This study is the first to explore the relative preference weights of the items of the OxCAP-MH, a capability well-being instrument that was originally developed for outcome measurement in this context, and the impact of mental ill-health experience on these. As opposed to other capability instruments with existing preference weight or value sets such as the ASCOT [17], ICECAP-A [7], ICECAP-O [15] and ICECAP-SCM [34], the high number of items posed a major challenge in case of the OxCAP-MH. Hence, this study only focused on the items without incorporating preferences related to levels. The overall study sample was balanced for gender and for differing mental ill-health experiences between the cohorts.
The results confirmed significantly different preference weights attributed to the 16 items of the OxCAP-MH. The RIS scores for the full cohort ranged from 0.76 to 15.72 and indicated very strong relative preference weights for Freedom of deciding for yourself and Limit daily activities, suggesting that these concepts have large spans within the ‘capability space’ covered by the OxCAP-MH. The relatively high importance of these items also demonstrates that respondents placed a much higher emphasis on individual aspects of capabilities than on community-based aspects, including the influence on Local decisions, in the Austrian context [18]. The most important items of the OxCAP-MH are those related to Nussbaum’s practical reasoning and overall health capabilities, whilst aspects related to emotions and control over one’s environment are regarded significantly less important. These results are in line with the findings of the BWS experiment eliciting preference weights for the ASCOT instrument for service users in Austria. Hajji et al. found that the most important choices were related to being meaningfully occupied during the day and having control over daily life, whilst the least important choices were associated with dignity and social participation [18]. These findings are also in line with the results of the UK preference weighting study of the ASCOT [17]. Conversely, the development of the initial UK value set of ICECAP-A in 2013 found that attachment and stability had slightly higher contribution to the capability space then the items of autonomy, achievement and enjoyment. This suggests that the different capability instruments either elicit different aspects of capabilities, or the actual wording of questions and the cultural context play a more significant role than previously thought.
With regard to differences across the cohorts, we could not find evidence that mental ill-health experience or gender were significantly associated with the relative importance of the OxCAP-MH items. While rank analysis revealed some differences across the cohorts, this was not to a significant extent. The magnitudes of the regression coefficients with respect to the mean values of the OxCAP-MH item scores were relatively small across cohorts, apart from the Suitable flat situation and Freedom of expression items. These findings suggest that potential future value sets for the OxCAP-MH elicited from the general population are likely to represent well also the direct preferences of people with lived experience of mental ill-health. These results somewhat contradict the findings that patient preferences differ from preferences derived from the general population for the EQ-5D-5L items [10]. In the study by Ludwig et al, 2021, patients in the EQ-5D-5L study gave more importance to mobility, self-care, or usual activities and less importance to pain/discomfort and anxiety/depression compared to the general population [10]. The differential findings between the two studies could be explained by the specific perception of mental health itself and the difference between health-related quality of life and capabilities as evaluative spaces.
Our study is unique by explicitly exploring the impact of ill-health experience on preferences in the context of a capability wellbeing instrument. Moreover, the study is based on a thoroughly designed BWS experiment and sophisticated statistical analysis. This takes into account the multivariate distribution with implicit correlation of outcomes, and robustness against model misspecification and violation of assumptions of linear regression, which probably implied larger estimated standard errors.
The interpretation of the study results, however, require the consideration of its limitations. One of the main limitations is the relatively small sample size. A larger number of observations would have strengthened the robustness of the statistical analysis, and also it would have enabled the formation of bigger groups for subgroup analyses, including latent class estimation. Moreover, we adjusted for gender, but not for other demographic variables because they were intrinsic to the definition of the population, for instance, in terms of experts’ age. This meant that the analysis compared the items between population cohorts in a simple form and did not artificially equalise their characteristics, which would have caused over-adjustment and could have induced bias. A further limitation is that the sample is not representative of the Austrian population because it is generally younger and all respondents live in or around Vienna. In the future, the current methods could be expanded and be developed into a full preference weight set using a representative Austrian general population sample. Finally, the research focused only on the capability dimensions of the OxCAP-MH themselves but not their levels due to the high number of item-level combinations. The assumption that levels carry proportionately constant weights may prove problematic. Similar BWS studies developing weights for capability instruments, including the ICECAP-A and the Carer Experience Scale, found that greater value was placed on differences between the bottom and middle levels of items than between the middle and top levels [7, 43]. Future research should address the issue of preferences related to levels, and if the current proportionally constant assumption proves violated, future preference weight set developments should incorporate also differential preference weights by levels. The indicative preference weight set could be used as current best proxy to calculate individual scores for different capability states in economic evaluations [42].