The minimal clinically important difference (MCID) and the patient acceptable symptom state (PASS) are cutoffs dichotomizing continuous values into variables such as improved or not improved (MCID), or in an acceptable state or not in an acceptable state (PASS), allowing to report the success rate (proportion of patients improved or in an acceptable state) in trial arms. The objective was to investigate the influence of the choice of MCID and/or PASS values on the difference in success rate between arms.
Analytic study was carried out. In two hypothetic trials (prespecified mean and standard deviation (SD) for control arm score, effect sizes [ES] of 0.25 and 0.5), we calculated the success rates in control and experimental arms for different MCID and PASS values, and derived the difference in success rates between arms.
For a 0-100 score and MCID values from -40 to -10, the difference in success rate between arms ranges from 7.9% to 9.9% (ES=0.25) and from 15.9% to 19.7% (ES=0.50). For PASS values from 20 to 50, the difference in success rate between arms ranges from 7.1% to 9.9% (ES=0.25) and from 15.6% to 19.7% (ES=0.50).
The MCID or PASS value has a low impact on the difference in the success rate between the arms in a trial.