Background: Few studies use modern approaches to develop a quality of life (QOL) questionnaire with acceptable construct validity, especially in Iran. Our main objective was to construct a new validated and uni-dimensional questionnaire, based on WHOQOL-100, using the Rasch analysis.
Methods: In a population-based, cross-sectional study in 2007, 500 Tehran residents aged?18 were randomly sampled. The Persian version of WHOQOL-100 was used to measure the participants' QOL. After using targeting and person fit analysis, we performed category/threshold ordering, item fit, and differential item functioning analyses, in succession. We used outfit or infit statistics>1.5 and<0.5 for detecting underfit and overfit items/persons, respectively. We also deleted items with disordered category and/or threshold. Person Separation Index and test reliability were also calculated in the datasets.
Results: Male to female ratio was 0.98 and the mean age (SD) of participants was 35.1 (12.8) years. Initial analyses showed significant differences in quality of life between age groups (P=0.002), educational levels (P=0.001), and cur-rent health status groups (P<0.001). We eliminated 67 persons for under-fitting, 38 items for category and/or thresh-old disordering, 6 items for under-fitting, and 26 items for item bias. Test reliability for the final 30-item scale was 0.89.
Conclusion: We prepared a shortened version of the WHOQOL-100 that is single construct, uni-dimensional and free of item bias or any disordering, according to the Rasch model.