Comparing GIS-based methods of measuring spatial accessibility to health services.

The inequitable geographic distribution of health care resources has long been recognized as a problem in the United States. Traditional measures, such as a simple ratio of supply to demand in an area or distance to the closest provider, are easy measures for spatial accessibility. However the former one does not consider interactions between patients and providers across administrative borders and the latter does not account for the demand side, that is, the competition for the supply. With advancements in GIS, however, better measures of geographic accessibility, variants of a gravity model, have been applied. Among them are (1) a two-step floating catchment area (2SFCA) method and (2) a kernel density (KD) method. This microscopic study compared these two GIS-based measures of accessibility in our case study of dialysis service centers in Chicago. Our comparison study found a significant mismatch of the accessibility ratios between the two methods. Overall, the 2SFCA method produced better accessibility ratios. There is room for further improvement of the 2SFCA method-varying the radius of service area according to the type of provider or the type of neighborhood and determining the appropriate weight equation form-still warrant further study.