The National Assessment of Educational Progress (NAEP) administered a writing task to nationally representative samples of 9-, 13-, and 17-year-old students in order to assess their abilities to use the conventions of written English. Approximately 2,000 students at each age level completed this task as part of NAEP's 1984 writing assessment. Subsamples were drawn from the total national sample. In order to have reasonable precision in comparisons between black students and white students, the black students were subsampled at a higher rate. In addition to measures of overall quality, each writing sample was analyzed for a variety of aspects of spelling, word choice, punctuation, and syntax. Experienced English teachers coded each sentence type and error on the papers. The analyzed papers were then entered into a computer-readable database. Analyses of data indicate that older students are more proficient than younger ones in their use of written language at both the sentence and the word levels: (1) older students use a greater proportion of complex sentences; (2) spelling improves markedly at the older ages; (3) students at all three ages make few errors in word choice or capitalization; and (4) the majority of students make very few punctuation errors. Because there is no consistent profile of the types of mistakes poor writers make, diagnosis of individual difficulties and instruction targeted at needed skills is recommended over large group drill and practice activities. The appendix contains the scoring guide outline. (LMO)