This article focuses on items to consider when selecting outcome measures for a clinical study. The choice of outcome measures depends largely on the research question and the study design. Sample-size requirements can vary greatly, depending on the type and the number of outcome measures selected. In this paper, we review the differences between categorical and continuous outcomes as well as the differences between primary and secondary outcomes and we discuss the concept of minimally important differences and the problems associated with composite outcomes. We also provide instruction on how to conduct and present a sample-size calculation.