The emerging role and benefits of boundary analysis in spatio-temporal epidemiology and public health

Abstract

This special issue on geographic boundary analysis in spatio-temporal epidemiology and public health seeks to expand awareness on the contributions and potential caveats of using geographic boundary analysis to study human health outcomes, correlates and determinants. Epidemiology has a strong clinical tradition focused on the individual, on the characteristics of the population from which that individual arises, and on the risk factors that individuals may have been exposed to during critical times over his or her life course. The emphasis is on the construction of insightful hypotheses whose validity may be formally evaluated and on the identification of factors that may increase or reduce disease risk. Quantitative methods play a role in estimation, to provide accurate and unbiased estimates of the incidence and prevalence of health events and associated risk factors; in hypothesis testing, to evaluate whether sub-groups with quantitatively different exposures have concomitant differences in disease risk; and in modeling, to provide a formal basis for prediction and model-based theory formulation and testing (Kuh and Ben-Shlomo 1997; Koopman, Chick et al. 2000). The geography of the individual and of the population whence that individual comes underpins many aspects of the life course that are of direct epidemiological interest including social networks, socio-economic status and deprivation, accessibility to resources, and many of the ingredients of living that are determinants of individual health status and outcomes. From this perspective, an increased understanding of the locations, extent, and strength of geographic boundaries can directly strengthen our understanding of the spatio-temporal epidemiology of disease and hence our ability to design, conduct and assess the effectiveness of public health interventions. Consider some examples. Obesity and overweight Overweight, obesity and inactivity are a rapidly growing problem in many post-industrial nations. The magnitude of the problem is astonishing. Since the 1980’s in the United States obesity has increased steadily across all states, genders, age groups, ethnicities, and education levels, with 31% of all US adults obese in 2004 (Mokdad AH 1999; Flegal KM 2002; Frank, Andresen et al. 2004). This “obesity epidemic” has strong spatial and temporal patterns, with southern states serving as the initial nexus, followed by rapid increases in obesity and overweight across the Southeast, Midwest and western states in ensuing decades. Recent studies have focused on how the local neighborhood environment, access to fast foods, absence of accessibility to grocery stores, travel patterns, and the availability of opportunities for routine exercise serve as predictors of the prevalence of overweight and obesity in local populations (Frank, Andresen et al. 2004). The built environment and other neighborhood characteristics appear to be risk factors, with perceptions of “unsafe neighborhood” associated with increased risk of overweight and obesity. If we accept that overweight and obesity have strong spatial and temporal structure that varies locally (e.g. neighborhoods) as well as regionally (e.g. states), questions such as “where are the neighborhood boundaries” and “are the borders of states meaningful in terms of risk of obesity and overweight” become directly relevant.