This study examines associations between area-level socioeconomic factors and the birth defect gastroschisis in order to further our understanding of the etiology of this condition. Specifically, this study explores how measuring socioeconomic conditions at different geographic scales affect the results of statistical models. A population-based case-control study of resident live births was conducted using data from the North Carolina Birth Defect Monitoring Program and the North Carolina composite linked birth files from 1998 through 2004. Neighborhood conditions potentially related to gastroschisis (poverty, unemployment, education, and racial composition) were measured using Census 2000 data and aggregated to several geographic scales. The Brown-Forsythe test of homogeneity of variance was used to select the neighborhood size by examining the effect of neighborhood size on variation in gastroschisis rates. To examine our assumptions about neighborhood size and neighborhood effects on gastroschisis, we estimated a series of logistic regression and multilevel logistic regression models. The Brown-Forsythe test suggested an optimal neighborhood size with a circular radius of approximately 2500 m, which was supported by the statistical analysis. Results indicate a weak association between living in a neighborhood characterized by high poverty and unemployment and an elevated risk of a gastroschisis-affected pregnancy after adjusting for individual-level risk factors. Cross-level interactions indicate that women in low poverty neighborhoods who do not rely on Medicaid have a significantly lower risk of gastroschisis. The choice of neighborhood scale influences model results suggesting that socioeconomic processes may influence health outcomes variably at different scales.
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