The minimal detectable change cannot reliably replace the minimal important difference
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OBJECTIVE: We compared the minimal important difference (MID) with the minimal detectable change (MDC) generated by distribution-based methods. STUDY DESIGN: Studies of two quality-of-life instruments (Chronic Respiratory Questionnaire [CRQ] and Rhinoconjunctivitis Quality of Life Questionnaire [RQLQ]) and two physician-rated disease-activity indices (Pediatric Ulcerative Colitis Activity Index [PUCAI] and Pediatric Crohn's Disease Activity Index [PCDAI]) provided longitudinal data. The MID values were calculated from global ratings of change (small change for CRQ and RQLQ; moderate for PUCAI and PCDAI) using receiver-operating characteristic (ROC) curve and mean change. Results were compared with five distribution-based strategies. RESULTS: Of the methods used to calculate the MDC, the 95% limits of agreement and the reliable change index yielded the largest estimates. In the patient-rated psychometric instruments, 0.5 SD was always greater than 1 standard error of measurements (SEM), and both fell between the mean change and the ROC estimates, on two of four occasions. The reliable change index came closest to MID of moderate change. CONCLUSION: For patient-rated psychometric instruments, 0.5 SD and 1 SEM provide values closest to the anchor-based estimates of MID derived from small change, and the reliable change index for physician-rated clinimetric indices based on moderate change. Lack of consistency across measures suggests that distribution-based approaches should act only as temporary substitutes, pending availability of empirically established anchor-based MID values.
