Mean number of adult endocrinologists per 100,000 population = 1.5663; R² = 0.76; adjusted R² = 0.75.

The managed care data used in this model were derived from the Interstudy Competitive Edge Part III report from July 1999 (12). The data included estimates of total population, total HMO enrollment, HMO and Medicaid enrollment, non-HMO and Medicaid enrollment, and uninsured population for 306 Metropolitan Statistical Areas (MSAs).

Population and median household income are from the 1996 census estimates, updated through the current population survey, as they appeared in the 1999 Area Resource File (ARF), produced by the Health Resources and Services Administration (13). Data in the ARF are listed at the FIPS (county) level but are labeled according to MSA as well. Our MSA-level figures for median household income were calculated by taking an average of incomes for all counties in a given MSA, weighted by the 1996 Census population of that MSA.

For general practitioners, general internists, and urologists, we used estimates from the 1995 and 1997 AMA Physician Characteristics and Distribution publications as they appeared in the 1999 ARF.

Elasticity is a measure that summarizes how one factor responds to a change in another factor. An elasticity of −0.3 for managed care penetration with respect to the demand for physician services, for example, means that a 10% increase in the managed care penetration rate results in a 3% decline in the demand for physician services.