic entomology and so there's a lot of people working on how to incorporate this into calculations especially when you have cases in um arizona and texas and nevada and all these places that get very very hot we for purposes of this class base temperature will only be at this minimum threshold here um what's important to note right is that Not all species will have the same temperature. The temperature that is considered base temperature is going to be species-specific, but also population-specific. So, for example, you have a species that exists in both Mexico and northern Maine.

Their geographic range spans the entirety of the U.S. and Mexico. The population that has lived their entire lives in Mexico and all of their generations before them have come out of Mexico is going to have a higher base temperature than those that have been living in northern Maine because the ones that have been living in northern Maine have been subjected to very very cold temperatures so they become more cold tolerant as generations go on. So if you take an intro to organisms we've talked about natural selection and how populations change over time based on environmental pressures.

That is why. So this is why we have to be very specific when we're looking at the species that we're looking at. That's why step one, identify who you have, and then we can start identifying this biological information. Okay, now as far as base temperature, you don't have to know anything other than what identifying that temperature is and I'm going to talk to you about what that is. Okay, so just know there's a temperature that lower temp that temperature just means that is when temperature stops or development stops.

Okay, so now we can start talking about how we establish this time of colonization. Right, so you are going to have a maggot that's collected from the scene. And your job is to establish when eggs were laid, right?

So every time we do this, it doesn't matter if, you know, the whole life cycle has been completed and we're looking at adults, it would kick all the way back around to establish when eggs were laid. Remember, we're always working with time of colonization, not postmortem interval when we're looking at insects. Okay, so how do we actually know the age of the larvae? It's kind of easy.

So this is a maggot right here. So right here are the mouth hooks. So this is the mouth and this is the posterior end.

Right. So this is the back end. Now, it's might be easy to see.

Maybe it's a little hard. These two little black dots. Right.

These are called spiracles. These spiracles, I like to akin them to nostrils. There are they are breathing holes.

OK, so these maggots breathe in part out the. back end of their body. This allows them to continually feed or exist in the maggot mass, right, and still be able to breathe. So when you're looking at the maggot, all you need to do is look at the posterior end, look at those slits or the the spiracles, and what you're looking for is the number of slits.

Okay, so you have three instars, right, for your your maggots. We're not talking about beetles right now, just by the way. This is strictly for our flies. And you'll look at the nostrils.

So this is one nostril. So this is actually the left nostril. This little button is actually called a button is towards the midline. So towards the center of the maggot. And if you see a singular use shape slit, you're looking at a first instar larvae.

So I always tell individuals, if there's one slit, first instar, right? So one. If there are two slits, super challenging, it's a second instar, right?

So two slits equals second instar. And if you have three slits, then you're looking at a third instar. So all you're looking for is these number of slits right here.

And they actually are called slits. It's not something super complicated. So if you have a maggot, in class, I will show you a picture of the posterior end of a fly. I want you to count the slits. If it is super hard to tell because these slits are so close together, um if it's one or if it's two it's likely one right because sometimes they fuse but look how much space there is and i know this is a drawing but it's fairly accurate um it's also important to note that these slits do not always look the same so sometimes you have um different so there's housefly larvae that have an s-shaped slit so we call that sinuous That's not something that you have to worry about.

The ones that you'll be working with in this class are going to be flat. But regardless, the rule is the number of slits equals the number of instars it is. OK, so calculating time of colonization.

Again, we are looking at the time of discovery to when the adult fly had laid the eggs. OK, so first. we're going to take the species and the age, then we're going to look at temperature, the environmental temperature in which the remains or the larvae are found, and then we're going to go back in time to this point, right? So we need now temperature. This temperature goes two different ways, right?

So you're going to get something called development data. This is something that is done specifically with a species. So you'll see here the species that we're working with is Chrysomia megacephala. And the temperature that they're using is 23.13 degrees Celsius.

This data set is generated inside a lab. So they had taken Chrysomia megacephala eggs. They put them in an incubator at this temperature.

And they counted, they went in and they checked every two hours what life stage was present over and over and over and over again until the cows come home. Okay. And then every single time the stage ended and the new stage started, they determined what that time was. Okay.

So what you see here is the different life stages. So it's just written out. Right. And so remember, you have your third instar larvae and your pre-pupae.

This is just the wandering third instar. So remember I told you there's a time where third instars stop feeding and they leave the body, but they're still maggots, right? They're just clearing out their crop, so their stomach, and then they're going to go pupate, okay? What's really important too is development tables will tell you what the calculation is.

