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When deciding between experimental approaches, researchers generally prioritize the within-subjects design if practical. A good rule of thumb is to opt for a within-subjects experiment—utilizing proper counterbalancing—if there is sufficient time to test each participant in all conditions and if there are no serious concerns about confounding carryover effects. If these conditions cannot be met, researchers should consider a between-subjects design instead.

Rule of Thumb for Experimental Design Choice

A between-subjects design becomes strictly necessary when a within-subjects approach is difficult or impossible to carry out. This typically occurs in two main scenarios: first, when severe time constraints make it unfeasible to test each individual in all conditions; and second, when an experiment involves a treatment meant to produce long-term behavioral changes. In the latter case, a successful initial treatment permanently alters the participants, making them unsuitable for subsequent testing in a control condition.

Situations Requiring a Between-Subjects Design

According to the general rule of thumb for experimental design choice, under which specific conditions should a researcher prioritize a within-subjects approach?

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