Which design primarily allows researchers to compare the effects of different treatments on the same subjects?

The repeated measures design is a powerful approach because it involves measuring the same subjects under different conditions or treatments. This design allows researchers to effectively control for individual differences, as each participant serves as their own control. By comparing the effects of different treatments on the same individuals, researchers can more accurately determine the treatment's impact, having already accounted for variations in participant characteristics that could affect outcomes.

This design is particularly advantageous when studying behavioral or psychological interventions, such as those implemented in music therapy, where individual responses may vary widely. It not only increases the statistical power of the analysis by reducing error variance associated with individual differences, but it also allows for the observation of changes over time or conditions, thereby gaining deeper insights into the effects of the treatment being assessed.