When the βX1 parameters are correlated with at least one of the sets of βXk parameters for k=2,3,…,K, the second term in Equation 13 now contributes to the value of covw(α,βX1). The InSIDE assumption for univariable MR‐Egger will therefore be automatically violated as the weighted covariance between α and βX1 will not equal zero, resulting in biased causal estimates of θ 1. If the InSIDE assumption holds for multivariable MR‐Egger, and βXk are included in the analysis model, then θ^1ME will still be a consistent estimate of θ 1. Hence, in this case, multivariable MR‐Egger should result in reduced bias compared with univariable MR‐Egger.
