Seven hundred and fifty compositions, randomly selected from National Assessment of Educational Progress essays written by 9-, 13- and 17-year-olds, were analyzed in a study of the skills that go into the writing of a good composition. The essays were first rated as high, medium, or low in quality. A total of 43 different indices reported or hypothesized to have a relationship with composition quality were then identified from a survey of past research and were applied to each of the sample compositions. The relationship of the indices with quality at the different age levels was then tested. Analysis of the results led to the identification of five composing skills related to writing quality: modification within sentences, subordination between sentences, sentence sense, grammar and usage, and vocabulary. The data suggest a rather logical progression in terms of the five skills and provide support for the claim that all five skills should be taught at each grade level with slightly different emphases. (Descriptions of the 43 indices used in the study and suggestions for teaching the five composing skills at different grade levels are included.) (GW) Aspect of National Assessment (NAEP) dealt with in this document: Results (Secondary Analyses).