So this is hours to complete the life stage, right? So this is 15 hours. between the time the study started until the time it finished the egg stage. So when you started seeing first instars, it was 15 hours. All right, and what I also want you to realize, because we're going to do accumulations, this 18 is not 18 to the start time of the study.

This is 18 hours from the time you saw first instar to when you no longer saw first instars. Okay. And this was 45 hours from the time that you stopped seeing first instars to the time that you started seeing third instars.

Right. So this is strictly for the second instar stage. It is not to the start of this study.

OK, it is very rare that that happens. What I also want you to note is there's always going to be a base temperature that is given to you in this class. In reality, they don't always give a base temp. Okay.

It's really annoying. Um, but in this case, I will always give you the base temperature. So it's not up for questioning or debate or, and I won't make you calculate it.

it's fine. Just always look for that base temperature. So the first thing we need to do, because we operate not in hours when we're working with time of colonization, we work with days.

And the reason being is because, especially since we don't have development data for every single location in the world, you have to operate at bigger time intervals to be more conservative, right? We don't want to cut off. a certain part of a day just because the calculation said it didn't go into that 30 minute mark, right?

So to be more conservative, we say it could have been any time during that entire day. So we'll talk about what that means. All I want you to know right now is you have to convert those hours to days because we're going to be using days in our calculations.

You'll also thank me later because we'll have to convert environmental data. and if you had to do hours that is so much calculations so convert to days all right and what you'll get are these decimals right so just to show you the calculation right we had 15 hours in our egg stage divide that by 24 we get 0.63 okay the next thing we have to do is calculate what is called the degree days remember our degree days is taking account temperature into time, right? So it's temperature and time. Because we're operating in days, we're going to be doing the number of days, which will be what we just calculated, right? And the temperature is going to be the temperature, the ambient temperature minus the base temperature.

So what this looks like, right? So we have our temperature that the study was at. minus base temperature, and then multiply that by the time interval. So when we do this first column, right, when we do this for the eggs, we get 8.21, all right? And then you can do this, like you can double check my math on all these individual ones, right?

But the more important thing is you have to go line by line in this. Do not do them all together. It'll be a really long calculation.

It should not. We're doing things truncated right now. Okay, so now we have to accumulate our degree days.

This is where students start getting a little confused. Okay, so we're going to walk through this fairly slowly. So what you need to do is take your previously accumulated degree days.

So that is in purple here. If we were going to do the accumulated degree days for the first Instar, then we have the degree days that we previously calculated for our first instar. We have to add that to the 8.21, okay? And then we get our accumulated degree days. What is really important is you'll note that we just shifted this 8.21 over.

So in that first stage, it'll always just shift over because there's nothing accumulated before it, right? It's just zero. So if you wanted to, you could just 8.21 plus zero.

but you can just shift it over right and so when we add these two things together we get 18.38 right and so you have to do this down again right so now you take that 18.38 you add it by 24.64 and then you get this 43.21 and then you can do this all the way down right so again still moving line by line but taking your previous calculation and adding it to the degree days you calculated in the previous step All right, and that's it. So remember the degree days is a day as accumulated degree days is a week, right? So now we've accumulated all this time. So if you had a pupae, right, the amount of accumulated degree days it took for a pupae to develop at this temperature was 266.97 days.

OK, that's a long time. right? But it is weighted by temperature. So this isn't actual days, right?

It's degree days. Okay. Now let's talk about environmental data.

Okay. So first step, do the development data, set that to the side. Okay.

We don't have to worry about that until the very end. So we do these steps independent of one another until it's time to compare them all together. All right.

So don't worry about the development data for now. we're going to move on to environmental data. Okay, the important thing, right, we always get environmental data from what's called NOAA. So this is the National Oceanic Administration.

So this is a government agency, right, that is one of their jobs is calculating the temperature for regions, right? So there's a bunch of different stations that are located all over the United States. Many of them are at airports. So I usually pull a lot of my data from the airport.

You don't have to worry about pulling this. I'm going to give you environmental data. What is really important is we work in the United States, right? And so a lot of this data is going to either be solely in Fahrenheit or is going to be given to you inherently in Fahrenheit.

Now, I usually pull it with Celsius because I have a way to do that. But for practice, I want you to get in this established idea, right, that you're going to have. Data that is in Fahrenheit and you have to convert it. Okay, so You can convert Fahrenheit to Celsius using Google.

There's also Excel that you can use there is a if you google how to convert Fahrenheit to Celsius. It's a very simple Equation that you can put into Excel and in that particular cell highlight what you want to convert and it will give you the answer Or you can use this calculation, right? So you have your temperature in Fahrenheit minus 32 multiplied by 0.5556.

