1. A method for analyzing differences in an outcome between a data set for a process and a subset of the data set, the method comprising a computer system automatically performing the following: processing a data set containing observations of the process, the observations expressed as values for a plurality of variables and for the outcome, wherein processing the data set determines behaviors for different variable combinations with respect to the outcome, the variable combinations defined by values for one or more of the variables, the subset defined as those observations for which one or more test variables take trial values; for pairs of a first variable combination and a second variable combination, wherein the test variables take the trial values in the second variable combination and the first variable combination is the same as the second variable combination except that the test variables are not specified as part of the first variable combination, estimating contributions of the pair to differences in the outcome between the data set and the subset, based on differences in the behaviors of the pair and also based on differences in populations of the pair; and reporting differences in the outcome between the data set and the subset based on the estimated contributions for the variable combinations.