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We measure health outcomes to help us make decisions about managing our patients. Outcome measures help us predict which patients will benefit most from a particular intervention and to document whether the patient improves after the intervention is provided. There is a wide range of important health outcomes, including disability and quality of life. Reliability is a necessary but not sufficient characteristic of an outcome measure. It also is necessary to determine whether the measure actually captures the aspect of the phenomenon of interest. Validity is not a characteristic of an instrument. It can be determined only in relation to a particular question as it pertains to a defined population. Finally, outcome measures used to evaluate changes in patients over time must be responsive in their ability to detect real change. It is important to understand the types and psychometric properties of outcome measures to select the measure best suited to a particular purpose.