Now, how you convert is going to change the ultimate answer, right? But because we're working in days, changes in decimals should not change the calendar days we are giving investigators. So that's also why we work in days, right? Okay, what you need to do regardless, right? So find your degree Celsius.

We're going to have to find the average temperatures for our environmental data. So usually you're going to be given two temperatures for every day, right? So you need to figure out that daily max temperature, the daily minimum temperature, divide it by two, and then you'll have your average temperature for that day.

Okay, I'm going to show you what this looks like. But you have to do this because we're not going to be working. It's not going to be the hottest temperature for the entirety of the day. Right. And it's not going to be the lowest temperature for the entirety of the day.

So we do need to find that average. OK, so what this looks like. Right. When you get your development data set, you're going to have that average or the maximum. You'll have to calculate this average.

OK. And again, you're doing this for every single individual day. Now, to be able to use this environmental data for time of colonization, you might have already started reading, we have to flip this.

Now, you might be like, why would we go backwards on a calendar, right? So why this is asterisked is because this is the date of discovery, right? So remember that when we're working with time of colonization, we are going backwards in time, okay? So what we need to do is start the first row should always be your date of discovery and you should be operating backwards. So you could see 3 uh 3-7 3-7 15.83 15.83 if you double checked that all of these would be the reverse order if that's what what is over here.

Okay that is step one. If you miss this step you're going to be wrong um every single time. right so this is a really really important step that you cannot miss and students oftentimes forget all right the next thing that we need to do is determine how many hours the insects were on the body that day now if it's not the date of discovery it will always be 24 because we're assuming the entirety of that day possibly had insects on them right because we weren't there We don't know all the contextual information. So if the body was exposed, right, and insects had access on 3-8, we don't know when. So we're just going to give the full 24 hours, okay?

And we'll talk about why that works in a little bit. But this date of discovery, we know that they weren't there the full 24 hours, right? Unless it was at 1159 that night, right?

So what we need to do is count how many hours it has been into the 21st day of this month, right? So how you do this, right, because we also work with police officers who, you know, grant us the opportunity to work in military time. So you might already been establishing that this is 1800 hours. Right. So 1800 hours, 18 hours have accumulated in the day.

OK, so this would be six o'clock at night. So if it's six o'clock, you have to convert it. Right.

You have to figure out how many. hours it's been since midnight, right? Because that's when the day starts.

All right, moving on. Now, we have to have a column for base temperature. Remember, we just worked with our development set and we had to incorporate base temperature into our development set.

We have to do that with our environmental data as well. Now, base temperature is going to be dictated based on your development data, okay? So your development data, if it says the base temp is five, your environmental data better have five in the base temp column.

In our example, it was 10. So we're going to put 10 in our column. All right, now we're going to do degree days just like we did for all of our life stages, just for every single day, right? So remember based degree days is temperature minus the base temperature.

multiplied by the days. So we're taking this average temperature that we calculated from our environmental data. Then we are subtracting this base temperature, and then we're multiplying it by the number of days.

Now, the trick is, right, that we have hours here, not days. So don't multiply it by 24, right? Because that's going to be 24 days.

Don't do that. So what we what I tend to do instead, because you will never forget this first step of figuring out how many hours was actually accumulated instead of just slapping a 24 on there. Take the number of hours and divide it by 24. Right.

That will give you days. If it was a day that we are assuming the full day. Right.

It's just 24 divided by 24. That's one. Right. But if it's the 18 divided by 24. Right. That will give you a different calculation. All right.

So in this section here, it actually looks like this. Right. Instead of just slapping a one down. If you don't want to do it this way, you can just put a one here. You just have to remember to put the decimal here on that first the date of discovery.

Right. OK. So then all we need to do is. accumulate those degree days right so we push this 1.88 over and then we add 1.88 to 7.5 and we get 9.38 then we take that 9.38 add it to 13.06 and then we get 22.43 so on so on and so forth right so we do that over and over and over and over again now if you're clever right you just do this in excel right excel you can make these equations right and it just spits it out for you I can give you a time of colonization estimate in less than five minutes because I have Excel sheets that do the thing for me.

At this stage, it'll be easier for you to do it by hand because you have a little bit more control. If you mess up on an Excel document and you don't know exactly what you're doing, your calculation will be wrong, but you won't know why. Okay, now, we have... both our development data that we just used, we just calculated all the degree days, right, and then two seconds ago we developed the environmental data, right? Now we need to look at our data here and compare these two things.

