External validity

External validity refers to the degree that the results of an experiment can be extended beyond the conditions of the experiment to conditions of everyday life.    The purpose of research, including experiments, is to learn something about the behavior of people.   Obviously, this knowledge is useful to the extent that it describes normal, everyday behavior.   But this objective is often undercut by efforts to achieve internal validity.    As conditions from everyday life are changed to reduce threats to internal validity, subjects in the experimental and control groups can become different from those in the general population.   Consequently, while results may have high internal validity, they may lack external validity, meaning that they cannot be generalized beyond the particular groups used in the experiment. In designing an experiment, each experimenter has to decide which requirement is more important, internal or external validity, and seek a balance between them.

Further variations in experimental design

So far we have discussed experiments in which an experimental variable is applied in the same way to all subjects in one experimental group.   These represent "all or none" applications of the experimental variable.   There are other ways to administer the experimental variable.   For example, instead of the all or none application, different levels or amounts of the experimental variable can be applied to two or more experimental groups.

As a way of illustration let's return to the example of reducing prejudice.   Instead of using only one experimental treatment and group, we could test the effects of three different ways of reducing prejudice. In this case, we would create three experimental groups, while still retaining one control group. Subjects would be randomly assigned to one of the three experimental or to the single control group. The three treatments could consist of the following:

1. Experimental group 1 in which subjects see a specially prepared film, are given material to read, and participate in discussion groups.
2. Experimental group 2 in which subjects are given the same material to read as given to members of group 1 and participate in discussion groups, just the same as those in group one, but do nott see the film.
3. Experimental group 3 in which subjects receive only the reading material.

In addition, we would create a randomly selected control group of about the size as each of the experimental groups. Pretest data would be obtained from subjects in all four groups.   The experiment would then be conducted and posttest measurements of prejudice would be obtained from subjects in the three experimental groups and in the control group.   With these data we could determine:

1. Whether each of the treatments was effective; that is, were prejudice measurements significantly lower among subjects in each experimental group as compared with those in the control group.
2. If the most extensive treatment, represented by group 1, was more effective than the less extensive treatments, as represented by groups 2 and 3.
3. If the combination of treatments, as applied to group 2 was more effective than just the single treatment, as applied to group 3.

The additional data obtained from groups with multiple treatments are frequently useful in making decisions involving development of programs.   For example, suppose we found that the treatment with the movies was not more effective in reducing prejudice than the combination of reading material and participating in discussion groups.   This information could help in developing programs to reduce prejudice by showing that use of films, which are more expensive to create and use, do not need to be used.