Minimal impact of response shift for SF-12 mental and physical health status in homeless and vulnerably housed individuals: an item-level multi-group analysis

Anne M. Gadermann, Richard Sawatzky, Anita Palepu, Anita M. Hubley, Bruno D. Zumbo, Tim Aubry, Susan Farrell, Stephen W. Hwang

The purpose of this study was to examine whether homeless or vulnerably housed individuals experienced response shift over a 12-month time period in their self-reported physical and mental health status. Data were obtained from the Health and Housing in Transition study, a longitudinal multi-site cohort study in Canada (N = 1190 at baseline). Multi-group confirmatory factor analysis (MG-CFA) and methods for response shift detection at the item level, based on the approach by Oort, were used to test for reconceptualization, reprioritization, and recalibration response shift on the SF-12 in four groups of individuals who were homeless (n = 170), housed (n = 437), or who reported a change in their housing status [from homeless to housed (n = 285) or housed to homeless (n = 73)] over a 12-month time period. Mean and variance adjusted weighted-least squares estimation was used to accommodate the ordinal and binary distributions of the SF-12 items. Using MG-CFA, a strict invariance model showed that the measurement model was equivalent for the four groups at baseline. Although we found small but statistically significant response shift for several measurement model parameters, the impact on the predicted average mental and physical health scores within each of the groups was small. Response shift does not appear to be a significant concern when using the SF-12 to obtain change scores over a 12-month period in this population.