A 2-year study of mathematics performance of sixth-grade students was conducted to: (1) determine the extent to which previously reported sex differences in California data exist and influence student response; (2) examine the predominant successful and unsuccessful strategies used by sixth-grade students in solving story problems; (3) discover to what extent boys and girls use different strategies in solving story problems; and (4) determine whether there are underlying cognitive differences in typical strategies of boys and girls and to identify these differences if they exist. Particular emphasis was given to analysis and classification of errors made by students solving a variety of mathematical items. The research consisted of analyzing responses from all sixth-grade students on previously existing data and analyzing children's (N=93) problem-solving behaviors obtained from individual interviews. Among the results reported are those indicating that girls were more likely to make errors leading to illogical responses and errors resulting from guessing strategies and that girls performed better on computation and worse than boys on story problems. Appendices include items used in interviews; children's responses to these items; and manuscripts, prepared for journal or book publication, which contain more technical detail about the research. (JN)