 The gravity model can be used to measure accessibility to services (e.x., access to health care).A special case of gravity model is the two-step floating catchment area method (2SFCA), which is popular in health care research.The gravity model was expanded by William J. Reilly in 1931 into Reilly's law of retail gravitation to calculate the breaking point between two places where customers will be drawn to one or another of two competing commercial centers.