This report presents the findings of an analysis of the Academic Performance Index (API) scores based on SATs taken in 1999, 2000, and 2001. It focuses on charter schools in California that serve students from low socioeconomic-status (SES) families. The purpose of the study was to see how standardized test scores from charter schools serving low-SES students compared with statewide averages. The method employed in the study was a longitudinal statistical analysis of test-results data available from California schools. The unit of analysis consisted of individual schools. The principle findings are as follows: (1) California charter schools are doing a better job of improving the academic performance of at-risk students than noncharter schools are; (2) student achievement in low-income charter schools is, on average, improving at a faster rate than in similar noncharter schools; (3) charter schools are serving a greater concentration of low-income students; (4) smaller schools tend to outperform larger schools in terms of student achievement growth; (5) socioeconomic status continues to influence student performance on standardized tests; and (6) factors also influencing performance include percentage of teachers on emergency credentials, high mobility rates, and high percentage of English-language learners. (Contains 16 references.) (WFA)