Summary of Rasch analysis
| Rasch statistics | K-BILD domains | |||
| Psychological | Breathlessness and activity | Chest symptoms | Total score | |
| Number of items | 7 | 4 | 3 | 15 |
| χ2 fit statistic | 18.2 | 8.2 | 7.9 | 41.0 |
| Degrees of freedom | 14 | 8 | 6 | 30 |
| p Value | 0.20 | 0.42 | 0.25 | 0.09 |
| Mean item fit residual (SD) | −0.1 (0.9) | −0.01 (0.9) | 0.4 (1.2) | 0.2 (1.1) |
| Mean person fit residual (SD) | −0.3 (1.1) | −0.5 (1.1) | −0.5 (1.3) | −0.2 (1.2) |
| PSI | 0.91 | 0.93 | 0.70 | 0.94 |
| Cronbach's α coefficient | 0.90 | 0.90 | 0.72 | 0.94 |
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 K-BILD discriminates subjects with differing levels of health status. Cronbach's α coefficient is a measure of internal reliability. A PSI and Cronbach's α coefficient ≥0.7 is considered acceptable.