A study examined whether patterns exist in the kinds and amounts of writing errors students make and whether teachers follow any sort of pattern in correcting these errors. Sixty compositions, gathered from a twelfth grade class taught by one teacher, were analyzed using the "McGraw-Hill Handbook of English." Student written errors were classified into five basic categories: (1) punctuation--commas, capital letters, and apostrophes; (2) subject-verb and pronoun agreement; (3) sentence structure errors--incomplete sentences and mistakes involving verbs, infinitives, conjunctions, and modifiers; (4) lexical and structural diction and spelling; and (5) miscellaneous, including syntax errors and letter format errors. Findings revealed that students made 3,870 English usage errors, of which one-third were corrected, and that no one manageable set of editorial usage rules could be used to improve significantly the writing of all of the students. In addition, the study established that the teacher followed no pattern for error correction. (Tables of data are included, and a checklist of errors is appended.) (JD)