All students were interviewed individually during sessions lasting approximately 60 minutes in Grades 3, 5, and 7, and 50 minutes in Grade 9. The outlines of the tasks appearing in the tables were supplemented by questions probing the reasons behind students' answers. All sessions were audio taped and later coded. All third graders were interviewed before fifth graders, fifth graders before seventh graders, and seventh graders before ninth graders. This procedure allowed us to gain good understanding of the capabilities of students at one developmenatal level before proceeding to the next and to make minor modifications in the interview formats as necessary. Thus, example, we decided to delete or change some of the tasks at Grade 9. (Some for influenced by of these decisions were also the shorter interview times we were allowed at Grade 9.)