Although many terms are used to describe practices of curricular differentiation, each has its limitations.
Tracking, the term used by the Congress in defining the committee's mandate, suggests the classic, rigid form of curricular differentiation in which a student's program or "track"—academic, general, or vocational—determines virtually every course that the student will take and at what level of difficulty. In recent decades, formal grouping systems this rigid have become less common in schools (Lucas, in press).
Ability grouping, a term used widely by scholars and practitioners, implies—incorrectly, in our view—that students are being grouped on the basis of "ability," a quality that some view as innate and immutable. As we will see, schools that group students usually do so on the basis of classroom performance and other measures of achievement that reflect acquired knowledge—something that can and does change over time—rather than ability. It is therefore misleading to use the term "ability grouping." Moreover, given the degree of racial and socioeconomic stratification that is often associated with grouping, it may reinforce false stereotypes to imply incorrectly that students in different groups are distinguished by ability. We find it more accurate to say that schools that group students typically try to do so by "skill level" or "achievement level" (Mosteller et al., 1996).
Homogeneous grouping is also a misnomer, based on studies of actual practice. The term "homogeneous" suggests that all the students in a given group are alike, or at least similar, in their achievement levels. Empirical studies cast doubt on this assumption, however. "Grouping's effect on reducing even cognitive diversity may be very small," report Oakes et al. in their comprehensive survey (1992:594). "Other studies document considerable overlap of students' skills and abilities among groups …. Thus the degree to which tracking reduces heterogeneity may be far less than we typically assume." For reasons discussed below, it appears that factors other than student achievement—scheduling constraints, parental interventions, and student choice, in particular—often help to determine who takes which classes. Although these other factors may be entirely legitimate, they often produce groupings that are not very homogeneous. In some circumstances, "it is unclear whether it is possible to organize classes that contain a narrow range of student ability" (Gamoran and Weinstein, 1998:387). At the same time, there is evidence