RESEARCH Open Access Accuracy of residential geocoding in the Agricultural Health Study Rena R Jones 1* , Curt T DellaValle 1 , Abigail R Flory 2 , Alex Nordan 1 , Jane A Hoppin 4 , Jonathan N Hofmann 1 , Honglei Chen 5 , James Giglierano 6 , Charles F Lynch 7 , Laura E Beane Freeman 1 , Gerard Rushton 3 and Mary H Ward 1 Abstract Background: Environmental exposure assessments often require a study participant’s residential location, but the positional accuracy of geocoding varies by method and the rural status of an address. We evaluated geocoding error in the Agricultural Health Study (AHS), a cohort of pesticide applicators and their spouses in Iowa and North Carolina, U.S.A. Methods: For 5,064 AHS addresses in Iowa, we compared rooftop coordinates as a gold standard to two alternate locations: 1) E911 locations (intersection of the private and public road), and 2) geocodes generated by matching addresses to a commercial street database (NAVTEQ) or placed manually. Positional error (distance in meters (m) from the rooftop) was assessed overall and separately for addresses inside (non-rural) or outside town boundaries (rural). We estimated the sensitivity and specificity of proximity-based exposures (crops, animal feeding operations (AFOs)) and the attenuation in odds ratios (ORs) for a hypothetical nested case–control study. We also evaluated geocoding errors within two AHS subcohorts in Iowa and North Carolina by comparing them to GPS points taken at residences. Results: Nearly two-thirds of the addresses represented rural locations. Compared to the rooftop gold standard, E911 locations were more accurate overall than address-matched geocodes (median error 39 and 90 m, respectively). Rural addresses generally had greater error than non-rural addresses, although errors were smaller for E911 locations. For highly prevalent crops within 500 m (>97% of homes), sensitivity was >95% using both data sources; however, lower specificities with address-matched geocodes (more common for rural addresses) led to substantial attenuation of ORs (e.g., corn <500 m OR obs = 1.47 vs. OR true = 2.0). Error in the address-matched geocodes resulted in even greater OR obs attenuation for AFO exposures. Errors for North Carolina addresses were generally smaller than those in Iowa. Conclusions: Geocoding error can be minimized when known coordinates are available to test alternative data and methods. Our assessment suggests that where E911 locations are available, they offer an improvement upon address-matched geocodes for rural addresses. Exposure misclassification resulting from positional error is dependent on the geographic database, geocoding method, and the prevalence of exposure. Keywords: Geocoding, Positional error, Rural location, Environmental exposure assessment, Accuracy * Correspondence: rena.jones@nih.gov 1 Occupational and Environmental Epidemiology Branch, Division of Cancer Epidemiology and Genetics, National Cancer Institute, National Institutes of Health, Department of Health and Human Services, 9609 Medical Center Drive, Rockville, MD, USA Full list of author information is available at the end of the article INTERNATIONAL JOURNAL OF HEALTH GEOGRAPHICS © 2014 Jones et al.; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Jones et al. International Journal of Health Geographics 2014, 13:37 http://www.ij-healthgeographics.com/content/13/1/37