Psychometric evaluation of the Positive Mental Health (PMH) scale using item response theory

The investigation of patient-reported outcomes and psycho-oncological interventions mainly focuses on psychological distress or psychopathology. However, the recognition of the equal importance of positive mental health (PMH) has increased lately. The PMH-scale is a brief questionnaire allowing to assess well-being in individuals in the general population and in patients. Previous studies evaluated the psychometric properties of the PMH-scale using classical test theory (CTT). This study is the first to investigate the PMH-scale in patients with cancer using item analysis according to the Rasch model.

In total, N = 357 cancer patients participated in the study. A Rasch analysis of the PMH-scale was conducted including testing of unidimensionality, local independence, homogeneity and differential item functioning (DIF) with regard to age, gender, type of cancer, the presence of metastases, psycho-oncological support, and duration of disease. Additionally, the ordering of the item thresholds as well as the targeting of the scale were investigated.

After excluding one misfitting item and accounting for local dependence by forming superitems, a satisfactory overall fit to the Rasch model was established (χ² = 30.34, p = 0.21). The new PMH-8 scale proved to be unidimensional, and homogeneity of the scale could be inferred. All items showed ordered thresholds, there was no further item misfit. DIF was found for age, but as the impact of DIF was not substantial, no adjustment related to the age-DIF had to be made. The Person Separation Index (PSI = 0.89) was excellent, indicating excellent discriminatory power between different levels of positive mental health. Overall, the targeting of the PMH-8 was good for the majority of the present sample. However, at both ends of the scale item thresholds are missing as indicated by a slight floor effect (1.4%) and a considerable ceiling effect (9.8%).

Overall, the results of the analysis according to the Rasch model support the use of the revised PMH-scale in a psycho-oncological context.