In our examples, we estimated the error structure with simple AR(p) models (mostly AR(1) or AR(2)) obtaining a good fit for time dependence. We also tried other ARMA models and obtained similar results. Our method can additionally be used to explore more complex dependence structures like heterogeneous covariances by counties or even spatio–temporal modelling. The iGLS procedure allows for more simplicity and flexibility in the estimation of the dependence structure at the cost of a light heavier computational work. Furthermore, in particular in the example provided, the iGLS allows us to specify a general dependence structure that can be adapted for every county rather than considering the same model for all counties or designing, by hand, the best structure for each county.