Fundamental to statistical analyses is the comparison of means of one variable from two or more populations. Population samples may be constructed (i.e., experimental and control groups), or they may be natural groupings (i.e., students at a particular grade in different years). If the populations are similar, the mean comparisons are straightforward; if not, the question arises as to whether the mean differences are due to differences in the variable or differences in the populations. Partitioning analysis is a way of distinguishing between these differences. This paper is a demonstration of how partitioning analysis can be used to help separate changes in reading and mathematical proficiency from changes in school populations over assessment years. (Contains 2 figures and 12 tables.)