world health organization quality-of-life scale (whoqol-bref): analyses of their item response theory properties based on the graded responses model.

objective: this study has used item response theory (irt) to examine the psychometric properties of health-related quality-of-life. method: this investigation is a descriptive- analytic study. subjects were 370 undergraduate students of nursing and midwifery who were selected from tabriz university of medical sciences. all participants were asked to complete the farsi version of whoqol-bref. samejima's graded response model was used for the analyses. results: the results revealed that the discrimination parameters for all items in the four scales were low to moderate. the threshold parameters showed adequate representation of the relevant traits from low to the mean trait level. with the exception of 15, 18, 24 and 26 items, all other items showed low item information function values, and thus relatively high reliability from low trait levels to moderate levels. conclusions: the results of this study indicate that although there was general support for the psychometric properties of the whoqol-bref from an irt perspective, this measure can be further improved. irt analyses provided useful measurement information and demonstrated to be a better methodological approach for enhancing our knowledge of the functionality of whoqol-bref.