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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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.


This was an interesting paper to research. We kept hitting on the inflation/deflation issue. It was only when we realized that a modification inspired by a technique used in spatial econometrics that we saw how simple and elegant the solution was. We have since used this technique in several applications. The method has also inspired further extensions that draw from the basic idea of proportional allocation.

Antonio Paez
McMaster University

Read the Original

This page is a summary of: Demand and level of service inflation in Floating Catchment Area (FCA) methods, PLoS ONE, June 2019, PLOS,
DOI: 10.1371/journal.pone.0218773.
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