Smallest detectable and minimal clinically important differences of rehabilitation intervention with their implications for required sample sizes using WOMAC and SF-36 quality of life measurement instruments in patients withosteoarthritis of the lower extremities

Objective. To discuss the concepts of the minimal clinically important diff erence (MCID) and the smallest detectable difference (SDD) and to examine t heir relation to required sample sizes for future studies using concrete da ta of the condition-specific Western Ontario and McMaster Universities Oste oarthritis Index (WOMAC and the generic Medical Outcomes Study 36-Item Shor t Form (SF-36) in patients with osteoarthritis of the lower extremities und ergoing a comprehensive inpatient rehabilitation intervention. Methods, SDD and MCID were determined in a prospective study of 122 patient s before a comprehensive inpatient rehabilitation intervention and at the 3 -month followup. MCID was assessed by the transition method. Required SDD a nd sample sizes were determined by applying normal approximation and taking into account the calculation of power. Results. In the WOMAC sections the SDD and MCID ranged from 0.51 to 1.33 po ints (scale 0 to 10), and in the SF-36 sections the SDD and MCID ranged fro m 2.0 to 7.8 points (scale 0 to 100). Both questionnaires showed 2 moderate ly responsive sections that led to required sample sizes of 40 to 325 per t reatment arm for a clinical study with unpaired data or total for paired fo llowup data. Conclusion. In rehabilitation intervention, effects larger than 12% of base line score (6% of maximal score) can be attained and detected as MCID by th e transition method in both the WOMAC and the SF-36. Effects of this size l ead to reasonable sample sizes for future studies lying below n = 300. The same holds true for moderately responsive questionnaire sections with effec t sizes higher than 0.25. When designing studies, assumed effects below the MCID may be detectable but are clinically meaningless.