This paper discusses the benefits of using generalizabilty theory in lieu of classical test theory. Generalizability theory subsumes and extends the precepts of classical test theory by estimating the magnitude of multiple sources of measurement error and their interactions simultaneously in a single analysis. Since classical test theory examines only one source of measurement error at a time (e.g. occasions, forms, or internal consistency), it is not possible to estimate the magnitudes of all sources of measurement error and the magnitude of measurement error interaction effects concurrently. As this paper explores the shortcomings of classical test theory and the strengths afforded by using generalizability theory, a small heuristic data set is used to make the discussion concrete. (Contains 9 tables and 28 references.) (SLD)