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Sensitivity to change
Patients with clinical disease activity who required an important therapeutic intervention because of unacceptable clinical disease activity were invited to complete the questionnaire during a second visit 4-24 weeks after the initiation for NSAIDS and 12-24 weeks after the initiation for DMARDS (methotrexate, sulfasalazine) or TNF blockers (sensitivity arm). Baseline assessments including the questionnaires and clinical assessments were performed in the clinic.
Statistical analysis
Reliability was assessed by the test-retest method at 4-7 days interval for patients who considered themselves in a stable state. It was measured using the intra-class correlation coefficient (ICC) as an estimate of the instrument reproducibility over time -assuming that no modification in health conditions had occurred. For testing sensitivity to change, patients were included in the analyses if they report improvement on a global change question. Results obtained before and after treatment were compared and sensitivity to change was measured using the standardised response mean (SRM), which is calculated by dividing the mean score change by the standard deviation (SD) of the change.
Construct validity was analysed between ASAS-HI score and several other health outcomes: patient global, pain, spinal pain, BASDAI, BASFI, ASDAS, SF-36, EQ-5D and HAD-S using Spearman’s correlation coefficient (considered good if ≥0.6). Internal consistency was evaluated by Cronbach’s α coefficients (adequate: ≥0.70) 14.
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