Communication Between Groups
Whenever the participants in on treatment condition are allowed to take with the participants in another condition, there is the potential for a variety of problems to develop. For example, a researcher may want to test the effectiveness of a new treatment for depression. Using a between-subjects design, the researcher randomly assigns half the clients of an inpatient facility to receive the new treatment and half to receive the standard treatment for depression. If the participants talk to each other, however, than those individuals receiving the old treatment may learn about the new treatment and may begin to use some elements from the new treatment. Diffusion refers to the spread of the treatment effects from the experimental group to the control group, which tends to reduce the difference between the two conditions. This is a threat to the internal validity of a between-subjects design because the true effects of the treatment can be masked by the shared information (that is, it appears that there is no difference between the groups because both groups are actually getting much of the same treatment).
Another risk is that an untreated group learns about the treatment being received by the other group, and demands the same or equal treatment.This is referred to as compensatory equalization. For example, in a study examining the effects of violent television viewing on boys in a residential facility, one Team of researchers faced this problem. The boys in the nonviolent television group learned that those in the violent television group were allowed to watch the television series Batman and demanded the right to watch it, too (Feshback & Singer, 1971). This threat commonly occurs in medical and clinical studies when one group receives a treatment drug and another does not. A similar problem arises when research are try to assess the effectiveness of large-scale educational enrichment programs (involving such improvements as computers in the classroom). Parents and teachers of the classes or schools that do not receive the enrichment (the control group) hear about the special program other classes or schools (the experimental group) receive, and demand that their children receive the same program or something equal in value. If the demand is met, the research study no longer has a no-treatment condition for comparison. Again, this is a threat to the internal validity of a between-subject design because it can wipe out the true effects of the treatment (that is, make it look like there no differences between the group on the dependent variable).
Finally, problems can occur when participants in an untreated group change their normal behavior when they learn about a special treatment that is given to another group. One possibility is that the untreated groups works extra hard to show that they can perform just as well as the individuals receiving the special treatment. This is referred to as compensatory rivalry. In this case, the performance observed by the researcher is much higher that would normally occur. It is also possible that the participants in an untreated group simply give up when they learn that another group is receiving special treatment. This is referred to as resentful demoralization. In this case, the untreated group becomes less productive and less motivated because they resent the expected superiority of the treated group. As a result, the effect of the treatment appears to be much greater that it really is.
In each case, internal validity is threatened because that observed difference between group can be explained by factors other than the effects of the treatment. The best way to minimize each of these threats to internal validity resulting from communication between the group is to separate the groups of participants as much as possible and keep them from being aware of one another. Notice that these problems are exclusive to between-subject experimental designs in which different groups of participants are used to compare different treatment conditions.
Leaning check Describe some of the problems that can arise when the participants in one treatment condition of a between-subject experiment are allowed to communicate with participants in a different condition.