Although many previous analytical studies indicate that physicians' assistants can substantially increase productivity and reduce costs, the utilization of physicians' assistants in ambulatory health care settings has grown at a disappointing rate. This apparent discrepancy may be explained in part through close examination of the models used in the analytical studies. This paper describes the limitations of previous studies and shows how analysis can be improved through the use of a combined optimization-simulation model. The model is applied to a hypothetical example to demonstrate how productivity and income benefits can be overstated, and to test the sensitivity of such benefits to a range of management policies for the ambulatory care practice regarding patient load, physical resources, appointment scheduling, and more flexible assignment of providers to patients.