To try to visualise the difference between internal and external validity, why not create a diagram using an image of a famous psychology lab? We chose Yale as we were working on Milgram at the time. Stick the image into your book or paste into a document if working online. Use a bright colour to draw a bigger ‘lab’ outline around the image.

Next, write internal validity inside the lab and external validity outside the lab as you think about the difference between the two concepts. Write a definition of internal validity inside the outline of the lab and think about all the things that can affect the dependent variable other the the independent variable, e.g. confounding variables including participant and situational variables, social desirability, evaluation apprehension, demand characteristics, experimenter effects, etc.

Next, draw a participant in the window of the lab and think about his or her behaviour whilst they are in this unusual and unfamiliar setting and how it might differ to their behaviour in their day life. Think about one of the participants you designed in the sampling lesson (I haven’t blogged about this yet, but will one day!) and that person’s job, family, background etc. Draw stick men to show the participant leaving the laboratory to go home. My participant looks a bit perplexed in the bottom corner as he thinks about the study he has participated in and whether his behaviour would have been the same in everyday life.

Finally, write the term and a definition for external validity outside of the lab outline. External validity is about whether the conclusions would be true outside the confines of the study itself and therefore relates also to era and population validity, i.e. would you get the same result at a different time (era) or with a different target population (population). External validity also includes ecological validity, meaning the extent to which the findings are true of real life (people in the real world ‘habitats’; what if the study took place in the ‘real world’ and participant didn’t know they were in a study because they were not in a lab at a university? Would the same conclusions be drawn if the task was something more in-keeping with what the participants normally do in their everyday lives as opposed to artificial tasks, designed by experimenters to control confounding variables? Would behaviour be the same at home with family, at the pub with friends, at work with colleagues? Experiments often eliminate the immediate social situations that we all find ourselves in  everyday. This can have benefits but also it can degrade the ecological validity of the study. Think about the specific aspects of the experimental set-up which may have been the active ingredient in triggering the observed behaviours.