Setting the minimal metrically detectable change on disability rating scales

Objective: To determine the minimal metrically detectable change (MMDC) on rating scales by comparing two methods: (1) using the reliability coefficient derived from an external study in the calculation of the standard error of measurement (psychometric method); and (2) examining the variability of scores in a stable subsample from a longitudinal study (empirical method). Design: Longitudinal survey. Setting: General community. Participants: Population-based representative sample of community-dwelling people older than 75 (n = 572). Main Outcome Measure: Disability as measured by the Functional Autonomy Measuring System (SMAF). Results: Using the psychometric method, a change in score of 3.7 on the SMAF was obtained using a reliability coefficient derived from an external test-retest study, and 5.2 using the reliability coefficient measured in a stable subsample of the longitudinal study. With the empirical method, a change of 5 points was established as the MMDC. Conclusion: The setting of the MMDC on a disability scale could be useful for calculating sample size or interpreting results from clinical trials because it helps to establish the minimal clinically important difference, which should be equal to or larger than the MMDC. (C) 1997 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation.