Over the next decade, Begle attempted to act on that credo by designing a series of structured teaching units to be used to control some of the variation that occurs when comparing teaching mathods. He also began a series of reviews of the empirical literature that culminated in a book issued after his death in March 1978 (Begle. 1979). In that book he classified nearly 7,000 studies according to variables (characteristics of teachers, students, materials, instruction, and so on) that might affect the learning of mathematics and then reviewed the main findings of the studies .