Using a GIS-based floating catchment method to assess areas with shortage of physicians.

This paper presents a geographic information system (GIS) based floating catchment method for identifying physician shortage areas. The traditional designation methods are primarily regional availability measures, which use administrative boundaries such as counties as the basic spatial units for calculating physician to population ratios and designate shortage based on those ratios. Such approaches have been criticized for their inability to account for either the spatial variations of population demand and physician supply within those boundaries or for population-physician interactions across them. The floating catchment method addresses the internal spatial distribution problem by deriving population data from a smaller unit, the census tract. The potential cross border patient-physician interaction is taken into consideration by using circles of reasonable radius around each census tract centroid as the basic spatial units, which can encompass areas on either side of an administrative border. By varying the radius of the catchment circle, this paper demonstrates that the physician to population ratio is scale dependent and that the greatest variability of the ratios and shortages occur at the most local scales (< 20 miles), which argues for using finer spatial resolution data in shortage designation practice.