outine measurement of the outcome of clinical care is increasingly considered important in health care. It is a key aspect of value-based health care, patient-centered care, and other quality-of-care initiatives. 1 For example, the Dutch government strives to have objective outcome data on 50 percent of all health care in 2022, 2 and in Sweden, outcome measurements have been part of a national registry for years. 3 The goals of routine outcome measurement are multiple, including improving communication and treatment guidance at the patient level, in addition to benchmarking of outcome at the level of individual clinicians or treatment centers. This benchmark information may help to establish priorities in resource allocation, and provide clinicians and managers with valuable feedback on performance. Furthermore, routine outcome measurement systems generate large data sets that can be used in scientific research. These "big data" can help provide knowledge on, for example, comparative effectiveness, predictive factors of outcome, and psychometric properties of measurement instruments. Although routine outcome measurement has been advocated for years, implementation in clinical practice is limited because of several