Food swamps and obesity revisited: a reproduction with public data

Cooksey-Stowers, Schwartz and Brownell (2017) reported that "food swamps", counties with many fast-food outlets and convenience stores relative to grocery stores, predict adult obesity better than food deserts, and that an instrumental-variables (IV) estimate using highway exits was about 20 times the ordinary least squares (OLS) estimate. I reproduced the analysis from public data, replacing the commercial exit data with a count of OpenStreetMap motorway junctions. The published sample sizes, R² and control coefficients were reproduced closely once store counts were taken from 2007, not 2009 as the paper states. OLS coefficients were 0.086 to 0.120 percentage points of obesity per unit of the retail food environment index (published 0.115 to 0.125). IV estimates were positive (1.02 to 1.81; first-stage F 28 to 35). But motorway junctions correlate 0.90 with county population. With linear population or the count of people over 65 as controls, the instrument stayed strong (F 19 to 35) and estimates fell to 0.73 to 0.93; with log population and state fixed effects, the first-stage F fell to 0.03 and the Anderson–Rubin confidence set became unbounded. Within counties, changes in the index from 2007 to 2016 were not associated with changes in obesity from 2009 to 2018 (−0.010 per unit, SE 0.012), although noise and a change of obesity source may bias this toward zero. The cross-sectional association reproduces; the published IV estimate of 2.604 does not, and the causal estimate depends on how county size is controlled. Preprint, not peer reviewed. Also published at https://x9m8.com/food-swamps-revisited/. The zip file contains the code, results and the two independent audits.