Whether students learn more in private schools is a point of controversy among researchers, though the argument for tuition tax credits for parents of private school students often rests on it. In different studies comparing private and public schools, the main area of disagreement is the extent to which nonschool factors affect achievement. The "Public and Private Schools" report concluded that private schools produce better cognitive outcomes than public schools, with half the difference attributable to students' backgrounds and half to private school practices; that greater cognitive growth occurs in private schools between sophomore and senior years; and that achievement levels vary less in Catholic schools than elsewhere. The study's methodology has been attacked on several grounds, but some groups of minority students do appear to do better in private schools. The quality of individual schools varies and recent school effectivess data suggest that good public and private schools share such characteristics as strong leadership, more homework, a supportive learning environment, and fair discipline. (MJL)