The National Education Longitudinal Study of 1988 (NELS:88) is the third major study in the National Center for Education Statistics program of longitudinal studies about the achievement and characteristics of elementary and secondary school students. NELS:88 began with a base year survey of eighth grade students in 1988, followed up at 2-year intervals in 1990, 1992, and 1994. It collected data from more than 20,000 students using a complex sampling design. This report is designed for researchers who may use NELS:88 to study the longitudinal progress of eighth graders through the next 4 years. It gives information about the measurement characteristics of survey items in the base year and first and second followups, emphasizing issues of nonresponse and the convergence of information from alternative sources. Nine separate sources of NELS:88 data are covered in the report. Discrepancies among the sources are analyzed in terms of reliability, validity, and communality. The following chapters are included: (1) "Introduction"; (2) "Methodology"; (3) "Comparison of Student and Parent Responses"; (4) "Comparison of Teacher and Student Responses"; (5) "Comparison of Second Follow-up Student Responses with Earlier Responses"; and (6) "Summary and Conclusions." Appendixes discuss the univariate distribution of the items, recoding of measurements for comparisons, and statistical measures of association and omission bias. (Contains 8 figures and 162 tables.) (SLD)