Meta-analysis on continuous outcomes in minimal important difference units: an application with appropriate variance calculations

ObjectiveTo compare results from meta-analyses for mean differences in minimal important difference (MID) units (MD_MID), when MID is treated as a random variable vs. a constant.Study Design and SettingMeta-analyses of published data. We calculated the variance of MD_MID as a random variable using the delta method and as a constant. We assessed performance under different assumptions. We compare meta-analysis results from data originally used to present the MD_MID and data from osteoarthritis studies using different domain instruments.ResultsDepending on the data set and depending on the values of rho and coefficient of variation of the MID (CoV_MID), estimates of treatment effect and P-values between an approach considering the MID as a constant vs. as a random variable may differ appreciably. Using our data sets, we provide examples of the potential magnitude. When rho = 0.5 and CoV_MID = 0.8, considering MID as a constant overestimated the treatment effect by 33–110% and decreased the P-value for heterogeneity from above 0.95 to below 0.08. When rho = 0.8 and CoV_MID = 0.5, the magnitude of the effects was similar.ConclusionsConsidering MID as a random variable avoids unrealistic assumptions and provides more appropriate treatment effect estimates.