In deciding whether to use a particular treatment for conditions such as depression, arthritis, or heart disease, clinicians and patients must balance the benefits against the side effects and burden. To make this trade-off, they must understand the likely degree of benefit in patients’ symptoms and perceived wellbeing, best undertaken using patient-reported outcomes (PROs). PROs are measures of any aspect of a patients’ health status that are obtained from direct patient inquiry without interpretation by a clinician or anyone else. PRO measures (PROMs) are increasingly used in clinical trials and systematic reviews to evaluate health care interventions, and information obtained from PROMs can guide clinical decisions and inform shared-decision making. The use of PROMs, however, involves challenges, the most important of which is deciding if a particular treatment effect is trivial, small but important, moderate or large. One way to make this judgment is to consider the minimal important difference (MID), the smallest change in a PROM score that is important enough that patients would consider a change in treatment to achieve that benefit. The number of published studies providing anchor-based MIDs for PROMs has grown rapidly over the last three decades, and researchers have proposed several anchor-based methods to derive MID estimates, each with its own merits and limitations. This thesis begins with the development of a framework to determine the extent to which the design and conduct of studies measuring anchor-based MIDs are likely to have protected against misleading estimates. Subsequently, this thesis presents a comprehensive inventory of empirically estimated anchor-based MIDs and their associated credibility for all PROMs published in the medical literature. Further, this thesis highlights critical issues that key stakeholders should consider, and demonstrates how the use of credible MIDs may inform the development of a clinical practice guideline in which PROs were identified as critically important. Finally, this thesis concludes with insights to improve the methodological quality and transparency for researchers in the PRO and MID field. / Thesis / Doctor of Philosophy (PhD)