The application of Rasch measurement theory to psychiatric clinical outcomes research

medical settings with the Patient Health Questionnaire (PHQ): a diagnostic meta-analysis. 25 Smith EV. Detecting and evaluation the impact of multidimensionality using item fit statistics and principal component analysis of residuals. Unlike many fields in medicine, most clinical outcomes in psychiatry are not directly observable and cannot be captured with diagnostic tests such as blood work or imaging. In recent years, the importance of the routine use of clinical outcome assessments (patient-reported outcomes, clinician-reported outcomes, observer-reported outcomes and performance outcomes) for measuring the symptoms of disease and treatment outcomes has been increasingly emphasised. 1 Clinical outcome assessments such as the Patient Health Questionnaire-9 (PHQ-9) 2 are now commonly used in clinical research and practice to provide an assessment of a patient's severity of mood and improvement in response to treatment. 3 More broadly, as the demand increases for a broad range of mental health services to be patient-centred, clinical outcome assessments are used to capture outcomes such as sustained symptom reduction, return to full functioning and optimal patient well-being. 4 To optimise mental healthcare, clinical outcome assessments used in psychiatry should be shown to be fit for purpose. They should appropriately capture the concept of interest (e.g. depression) in the context of use (e.g. B 2016 The Authors. This is an open-access article published by the Royal College of Psychiatrists and distributed under the terms of the Creative Commons Attribution License (http:// creativecommons.org/licenses/by/ 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Summary This commentary argues the importance of robust, meaningful assessment of clinical and functional outcomes in psychiatry. Outcome assessments should be fit for the purpose of measuring relevant concepts of interest in specific clinical settings. As well, the measurement model selected to develop and test assessments can be critical for guiding care. Three types of measurement models are presented: classical test theory, item response theory, and Rasch measurement theory. To optimise current diagnostic and treatment practices in psychiatry, careful consideration of these models is warranted..