r a square root transformation. Here we respect the nature of the outcome, and analyze it as a count. In the <code>aids</code> data frame the <code>CD4</code> variable is the already square root transformed CD4 cell counts. Hence, as a first step we transform it to the original scale. For the mean structure we only include the follow-up time as a categorical variable. For the working correlation we solve the GEEs assuming the independence, exchangeable and AR1 structures (the data are already ordered with r