Quantifying the magnitude of environmental exposure misclassification when using imprecise address proxies in public health research.

In spatial epidemiologic and public health research it is common to use spatially aggregated units such as centroids of postal/zip codes, census tracts, dissemination areas, blocks or block groups as proxies for sample unit locations. Few studies, however, address the potential problems associated with using these units as address proxies. The purpose of this study is to quantify the magnitude of distance errors and accessibility misclassification that result from using several commonly-used address proxies in public health research. The impact of these positional discrepancies for spatial epidemiology is illustrated by examining misclassification of accessibility to several health-related facilities, including hospitals, public recreation spaces, schools, grocery stores, and junk food retailers throughout the City of London and Middlesex County, Ontario, Canada. Positional errors are quantified by multiple neighborhood types, revealing that address proxies are most problematic when used to represent residential locations in small towns and rural areas compared to suburban and urban areas. Findings indicate that the shorter the threshold distance used to measure accessibility between subject population and health-related facility, the greater the proportion of misclassified addresses. Using address proxies based on large aggregated units such as centroids of census tracts or dissemination areas can result in very large positional discrepancies (median errors up to 343 and 2088 m in urban and rural areas, respectively), and therefore should be avoided in spatial epidemiologic research. Even smaller, commonly-used, proxies for residential address such as postal code centroids can have large positional discrepancies (median errors up to 109 and 1363 m in urban and rural areas, respectively), and are prone to misrepresenting accessibility in small towns and rural Canada; therefore, postal codes should only be used with caution in spatial epidemiologic research.

Linking childhood obesity to the built environment: a multi-level analysis of home and school neighbourhood factors associated with body mass index.

OBJECTIVES: This study examines environmental factors associated with BMI (body mass index) levels among adolescents with the aim of identifying potential interventions for reducing childhood obesity. METHODS: Students (n=1,048) aged 10-14 years at 28 schools in London, ON, completed a survey providing information on age, sex, height, weight, home address, etc., which was used to construct age-sex adjusted BMI z-scores. The presence of recreation opportunities, fast-food outlets and convenience stores was assessed using four areal units around each participant's home and school neighbourhood: "circular buffers" encompassing territory within a straight-line distance of 500 m and 1000 m; and "network buffers" of 500 m and 1000 m measured along the street network. School neighbourhoods were also assessed using school-specific "walksheds". Multilevel structural equation modeling techniques were employed to simultaneously test the effects of school-environment (Level 2) and home-environment (Level 1) predictors on BMI z-scores. RESULTS: Most participants (71%) had a normal BMI, 16.9% were overweight, 7.6% were obese, and 4.6% were considered underweight. Multilevel analyses indicated that built environment characteristics around children's homes and schools had a modest but significant effect on their BMI. The presence of public recreation opportunities within a 500 m network distance of home was associated with lower BMI z-scores (p<0.05), and fast-food outlets within the school walkshed was associated with higher BMI z-scores (p<0.05). CONCLUSION: Interventions and policies that improve children's access to publicly provided recreation opportunities near home and that mitigate the concentration of fast-food outlets close to schools may be key to promoting healthy lifestyles and reducing childhood obesity.