Psychometric validation of a multi-dimensional capability instrument for outcome measurement in mental health research (OxCAP-MH)

Abstract

Background: Patient reported outcome measures (PROMs) are widely used in mental healthcare research for quality of life assessment but most fail to capture the breadth of health and non-health domains that can be impacted. We report the psychometric validation of a novel, multi-dimensional instrument based on Amartya Sen's capability approach intended for use as an outcome measure in mental health research. Methods: The Oxford Capabilities Questionnaire for Mental Health (OxCAP-MH) is a 16-item self-complete capability measure that covers multiple domains of functioning and welfare. Data for validation of the instrument were collected through a national randomised controlled trial of community treatment orders for patients with psychosis. Complete OxCAP-MH data were available for 172 participants. Internal consistency was established with Cronbach's alpha; an interclass correlation coefficient was used to assess test-retest reliability in a sub-sample (N = 50) tested one week apart. Construct validity was established by comparing OxCAP-MH total scores with established instruments of illness severity and functioning: EuroQol (EQ-5D), Brief Psychiatric Rating Scale (BPRS), Global Assessment of Functioning (GAF) and Objective Social Outcomes Index (SIX). Sensitivity was established by calculating standard error of measurement using distributional methods. Results: The OxCAP-MH showed good internal consistency (Cronbach's alpha 0.79) and test-retest reliability (ICC = 0.86). Convergent validity was evidenced by strong correlations with the EQ-5D (VAS 0.52, p < .001) (Utility 0.45, p < .001), and divergent validity through more modest associations with the BPRS (-0.41, p < .001), GAF (0.24, p < .001) and SIX (0.12, p = ns). A change of 9.2 points on a 0-100 scale was found to be meaningful on statistical grounds. Conclusions: The OxCAP-MH has demonstrable reliability and construct validity and represents a promising multi-dimensional alternative to existing patient-reported outcome measures for quality of life used in mental health research.