Many research studies involve comparing at least two conditions on one or more dependent variables. Two major categories of such studies are within-subjects (also called repeated measures) and between-subjects designs. When designing this type of comparison study, researchers must decide whether each condition will be administered to all participants (within subjects) or whether each condition will be administered to a separate group of participants (between subjects). In a within-subjects research design, every participant in the research study experiences all conditions.

[Page 3736]For example, consider a researcher who is interested in comparing the effects of two behavioral treatments for children with autism. Choosing a within-subjects research design would mean that each child enrolled in the research study would be tested after experiencing Treatment A and then tested ...