### Abstract

#### Objective

To discuss the concepts of the minimal clinically important difference (MCID) and the smallest detectable difference (SDD) and to examine their relation to required sample sizes for future studies using concrete data of the condition-specific Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) and the generic Medical Outcomes Study 36-Item Short Form (SF-36) in patients with osteoarthritis of the lower extremities undergoing a comprehensive inpatient rehabilitation intervention.

#### Methods

SDD and MCID were determined in a prospective study of 122 patients before a comprehensive inpatient rehabilitation intervention and at the 3-month followup. MCID was assessed by the transition method. Required SDD and 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 points (scale 0 to 10), and in the SF-36 sections the SDD and MCID ranged from 2.0 to 7.8 points (scale 0 to 100). Both questionnaires showed 2 moderately responsive sections that led to required sample sizes of 40 to 325 per treatment arm for a clinical study with unpaired data or total for paired followup data.

#### Conclusion

In rehabilitation intervention, effects larger than 12% of baseline score (6% of maximal score) can be attained and detected as MCID by the transition method in both the WOMAC and the SF-36. Effects of this size lead to reasonable sample sizes for future studies lying below n = 300. The same holds true for moderately responsive questionnaire sections with effect sizes higher than 0.25. When designing studies, assumed effects below the MCID may be detectable but are clinically meaningless.