Construct validity of the SF12 v.2 for the homeless population with mental illness: An instrument to measure self-reported mental and physical health

Background: Self-reported health measures are important indicators used by clinicians and researchers for the evaluation of health interventions, outcome assessment of clinical studies, and identification of health needs to improve resource allocation. However, the application of self-reported health measures relies on developing reliable and valid instruments that are suitable across diverse populations. The main objective of this study is to evaluate the construct validity of the SF-12v.2, an instrument for measuring self-rated physical and mental health, or homeless adults with mental illness. Various interventions have been aimed at improving the health of homeless people with mental illness, and the development of valid instruments to evaluate these interventions is imperative. Study Design: We measured self-rated mental and physical health from a quota sample of 575 homeless people with mental illness using the SF-12v2, EQ-5D, Colorado Symptoms Index, and physical/mental health visual analogue scales. We examined the construct validity of the SF-12v2 through confirmatory factor analyses (CFA), and using ANOVA/correlation analyses to compare the SF-12v2 to the other instruments to ascertain discriminant/convergent validity. Results: Our CFA showed that the measurement properties of the original SF-12v2 model had a mediocre fit with our empirical data (χ2 = 193.6, df = 43, p < .0001, CFI = 0.85, NFI = 0.83, RMSEA = 0.08). We demonstrate that changes based on theoretical rationale and previous studies can significantly improve the model, achieving an excellent fit in our final model (χ2 = 160.6, df = 48, p < .0001, CFI = 0.95, NFI = 0.95, RMSEA = 0.06). Our CFA results suggest that an alternative scoring method based on the new model may optimize health status measurement of a homeless population. Despite these issues, convergent and discriminant validity of the SF-12v2 (scored based on the original model) was supported through multiple comparisons with other instruments. Conclusion: Our study demonstrates for the first time that the SF-12v2 is generally appropriate as a measure of physical and mental health status for a homeless population with mental illness.This research has been made possible through a financial contribution from Health Canada and funded by the Mental Health Commission of Canada