SEELS was a six-year study of a nationally representative sample of students receiving special education who were ages 6-12 at the inception of the study.

Students were selected randomly from those receiving special education in about 300 school districts nationwide. SEELS was a conceptually ambitious project, collecting descriptive and explanatory information about the characteristics of students as they transitioned from elementary to middle and middle to high school. It documented the educational services they received and their academic, social, and vocational development.

Statistical summaries generated from SEELS generalized to special education students nationally as a group, to each of the federal special education disability categories, and to each single-year age cohort. A major analytic focus of SEELS was to identify combinations of curricula, instruction, and other services that correlate with developmental growth in key areas and with school success. Data were being collected repeatedly through telephone interviews with parents, surveys of school staff, direct assessments of students' academic skills and knowledge (with accommodations), and collection of transcripts for middle and high school students.