Recent research by Coleman, Hoffer, and Kilgore on the effectiveness of public and private schools may be seriously flawed because of its neglect of input-level differences in student performance and its reliance on cross-sectional testing data as the criterion measure. The sample used by Coleman and his colleagues from the High School and Beyond (HSB) data set was limited to seniors and to fewer schools than the more complete data in the National Longitudinal Study (NLS) of the High School Class of 1972. Whereas it is true that a comparison of mean scores for public and Catholic schools in both the NLS and HSB consistently favors Catholic schools, such a comparison may be inappropriate because private sector schools tend to attract students who are in an academic tract, but public schools must take anyone. The differences between public and Catholic test score results become markedly slimmer when academic and general track students are compared separately. When adjusted for differences in enrollment proportions in the two tracks, the figures give only two significant advantages to Catholic schools--in the verbal SAT among academic-track students and in the verbal followup test among general-track students. The differences between public and Catholic schools in achievement scores become insignificant after the variables of student selection and background characteristics are statistically controlled. There is thus little reason to believe that Catholic schools are more effective than public schools in promoting cognitive development. (Author/JW)