Model diagnostic plots for repeated measures data using the generalized estimating equations approach

The generalized estimating equations (GEE) approach has been widely used to analyze repeated measures data. However, in the absence of likelihood ratio tests, model diagnostic checking tools are not well established for the GEE approach, whereas they are for other likelihood-based approaches. Diagnostic checking tools are essential for determining a model’s goodness of fit, especially for non-normal data. In this paper, we propose simple residual plots to investigate the goodness of fit of the model based on the GEE approach for discrete data. The proposed residual plots are based on the quantile–quantile (QQ–QQ) plots of a χ2χ2-distribution, and are particularly useful for comparing several models simultaneously.