Summary of Rasch analysis of KSQ modules
| Rasch statistics | KSQ modules | ||||
|---|---|---|---|---|---|
| General health status | Lung | Skin | Eye | Medication | |
| Number of items | 10 | 6 | 3 | 7 | 3 |
| χ2 fit statistic, p value | 0.310 | 0.157 | 0.975 | 0.578 | 0.224 |
| 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 index | 0.90 | 0.78 | 0.79 | 0.83 | 0.70 |
| Cronbach's α coefficient | 0.93 | 0.86 | 0.84 | 0.88 | 0.70 |
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.