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 (P0.05). 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.