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.
Department of Dermatology, Erasmus Medical Center, Rotterdam, The Netherlands.
Tamar E C
TE
Nijsten
Francesca
F
Sampogna
Mary-Margaret
MM
Chren
Damiano D
DD
Abeni
D000328:Adult; D003872:Dermatitis / Q000523:psychology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011594:Psychometrics / Q000379:methods*; D011788:Quality of Life; D011795:Surveys and Questionnaires