Producing sufficient numbers of graduates who are prepared for science, technology, engineering, and mathematics (STEM) occupations has become a national priority in the United States. To attain this goal, some policymakers have targeted reducing STEM attrition in college, arguing that retaining more students in STEM fields in college is a low-cost, fast way to produce the STEM professionals that the nation needs (President's Council of Advisors on Science and Technology [PCAST] 2012). Within this context, this Statistical Analysis Report (SAR) presents an examination of students' attrition from STEM fields over the course of 6 years in college using data from the 2004/09 Beginning Postsecondary Students Longitudinal Study (BPS:04/09) and the associated 2009 Postsecondary Education Transcript Study (PETS:09). In this SAR, the term STEM attrition refers to enrollment choices that result in potential STEM graduates (i.e., undergraduates who declare a STEM major) moving away from STEM fields by switching majors to non-STEM fields or leaving postsecondary education before earning a degree or certificate. The purpose of this study is to gain a better understanding of this attrition by: (1) determining rates of attrition from STEM and non-STEM fields; (2) identifying characteristics of students who leave STEM fields; (3) comparing the STEM coursetaking and performance of STEM leavers and persisters; and (4) examining the strength of various factors' associations with STEM attrition. Appended are: (1) Glossary; (2) Technical Notes and Methodology; (3) Classification of Major Field of Study in BPS:04/09; and (4) Classification of Postsecondary STEM Courses in BPS:04/09. (Contains 7 tables, 12 figures, and 29 footnotes.) [This report was prepared for the National Center for Education Statistics under Contract No. ED-07-CO-0104 with RTI International.]