Research key concepts

External validity refers to how well data and theories from one setting apply to another. This question is usually asked about laboratory research: Does it apply in the everyday "real" world outside the lab? The figure at the right summarize external and internal validity and the relation between the two. The green ellipse represents internal validity, and the blue rounded rectangle around it represents external validity,

For example, a laboratory study tested people's helping behavior (Latan‚ and Darley, 1976). It showed that a participant in the experiment is more likely to help someone who appeared to be sick if the participant thinks s/he is the only other person in the lab. The more other people s/he thinks are in the lab, the less likely s/he is to help. If participants thought they were the only one, almost all went for help. If participant thought one other person was also in the lab, about 2/3 went for help. If participants thought there were three others in the lab area, only 1/3 went for help.

It turns out that a similar pattern emerges when you look at news reports of violent attacks on people.So the effect studied in this laboratory setting also appeared outside the laboratory. Thus, the laboratory setting clearly had external validity.

External validity refers to the extent that the outcome of any research study would apply to other similar situations in the real world.