Longitudinal measurement invariance in prospective oral health-related quality of life assessment

Background: Prospective assessments of oral health-related quality of life (OHRQoL) changes are prone to response shift effects when patients reconceptualize, reprioritize, or recalibrate the perceived meanings of OHRQoL test items. If this occurs, OHRQoL measurements are not "invariant" and may reflect changes in problem profiles or perceptions of OHRQoL test items. This suggests that response shift effects must be measured and controlled to achieve valid prospective OHRQoL measurement. The aim of this study was to quantify response shift effects of Oral Health Impact Profile (OHIP) scores in prospective studies of prosthodontic patients. Methods: Data came from the Dimensions of Oral Health-Related Quality of Life Project. The final sample included 554 patients who completed the OHIP questionnaire on two occasions: pre- and post-treatment. Only items that compose the 14-item OHIP were analyzed. Structural equation models that included pre- and post-treatment latent factors of OHRQoL with different across-occasion constraints for factor loadings, intercepts, and residual variances were fit to the data using confirmatory factor analysis. Results: Data fit both the unconstrained model (RMSEA =.038, SRMR =.051, CFI =.92, TLI =.91) and the partially constrained model with freed residual variances (RMSEA =.037, SRMR =.064, CFI =.92, TLI =.92) well, meaning that the data are well approximated by a one-factor model at each occasion, and suggesting strong factorial across-occasion measurement invariance. Conclusions: The results provided cogent evidence for the absence of response shift in single factor OHIP models, indicating that longitudinal OHIP assessments of OHRQoL measure similar constructs across occasions.