The Trends in International Mathematics and Science studies provide country-level data for tracking changes in student achievement over time. In this paper the author has developed a method for identifying and monitoring trends in student achievement above or below any specified cut-point on these tests. The method involved the use of the Foster, Greer, and Thorbecke indices, as well as a modified version of these indices. The ability to identify and monitor trends in student achievement at various cut-points on the test should prove useful to policy analysts as well as to governmental and international funding agencies wishing to obtain data on the effectiveness of various programs and policies. (Contains 3 tables.)