1. A method for identifying causes of time variations in a data set for a process, the method comprising a computer system automatically performing the following: processing a data set containing observations of the process taken at different times, 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 at different times with respect to the outcome, the variable combinations defined by values for one or more of the variables; estimating time variations in the contributions of the variable combinations to the outcome, based on time variations in the behaviors of variable combinations and also based on time variations in populations of the variable combinations; and reporting time variations based on the estimated time variations for the variable combinations.