Abstract
Background: There is increasing evidence that assessing outcomes in terms of capability wellbeing provides information beyond that of health-related quality of life measures for evaluation in mental health research. This paper aimed to contribute to the utilisation of capability-based measures relative to health status measures in mental health research, by exploring the empirical relationship between the OxCAP-MH, the ICECAP-A, and the EQ-5D instruments against condition-specific measures and each other for schizophrenic patients with depression. Methods: Using trial data for 100 patients from the UK, the properties of the instruments were compared in terms of convergent validity, including correlations between the OxCAP-MH, the ICECAP-A, the EQ-5D-5L descriptive system and the EQ-5D VAS scores; and a line of central tendency between two variables was plotted on locally weighted smoothing curves. Exploratory factor analysis (EFA) investigated the extent to which the instruments measured complementary or overlapping constructs. Responsiveness was assessed in terms of standardised response mean and correlation between change scores (baseline to endpoint) of the instruments. Results: Correlation between the OxCAP-MH and ICECAP-A baseline scores was strong (0.682) and between change scores was moderate (0.401). The baseline scores of both capability instruments correlated more with condition-specific (0.481-0.718) than with generic (0.344-0.425) instruments. Their change scores weakly correlated with change scores of the generic health-related scales (0.183-0.247), but moderately with those of condition-specific instruments (0.339-0.557). The EFA found that while the EQ-5D-5L descriptive system loaded onto one factor, the items of the ICECAP-A loaded onto two additional factors and the items of the OxCAP-MH loaded onto three additional factors. Conclusions: The capability instruments had stronger convergent validity with each other than with any of the other instruments. Assessing outcomes in terms of capabilities for schizophrenic patients with depression proved to capture broader relevant information than the EQ-5D-5L both by the OxCAP-MH and the ICECAP-A, albeit to different extent. When comparing the two capability instruments, the OxCAP-MH tended to have stronger correlations with condition-specific instruments most likely due to its origin from mental health outcome measurement, while the ICECAP-A had slightly stronger correlation with the EQ-5D VAS.
Do alternative capability instruments capture differing aspects of mental health and quality of life in comparison to the EQ-5D for schizophrenic patients with depression?
