Comment on 'Small sample GEE estimation of regression parameters for longitudinal data'.

In longitudinal studies, the generalized estimating equation (GEE) estimator of the parameters of a marginal model is known to be consistent even if the working intra-subject covariance matrix is incorrectly specified. Recently, a small sample correction for the bias of the GEE estimator has been proposed. We show that this correction formula relies on the correct specification of the working intra-subject covariance matrix. We provide a revised formula that is valid under misspecification and develop the R package 'BCgee' to ease the practical use of the formula. Copyright © 2017 John Wiley & Sons, Ltd.