The National Assessment of Educational Progress (NAEP) has measured students' progress toward higher achievement since 1969, assessing students in public and nonpublic schools in various subject areas on a regular basis. The NAEP has also collected background information about students to provide a context for interpreting assessment results and documenting the implementation of educational reform. This addendum is a revision and full reprint of sections of the report: (1) the Executive Summary; (2) Chapter 1; (3) Chapter 2; and (4) Appendix A of "NAEP 1996 Trends in Academic Progress" (NCES 97-985). It contains revisions to the text, tables, and figures of the Long Term Trend Science results. The material in this addendum replaces the corresponding sections of the original report. The original report presented the results of science, mathematics, reading, and writing trend assessment, charting them back to the year in which the assessment was first given. Trends are discussed for students at ages 9, 13, and 17, in grades 4, 8, and 11. Trends in average performance differences between White and Black, White and Hispanic, and male and female students are also discussed. The overall pattern of science performance has been one of decline followed by recent improved performance. The long-term science trend assessment contains a content dimension and a cognitive dimension. Because it represents educational objectives that were established in 1969 for 17-year-olds and 1970 for 9- and 13-year-olds, the long-term science trend assessment may represent a more constrained view than the 1996 main science assessment, as this addendum clarifies. (Contains 9 figures and 18 tables.) (SLD)