The strength and affluence of an association is related to the size of its membership and its share of those eligible for membership. Associations, therefore, have an incentive not only to recruit new members, but also retain those who already belong. However, less is known about the exogenous economic factors that influence an individual's membership decision. Using time-series data from 1977 to 1992 and linear regression techniques, this thesis identifies and measures the economic factors important to the physician membership percentage of the American College of Radiology. The results of this thesis will provide the leadership of the American College of Radiology useful insight into the impact certain variables have on the membership. / Master of Arts