A policy of reducing class size seems sensible, but might have costly, far-reaching implications for students, teachers, and public support for schools. The purpose of this research brief is to summarize 100 class size research studies from 1950 to 1985 and to regroup them into 18 areas of concern. The cluster analysis approach was designed to make this research more accessible and useful to school officials making class size decisions. (Three major 1978 syntheses had created substantial debate and controversy by treating class size as an isolated variable.) In contrast, clustering research by grade level, subject area, definition, pupil characteristics, teacher opinion, and cost implications helps decision-makers determine the best possible use of additional resources. This analysis dispels the concept of an "optimum" class size. The relationships between class size and student achievement vary greatly across grades, among subject areas, and by instructional methods. In general, the effects of smaller classes on student learning, attitudes, and behavior decreases as grade level increases. Findings indicate that disadvantaged or minority students achieve more academically in smaller classes. The same holds true for students with below average academic ability. Included are numerous tables and three appendices listing over 200 class size studies and 83 additional references. (MLH)