All right, so up here is an actual picture of a spherical. So I said that picture was pretty accurate. This is a actual spherical that you'd be looking for. So hopefully you've counted the number of slits already.

Right. So then we have one, two, three. Hopefully you've realized that this is our third instar.

Right. We are going to assume that that crop is full. Right.

I would tell you if it was. Right. So what this will look like in class.

I will give you a picture of a fly. I will tell you that species of fly. Remember, you have to have the correct development data for that species and you'll have to age it.

Right. So hopefully you've already aged it. We have chrysomia megacephala. We already worked with the chrysomia megacephala development set. We're good to go.

right? And so now this is what the tricky part is. We know we have a third instar, okay? And we're saying this is a third instar that's feeding, okay?

You have to remember that this data set was established to complete the life stage, okay? So what I was telling you before, right, that zero is the start of the study and that 15 is the end of that life stage, right? So anywhere between 0 and 15 hours. So if we went in at two hours, it would still be an egg, right?

So we need to get that entirety of our sample, right? So what this means is we have to get this accumulated degree day for the end of the third instar and the end of the second. Because remember, the end of the second instar also means it's the start of the third instar, okay?

So always what I always remember, right, you take your known age and include the previous one, right? So for the purposes of this class, always operate with that mentality, right? So we have third and star, so grab third and star and grab that previous one for this reason, right?

Because we want that beginning and end of that third and star. Okay, so let's just pull this over here. And now we're going to look at our environmental data set.

At this point, there is no more math. We are just comparing our development data set. So we know that at the temperature in which we had the data set, that it will take 78.90 accumulated degree hours to end the third instar and 43.02 to start it. So this is our range of accumulated degree days.

So what I want you to do is pop over here and look to see if you can find the upper bound of the 78.90, right, of when our third instar will end. OK, so take a second and look at that. And then I want you to find the lower bound.

All right. Do you think you have your guess? So for our lower bound, right, we have 51.32 for our lower bound. Now, why would that be?

So we have 37.15. Why would we not include that, right? So just like our development set, right, this time is from the date of discovery all the way to midnight. And this is midnight to midnight, midnight to midnight. So this day ends at 37.15.

So this 43 that we're looking for is not included on the 18th, but it is included sometime on the 17th. Do you see what I mean there? So now if we keep going forward, right, hopefully you can estimate that we're also going to go with.

the 14th, right? Because this has 84.38, all right? So now we don't just give these times, right?

Because it could be any time between this day and this day. So we fill in that timeline, so it is a full range. It's not just saying it's on day seven and then day 14. It's all the days in between as well. And then what we can do is just kick across and see what calendar days these dates are or these accumulated degree days are okay so how this ends up being written out is time of colonization likely likely occurred sometime between the 14th of march and the 17th of march in the year 2021 okay now i say likely right because we could have missed an insect that had already finished developing Right. We could be biased in the way that whoever sampled the larvae did.

Right. There's a lot of things that we can't control. So we leave it up to interpretation a little bit. Right.

But based on the information we do have, this is our best estimate. OK. Usually it is a range like this.

Sometimes there has been only one case that I had it within a 24 hour period. Like it was one day. There was no way the entire life stage.

was accumulated in a single day. That only happened once, and it was with eggs. So it was a very early...

stage of development, the later that you get in stages of development, you'll notice, you may have noticed, right, the development, the time, those hours get longer and longer. Okay, that's a general rule. So sometimes I'll get these ranges. Sometimes I'll have ranges that are multiple weeks, especially if we're working with beetles, not so much with flies. Flies, it's usually within a couple days or a few days.

So this range is not unheard of. Okay. So that's it.

It's fairly simple. Once you get past all the noise, right, if you can keep your system and your process pretty straight lined and focus on one thing at a time, it does become fairly simple. If you try thinking about everything all at once, it becomes super jumbled and mess in your brain.

It took me a very long time when I was actually first starting to really take the time and separate those two things because I was trying to figure it all out at once. Don't do that. It is far too complicated. But once you separate it out, it is a very, very simple calculation. Just take it step by step by step.

All right. At this point, you should be able to do your assignment that's affiliated with this lecture. You are able to answer that as many times as you need to get the correct answer. OK, so I want you to use this video.

a way to practice. Hopefully you've been taking notes so that way it is easier for you to remember the steps later on. But if you have any questions please make sure to reach out to me for any help.

Transcript for:Forensic Analysis of Time of Death

Transcript for:
Forensic Analysis of Time of Death