1. A method of selecting node variables for use in building a binary decision tree, comprising the steps of: (a) providing an input data set including a plurality of input variables and an associated decision state; (b) calculating a statistical measure of the significance of each of the input variables to the associated decision state; (c) averaging the statistical measures for each of the input variables and to form an averaged statistical measure for each input variable; (d) selecting the input variable with the largest average statistical measure; and (e) using the selected input variable as a node variable for splitting the input data set into two subsets that are used in building the binary decision tree.