1. A predictive modeling based focus error prediction method, comprising: obtaining wafer geometry measurements of a plurality of training wafers; grouping the plurality of training wafers to provide at least one training group based on relative homogeneity of wafer geometry measurements among the plurality of training wafers; for each particular training group of the at least one training group, developing a predictive model for that particular training group utilizing non-linear predictive modeling, the predictive model establishing correlations between wafer geometry parameters and focus error measurements obtained for each wafer within that particular training group; and utilizing the predictive model developed for a particular training group of the at least one training group to provide focus error prediction for an incoming wafer belonging to that particular training group.