Under what circumstances is a within-subjects design not a good choice for a research study?

What will be an ideal response?

A within-subjects design is not a good choice when one or more of the treatments has a permanent or long lasting effect on the participants which may influence their scores in later treatments. In this situation, the results are guaranteed to be distorted by carry over effects and it will be essentially impossible to determine which differences were caused by the treatments and which were caused by carry over effects. Also, it is often a good choice to use a between-subjects design in situations where individual differences are small. In this case, the individual differences will not generate real problems for the between-subject design, and this design completely eliminates the threats from time-related factors that are always a part of a within-subjects study.