This paper presents the results of a study designed to provide an empirical test of a learning theory proposed by McKenzie and Staaf in which the student effort enters as an argument in the learning achievement function. The model states that final achievement in economics is equal to the initial stock of knowledge plus improvement, where improvement is given by the product of effort times the learning rate or aptitude. The data source was the entire population of students enrolled in a principles of economics course in two universities. Pre- and posttests were given to all students, with additional academic and biographical data obtained from university records. Complete data were obtained for 689 students. Two findings are apparent in the results: first, the inclusion of an empirical measure of effort is shown to be a statistically significant variable explaining both learning achievement and improvement. Second, a substitute for the initial stock of knowledge proved not to be a significant variable in explaining learning improvement. Results indicate the necessity of including a measure of effort in further research (and its omission from previous studies calls into question the validity of the results derived therefrom). (Author/JK)