Table 2

Summary of Rasch analysis of KSQ modules

Rasch statisticsKSQ modules
General health statusLungSkinEyeMedication
Number of items106373
χ2 fit statistic, p value0.3100.1570.9750.5780.224
Mean item fit residual (SD)−0.01 (1.5)−0.11 (1.3)0.33 (0.8)0.14 (0.7)0.19 (1.9)
Mean person fit residual (SD)−0.3 (1.1)−0.3 (0.9)−0.3 (1.2)−0.2 (1.0)−0.2 (0.7)
Person separation index0.900.780.790.830.70
Cronbach's α coefficient0.930.860.840.880.70
  • χ2 statistic assesses item–trait interaction; non-significance indicates a good fit to the Rasch model (p>0.05). The mean person and item fit residuals are item–person interaction statistics that are z transformed; a good fit to the Rasch model is when they approximate to a mean (SD) of 0 (1). The person separation index (PSI) assesses how well KSQ discriminates subjects with differing levels of health status. The Cronbach's α coefficient is a measure of internal reliability. A PSI and Cronbach's α coefficient ≥0.7 is considered acceptable.

  • KSQ, King's Sarcoidosis Questionnaire.