Fit for purpose and modern validity theory in clinical outcomes assessment.

PURPOSE: The US Food and Drug Administration (FDA), as part of its regulatory mission, is charged with determining whether a clinical outcome assessment (COA) is "fit for purpose" when used in clinical trials to support drug approval and product labeling. In this paper, we will provide a review (and some commentary) on the current state of affairs in COA development/evaluation/use with a focus on one aspect: How do you know you are measuring the right thing? In the psychometric literature, this concept is referred to broadly as validity and has itself evolved over many years of research and application. REVIEW: After a brief introduction, the first section will review current ideas about "fit for purpose" and how it has been viewed by FDA. This section will also describe some of the unique challenges to COA development/evaluation/use in the clinical trials space. Following this, we provide an overview of modern validity theory as it is currently understood in the psychometric tradition. This overview will focus primarily on the perspective of validity theorists such as Messick and Kane whose work forms the backbone for the bulk of high-stakes assessment in areas such as education, psychology, and health outcomes. CONCLUSIONS: We situate the concept of fit for purpose within the broader context of validity. By comparing and contrasting the approaches and the situations where they have traditionally been applied, we identify areas of conceptual overlap as well as areas where more discussion and research are needed.