| MEASUREMENT PROPERTIES | |
| Limitations | >=5 European languages (English) |
| Observations | |
| 1. RELIABILITY | |
| A. Internal Consistency | Tested |
| Cronbach's (Describe) |
Cronbach's α = 0.87 |
| B. Reliability intraobserver or test-retest | Tested |
| Continuous scores: intraclass correlation coefficient (ICC) Dichotomus: Cohen kappa (Describe) |
Test–retest reliability was good when scores were repeated after a 24-hour interval (r = 0.96, p<0.001) in inpatients with AS. ICC = 0.86 |
| C. Reliability interobserver or Measurement error | Tested |
| Standard error of measurement (SEM), smallest detectable change (SDC) or Limits of agreement (LoA) (Describe) |
ICC = 0.99 (2 independent observers - rheumatologists, not patient-scored) |
| 2. VALIDITY | |
| A. Content validity: face validity | Tested |
| Expert opinion (relevance and comprehensiveness) (Describe) |
Items were generated in an expert group of 3 rheumatologists experienced in the management of patients with AS. Patients were not involved in the development process. |
| B. Construct Validity: Structural validity |
Tested |
| Hypotheses-testing | Tested |
| Cross-cultural validity | Not Tested |
| Brief Description |
Principal component analysis was used to refine items. DFI discriminates between AS inpatients and outpatients, AS patients with high and low disease activity, and AS smokers and non-smokers. |
| C. Criterion validity | Tested |
| Comparison with a 'gold standard' Continuous scores: correlations, ROC curves Dichotomus: Sensitivity & Specificity (Describe) |
Criterion validity was assessed using a multiple regression model, including the DFI score, and clinically measured morning stiffness, number of nocturnal awakenings, chest expansion, Schober’s test, hand-ground distance, self-physiotherapy, and pain. The correlation coefficient between the independent variables and the functional index (dependent variable) was R2 = 0.41. |