in a previous session we introduced The Run chart now we want to start turning our attention to thinking about the control charts which classically are known as shoe heart charts in honor of Dr Walter shuart so you remember the Run chart we had an X and A Y AIS time is always shown on the horizontal axis and the measure of improvement or our metric is shown on the vertical or Y AIS you can have patients months day time Monday Tuesday Wednesday all along the horizontal axis whether you're doing a run chart or control chart you remember the Run chart was a plot of the data over time and we put the median now the median x with a tilte above it gives us a center point it's also known as the 50th percentile what we're going to do on the control chart however is we're still going to have time on the horizontal the measure of interest on the vertical we're going to collect our data plot our dots but now instead of putting the median as the center line we're going to put the mean which is shown as x with a line above it otherwise known as xar now on the Run chart we didn't have control limits that's one of the advantages of moving to a control chart you end up getting what are called the upper control limit and the lower control limit these two boundaries help to define the variation in the process the tighter the variation the tighter the control Limits The Wider the variation the further apart these are you do not dictate or determine the upper and lower control limits the variation that in the data determines how wide or narrow these lines are now one of the things to realize is that these upper and lower control limits are known by specific names classically they are called Sigma limits so you will see them designated as SL in some software Sigma limits sometimes you'll see a symbol so that Sigma which is a symbol in basic statistics with a hat over it the hat is an estimate and this symbol Sigma is often times used to denote the standard deviation well these control limits are not standard deviations that's one thing to clearly remember they are not standard deviations they are Sigma limits and the way that is shown is with this little hat saying that these limits are estimat of the dispersion in the data the standard deviation is an estimate the range the difference between the Min and the max is also an estimate of the variation the standard deviation is another way to depict variation the key is that the standard deviation is a single number it's a statistic that's trying to give you the average dispersion in a whole group of data control limits are actually boundaries of a process that keeps changing over time so this is a fairly technical point but it's not that difficult to grasp the key thing is realizing that the standard deviation is not the same as a sigma limit and if you want to know more about that what you need to do is grab a book on statistical process control statistics and it will go into greater detail we don't have time to cover that today so basically the elements of the control chart look like a run chart except we're going to again replace the median with the mean we get the upper and lower control limit that tell us the boundaries of variation in the data we have data plotted over time and we have our measure on the vertical now another thing to realize is that with a run chart you can get away with less data than you can on a control chart you can make a run chart with upwards of around 10 data points when you move to a control chart you need to have at a minimum about 15 data points and preferably around 20 the reason being that the mean is more sensitive to point to-point variation than the median and so we need to have a little bit more data now think of this if you're collecting data monthly you're going to need 15 to 20 months of data before you can get enough data to make a chart that's why often times when we starting out with our Improvement initiatives we end up using the median and the Run chart because we can start with a little bit of data particularly if we're just getting a project underway so those are the basic elements of the control chart and the little bit of the difference between a run chart what we're going to do in the next piece of this video is to describe how we actually analyze and interpret a control chart