Many secondary schools now base track levels on course difficulty, with tracks such as basic, honors, or college-prep. Public schools might track in terms of high, average, or lower ability. As noted by Oakes and Martin, "school policies determine three structural qualities of the tracking system: "extensiveness" (the number of subjects tracked and the type of distinct curricula offered); "specificity" (the number of track levels offered); and "flexibility" (whether students move from one track to another)". Although, in theory, track assignment is based on academic ability, other factors often influence placement.