1. An incremental effect modeling apparatus comprising: a computing device processor; a storage device; and an incremental effect modeling application stored in said storage device and configured for operation on said computing device processor, said application configured to: determine a set of model variables from a plurality of predictor variables that are decision relevant and optionally time invariant, wherein the predictor variables are associated with a response variable or a performance variable and a decision variable for a population, wherein the decision variable defines treated observations and control observations in the population; determine a function describing a relationship between an incremental effect and the model variable, wherein the incremental effect is defined as the difference between performance of an individual under treatment and the performance of the individual under no treatment; rank order the treated and control observations in the population based on a score from low to high, wherein the score is a weighted sum of a set of functions created from the model variables; determine a cumulative incremental effect at increasing percentages of ranked observations with the lowest score values, based on the combined test and control data; determine an incremental effect area index based on the cumulative incremental effect as a function of the increasing percentages of ranked observations with the lowest score values; and iterate through the functions of the model variables for weight searching until the incremental effect area index is maximized to determine the incremental effect model.