An evaluation of edge effects in nutritional accessibility and availability measures: a simulation study

Abstract

BackgroundThis paper addresses the statistical use of accessibility and availability indices and the effect of study boundaries on these measures. The measures are evaluated via an extensive simulation based on cluster models for local outlet density. We define outlet to mean either food retail store (convenience store, supermarket, gas station) or restaurant (limited service or full service restaurants). We designed a simulation whereby a cluster outlet model is assumed in a large study window and an internal subset of that window is constructed. We performed simulations on various criteria including one scenario representing an urban area with 2000 outlets as well as a non-urban area simulated with only 300 outlets. A comparison is made between estimates obtained with the full study area and estimates using only the subset area. This allows the study of the effect of edge censoring on accessibility measures.ResultsThe results suggest that considerable bias is found at the edges of study regions in particular for accessibility measures. Edge effects are smaller for availability measures (when not smoothed) and also for short range accessibilityConclusionsIt is recommended that any study utilizing these measures should correct for edge effects. The use of edge correction via guard areas is recommended and the avoidance of large range distance-based accessibility measures is also proposed.

Scale effects in food environment research: Implications from assessing socioeconomic dimensions of supermarket accessibility in an eight-county region of South Carolina

“…This analysis was part of a large methodological study of the food environment in South Carolina (Liese et al, 2010, 2013; Van Meter et al, 2010, 2011). The study area consisted of a contiguous geographical area and encompassed eight counties in the Midlands region of the state.…”

“…The availability and accessibility measures utilized in this analysis were supermarket density, cumulative opportunity and distance to nearest supermarket. Supermarket density is an availability measure and is defined as the number of food outlets adjusted for the geographic area of the specific geographic unit, i.e., census tract or block group (Van Meter et al, 2010, 2011; Thornton et al, 2011). Density is useful when explaining the distribution of features across areas (Thornton et al, 2011).…”

“…The simplest measure of access—distance or proximity to the nearest supermarket—has been most commonly used in food environment research. However, accessibility has also been characterized by several other measures, including the cumulative opportunity index (Thornton et al, 2011; Van Meter et al, 2010, 2011). …”

“…The objective of this study, therefore, was to evaluate the influence of two commonly used administrative-based geographic definitions (i.e., census tract and block group) on the relationships between GIS-derived supermarket measures and common U.S. Census–based socioeconomic characteristics using data from a 2010 South Carolina food environment study (Liese et al, 2010, 2013; Van Meter et al 2010, 2011). Supermarkets were selected for this analysis because compared with other food retailers, they provide access to a greater quantity, variety and quality of food items (Franco et al, 2008; Block and Kouba, 2006) and have been used as a major criterion of the quality of the food environment in many studies (KriŸan et al, 2014; Larson et al, 2009; Bader et al, 2010; U.S.…”

Where Are the Food Deserts? An Evaluation of Policy-Relevant Measures of Community Food Access in South Carolina

“…Both measures were susceptible to influences of edge effects as both were based on the aggregation of stores to census tracts. 18 Our replication of the two measures used census tract boundaries corresponding to the year 2010 and corresponded to the methodology used by the 2013 USDA ERS Food Access Research Atlas 5, 8, 14 and the 2013 CDC report. 7 Both measures relied on census tract geographies, which, like any arbitrary areal unit, are subject to boundary and modifiable areal unit problems.…”