Random assignment thus plays a central role in justifying our cause-and-effect claims. But the psychologist’s tool kit includes another technique for ensuring that the experi-mental and control groups match each other at the start of the experiment. This tech-nique involves using the same people for the two groups, guaranteeing that the two “groups” are identical in their attitudes, backgrounds, motivations, and so forth. An experiment that uses this technique of comparing participants’ behavior in one setting to the same participants’ behavior in another setting is said to use within-subject com-parisons. This kind of experiment differs from the other designs we’ve considered sofar, which use between-subject comparisons.
Within-subject comparisons are advantageous because they eliminate any question about whether the experimental and control groups are fully matched to each other. But within-subject comparisons introduce their own complications. For example, let’s say that participants are first tested in the proper circumstances for the control condition and then tested in the circumstances for the experimental condition. In this case, if we find a differ-ence between the conditions, is it because of the experimental manipulation? Or is it because the experimental condition came second, when participants were more comfort-able in the laboratory situation or more familiar with the experiment’s requirements?
Fortunately, we can choose from several techniques for removing this sort of concern from a within-subjects design. In the example just sketched, we could run the control condition first for half of the participants and the experimental condition first for the other half. That way, any effects of sequence would have the same impact on both the experimental and control data, so any effects of sequence could not influence the com-parison between the conditions. Techniques like this enable psychologists to rely on within-subject designs and can remove any question about whether the participants in the two conditions are truly comparable to each other.