Commonality analysis was used to look for school effects in gains in reading test scores for 877 fourth to sixth grade children in Elementary Secondary Education Act Title I remedial reading programs. The four groups of predictor variables that were investigated were background, mental ability, parental involvement, and school program. Commonality analysis uses multiple regression procedures to partition the variance among the four groups. The criterion scores were vocabulary and comprehension scores from the Gates-MacGinitie Reading Test post-tests. Pre-test scores were included in the mental ability group of predictors. The results indicate the proportion of explained variance unique to each set of predictor variables plus the proportion common to the eleven combinations of sets. The interpretation and the advantages of commonality analysis, as well as the interpretation of negative commonalities were discussed. The technique of commonality analysis was judged to have several advantages over more traditional types of analysis. (Author/CTM)