what is internal validity imagine the following experiment scientist was interested in understanding what factors were important for a long lifespan they thought that drinking water was important so they decided to get a bunch of people together in a room and gave them all a glass of water the scientists told them to wait around in the room and that they'd come back in 80 years to check in on them sure enough the research team checked back in after the 80 years and found the subjects had all died on the basis of this the researcher concluded water must be deadly of course this conclusion is a valid in this experiment the dependent variable that is the variable the experiment was trained to measure his lifespan lengths and the independent variable that is the variable that was changed by the experimenter was drinking a glass of water one of the things needed for a conclusion to be valid is that the independent variable has to be the only thing that influences the dependent variable in this lifespan experiment there are of course many other factors besides the independent variable that influence the dependent variable the least of which is locking people in a room and not feeding them 280 years this is what internal validity is all about it's the extent to which you're able to say that no other variables apart from the independent variable cause the resultant change in the dependent variable factors other than the independent variable that may influence the outcome of the experiment are called compounding variables so for the example lack of food is a confounding variable in this lifespan study even variables other than the independent variable can influence the dependent variable in this lifespan study this study has poor internal validity the best way to insure an experiment has good internal validity is to make sure there is a control group that gets treated the exact same way as the treatment group just without getting the treatment for example in the lifespan study the influence of confounding variables could be probed by inclusion of a control group that perhaps were also locked in the room for 80 years without but were not given a glass of water to drink at the start they would presumably have suffered the same fate as the group that drank the water revealing to the experimenter that something other than drinking water of course the death of the participants every time you assess a research study you should make sure there is a control group that receives everything the experimental group gets just without the experimental intervention this will help you judge if the experiment has good internal validity or not