New methods can extend the use of minimal important difference units in meta-analyses of continuous outcome measures.

OBJECTIVE: For continuous outcomes measured using instruments with an established minimally important difference (MID), pooled estimates can be usefully reported in MID units. Approaches suggested thus far omit studies that used instruments without an established MID. We describe an approach that addresses this limitation. STUDY
DESIGN: Using the ratio of MID to standard deviation in the trials with an established MID, we imputed the MID for instruments without an established MID and pooled across all trials. We applied this approach to two meta-analyses.
RESULTS: In 20 trials of respiratory rehabilitation, the pooled estimate did not differ significantly between trials with an established MID and those without an established MID (interaction P=0.23). The same was true for 52 trials examining amitriptyline vs. other antidepressants (interaction P=0.54). In the respiratory example, the addition of trials without an established MID led to little change in point estimates or confidence intervals (CIs, more data balanced by more heterogeneity in a random effects model). In the antidepressant example, the additional trials resulted in an identical point estimate with a narrowing of the CI.
CONCLUSION: Our method allows estimates of a pooled effect in MID units using both trials with and without an established MID.