The National Assessment of Educational Progress (NAEP) is an ongoing, congressionally mandated national survey of the knowledge, skills, and understanding of young Americans in major subjects usually taught in school. This document is a guide to the files of the first NAEP Trial State Assessment (1990). The Trial State Assessment collected information on the mathematics knowledge, skills, understanding, and attitudes of a representative sample of about 100,000 students in public schools in 40 jurisdictions. The complexity of the assessment means that data file users need some special instructions. Following an overview of the NAEP system and the Trial State Assessment in the first chapter, the second chapter outlines special features of the assessment. The secondary-use data files for each state contain data for students, teachers, schools, and excluded students in the state and in the sample from the national mathematics assessment that was used for comparisons between the nation and the states. Other chapters discuss: (1) instrument design; (2) sample selection and weights; (3) data collection, processing, scoring, and database creation; (4) reporting subgroups and other variables; (5) NAEP scaling procedures; (6) conducting statistical analysis with NAEP data; (7) content and format of files, layouts, and codebooks; and (8) working with the Statistical Package for the Social Sciences and the Statistical Analysis System. Appendixes provide a history of the NAEP, the item response theory parameters for each cognitive item, and a glossary of terms. (Contains 29 tables and 22 references.) (SLD)