External Validity - The Basic Dilemma of The Social Psychologist

The Basic Dilemma of The Social Psychologist

When conducting experiments in psychology, there is always a trade-off between internal and external validity—that is between

1. having enough control over the situation to ensure that no extraneous variables are influencing the results and to randomly assign people to conditions, and
2. ensuring that the results can be generalized to everyday life.

One way to increase external validity is by conducting field experiments. In a field experiment, people's behaviour is studied outside the laboratory, in its natural setting. A field experiment is identical in design to a laboratory experiment, except that it is conducted in a real-life setting. The participants in a field experiment are unaware that the events they experience are in fact an experiment. The external validity of such an experiment is high because it is taking place in the real world, with real people who are more diverse than a typical university student sample.

Experimentes Latane and Darley (1970) tested their hypothesis about group size and bystander intervention in a convenience store outside New York City. Two "robbers" -- with the full knowledge and permission of the cashier and manager of the store -- waited until there were either one or two other customers at the checkout counter. Then they asked the cashier to name the most expensive beer the store carried. The cashier answered the question and then said he would have to check in the back to see how much of that brand was in stock. While the cashier was gone, the robbers picked up a case of beer in the front of the store,put the beer in their car, and drove off.

Because the robbers were intimidating, no one attempted to intervene directly to stop the theft. But when the cashier returned, fewer people reported the theft when there was another witness/customer in the store than when they were alone. Real life studies can best be captured by doing field studies, but it is difficult to control all extraneous variables in such studies.

The trade-off between internal and external validity is referred to as the basic dilemma of the social psychologist.

To maximize both, Wendy Josephson conducted a study on the relation between television violence and aggressive behaviour. In this study, boys in grades 2 and 3 from 13 schools in Winnipeg watched either a violent or a nonviolent television show. Internal validity was achieved by controlling the television show the participants watched. Josephson ensure that the violent and nonviolent shows were equivalent in terms of excitement, liking, and physiological arousal. This level of control ensured that any differences in subsequent behaviour between the two groups were because of differences in violent content, rather than other variables that might be associated with violent programming, such as excitement. Internal validity was further enhanced by random assignment of participants to either the violent or nonviolent condition. External validity was maximized by having the participants play floor hockey in their school gymnasium after they had finished viewing the television segment.

The observers recorded instances of aggression after the boys had viewed the shows. The observers were unaware whether the boys had seen the violent or the nonviolent show. To make the observation as natural as possible, participants were told that observers would be doing "play by plays" just the way they do in real hockey games. The observers spoke into microphones, noted the number on a child's jersey, and recorded the kind of aggression that occurred. The results indicated that exposure to violent programming did increase aggression—but only among boys who were predisposed toward aggression.

Both internal and external validity are not captured in a single experiment. Social psychologists opt first for internal validity, conducting laboratory experiments in which people are randomly assigned to different conditions and all extraneous variables are controlled. Other social psychologists prefer external validity to control, conducting most of their research in field studies. And many do both. Taken together, both types of studies meet the requirements of the perfect experiment. Through replication, researchers can study a given research question with maximal internal and external validity.