The applicability of existing instruments is marred by the lack of easily administrable, multi-dimensional instruments that cover all the culturally relevant domains of mental health. In this study, we demonstrated the validity and reliability of the PMH instrument using a series of studies in the local multiethnic population. Content of the PMH instrument was strengthened by identifying the components of the instrument through studies directly conducted among the end users. Though this method is now largely advocated for instrument content development, many of the available measures for well-being and patient reported outcomes have been developed by reducing item pools created from existing instruments [41, 42], hence the content of our PMH measure encompasses experiences that are of relevance to the general population in Singapore.
Factor analysis uncovered six important dimensions of mental health in Singapore. Much attention was given to understanding the content in the factors before naming them. The assessment was theory-driven where we compared and contrasted the item content with the definitions of key domains from the extant well-being literature as well as looked at the content of the available measures. While reviewing the 'General coping' items, we observed a mixture of active coping and avoidance. The domain had items such as 'I try to see the looking at humorous side' and 'I tell myself that things would get better', which are not direct acts of coping, yet contribute to the process, hence we used the General coping instead of active or passive coping. Interpersonal skills, Emotional support, and Global affect were named based on the item structure and comparison with other definitions. There is an overlap of the theories on personal growth, autonomy and environmental mastery (EM), however, EM involves much more than just these two aspects . The basis of EM is to be able to control situations surrounding the individual and turning the situation in favor of his/her needs. While we observed 'feeling in control' in the domain, the later was not evident. The content was also more comparable with definitions of autonomy and personal growth [20, 43] and hence we labeled this domains as 'Personal growth and autonomy'.
Some of the dimensions are close to those reported in the literature, such as autonomy, personal growth, coping and support. While others such as interpersonal skills and spirituality emerged salient in the local population. These findings strongly justify our decision to develop a new measure directly in the local population instead of using existing measures. The role of spirituality in achieving PMH and particularly its interaction with other domains has been under explored, and as such, this instrument may be of interest to assess the socio-cultural aspects and influence on PMH.
In this study we conducted item reduction for the PMH instrument and demonstrated its factor structure using a series of psychometric analyses. While we used quota sampling strategy where we oversampled participants from the minority ethnic groups (Malay and Indian), weighted analysis was not conducted as the purpose of oversampling was to get enough power for statistical analyses on cultural difference tested in the CFA and IRT-DIF and these inferences could not be drawn using weighted analysis. We also did not devise weighted summation scores as we wanted to preserve the variation in the original data. Furthermore, they are not always applicable while applying the scale in populations other than the one they were derived from .
A key feature of this instrument remains the use of both classic and modern test theory practices to select items for inclusion in the PMH instrument. As the results showed, classic approaches to item reduction enabled the removal of most items, but failed to discriminate adequately between items with similar properties. This was addressed by the use of IRT which enabled us to examine the reasons for these issues in more detail. Another important aspect of this study is that we used IRT models that provide up to three parameters and allows for analysing response scales with multiple options . In addition, these various approaches to item reduction ensured that the item correlations were substantially reduced and reflected construct validity by reduced redundancy.
The multi-dimensional PMH instrument has high internal reliability and fulfills assumptions with convergent and divergent validity. IRT thresholds (Table 5 theta < 0) and location estimates (peak in the negative zone) suggest superior accuracy in measuring mental health of individuals with below average levels of the domains General coping, Personal growth and autonomy, Spirituality, Interpersonal skills, and Emotional support. Global affect however, functions in the opposite direction as it will potentially provide more information on individuals above the average (theta > 0) and was slightly reduced when the theta was greater than 1.
A recent study of the SWEMWB indicated a multi-dimensional structure for the shortened well-being measure . In an earlier study on the longer version of this scale (WEMWB), differing strengths of association with convergent validity variables were reported . The correlations observed for our instrument were lower than those reported for WEMWBS. For example, WEMWBS demonstrated significantly strong correlation with EQ5D VAS (0.43) and SWLS (0.73)  where as the global PMH instrument measure showed a significant correlation of 0.39 and 0.53 with EQ5D VAS and SWLS, respectively. The differences in magnitude of correlations could be attributed to the study sample - WEMWBS estimates were established in a student population while ours were in a community sample, who were considerably older (mean age 41 y) and age is often associated with lower life satisfaction . In our sample SWEMWBS score was significantly correlated (r = 0.380, p < 0.01) with SWLS. However, we couldn't investigate it's relation with EQ VAS as the questionnaires were administered in different groups of the sample.
The multi-dimensional PMH instrument developed in this study has several strengths. First, the conceptual framework of the instrument was based on qualitative and quantitative tests in the target population. The instrument includes six dimensions which encompass the notion that mental health can be achieved by the balance and strengths of multiple domains, and while an individual may not be equipped with all the components of mental health, an optimum level can be achieved through further strengthening the stronger components. Furthermore, the acceptability of the instrument is high (as evidenced by the low rates of missing data) across the different age, gender and ethnic groups for all domains.
While the PMH instrument demonstrated superior reliability and validity in the study population, some limitations of this study should be addressed. The study was limited to English speaking adults, aged 21 - 65 years. Therefore, other dimensions of mental health in the wider population such as the adolescents or the elderly may have fallen outside the scope of this study. Measures like EQ5D VAS and SWLS have been previously employed for validation of the WEMWBS , but most of the other converging validity measures were selected based on the expected performance. Nevertheless our study provides evidence that they can be reliably used to assess domains of PMH. We also observed significant but lower correlations with the shorter measures for some subscales. For example, association between the one-item general happiness measure and Spirituality were low (0.21, p < 0.01) while the two-item Brief Cope subscale scores were low across all domains. The use of few-item criterion measures may have constrained the strength of the associations between these criterion measures and the six separate factors of PMH. Although the length of the instrument (47 items) was not a limitation in our study, a shorter measure would be more appropriate in other settings and is planned as part of future research. We did not establish the test retest reliability of the measure. We were also unable to obtain information on attrition rates for the surveys as many participants were unwilling to provide basic background information upon declining participation.