Note that two types of correlations exist in the above GEE models: correlation between different types of HPV infection, and correlation between the same types of HPV infections at different visits. Standard GEE methods do not easily enable construction of different correlation structures for these two types of correlation(13,14): only one working correlation structure can usually be used in a single model. Based on prior experience(8), we recommend using either an independent or exchangeable working correlation as an approximation for both correlations.
