This report reviews data generated during the first 3 years of California's Class Size Reduction (CSR) implementation. It focuses on a subpopulation of students who have had anywhere from zero to 3 years of reduced class-size experience, beginning in the first, second, or third grades, with some having returned to large classes in the fourth grade. The analysis is based on complete student, classroom, and teacher records from 15,267 third- and fourth-grade students in 546 classrooms from 72 schools in 7 southern California school districts. The data include reading, mathematics, and language test scores from the Stanford Achievement Test, as well as 36 variables covering student demographics, school assignments, classroom contexts, and teacher characteristics. The review found that since CSR was fully implemented the children first given exposure to smaller classes were not a representative sample of California's public school children. Positive achievements in mathematics accompanied CSR students, whereas language arts and reading showed trivial improvement. The report cautions that it is difficult to assess CSR's impact for a number of reasons, particularly the fact that CSR was initiated in concert with many other reforms, making it impossible to assess which changes resulted in student gains. Detailed information on the study sample is provided. (Contains 44 references, 7 tables, and 3 figures.) (RJM)