Outcome measures in clinical trials traditionally have been objective evaluations such as assessments of physical function (e.g., survival, lung capacity) or biological markers of response to treatment (e.g., blood cell counts, tumor size, etc.). For example, in a clinical trial examining the effectiveness of a new asthma medication, key outcomes might include measures of an individual’s respiratory capacity. Similarly, a study evaluating the efficacy of a new heart disease treatment might use outcomes such as myocardial infarction or longterm survival, depending on the goal of the treatment. While these measures of an individual’s biological status are important, they do not give an indication of the individual’s perception of treatment-associated benefits such as emotional well-being, physical function, or performance of daily activities. A group of instruments called “patient-reported outcomes” (PROs) has emerged during recent decades to fill this gap in the comprehensive assessment of treatment effectiveness.