A method to correct bias in geographical accessibility calculations

What is it about?

Accessibility analysis is widely used to understand how transportation and land use interact to satisfy the needs of the population. Accessibility is calculated as the sum of opportunities that can be reached given a certain cost. A family of accessibility measures is called Two-Stage Floating Catchment Areas, because it first calculates the population within the service area of a point of service, say a clinic. This population is used to calculate a level of service, basically a provider to population ratio. Then, in the second stage the provider to population ratio is allocated to the population. Two-Stage Floating Catchment Areas give intuitive results in service level per capita, but they suffer from inflation or deflation caused by counting the same population multiple times, and then allocating the same service multiple times. In this paper a revised version of the method allocates both population and service proportionally to avoid multiple counting. As a result, the population and level of service are preserved.

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Photo by Alexander Schimmeck on Unsplash

Photo by Alexander Schimmeck on Unsplash

Why is it important?

The modification to the method of Two-Stage Floating Catchment Areas effectively resolves an issue with accessibility analysis that has intrigued researchers for years. The research shows how multiple counting in the population and then level of service do not cancel out, and the inflation or deflation are also not uniform, thus introducing spatial biases that can lead to misleading policy recommendations. By using proportional allocation factors not only the inflation/deflation are resolved, but the results become more intuitive and easier to interpret.

Perspectives