Testing and reducing Skindex-29 using rasch analysis: Skindex-17

The Skindex is a well-studied dermatology-specific health-related quality of life (HRQOL) instrument. The objective of this study was to test Skindex-29 using Rasch analysis and, if necessary, to refine it so that it would fit this item response theory based model. The Skindex-29 of 454 Italian dermatological patients was subjected to Rasch analysis to investigate threshold order, differential item functioning (DIF), and item and overall fit to the model. The Skindex-29 did not fit the Rasch model (P<0.001). The 5-point scoring system was re-grouped into three categories and demonstrated logical response order for all but one item. Rasch analyses of a combined emotion and social functioning subscale of Skindex-29 resulted in a 12-item psychosocial subscale. Five of seven items were retained in a symptoms subscale. Both subscales fitted the model (P=0.32 and 0.13, respectively) without significant individual item misfit or DIF (P>0.05). Classical psychometric properties such as response distribution, item-rest correlation, item complexity, and internal consistency of the two subscales of Skindex-17 were at least adequate. The Skindex-17 is a Rasch reduced version of Skindex-29, with two independent scores that can be used in the measurement of HRQOL in dermatological patients.

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