 The Cosmin Checklist was developed in an international Delphi study to evaluate the methodological quality of studies on measurement properties of health-related patient-reported outcomes, HRPROS. This paper explains the design requirements and preferred statistical methods for which no evidence is available in the literature, or on which the Delphi panel members had substantial discussion, including internal consistency, content validity, hypotheses testing as an aspect of construct validity, criterion validity, and responsiveness. The paper aims to enhance the acceptance and use of the Cosmin Checklist by providing a better understanding of the rationale behind the items. This article was authored by Patrick Donald L, Alonso Xordi, Stratford Paul W, and others.