Academic journal article Environmental Health Perspectives

Vulnerability-Based Spatial Sampling Stratification for the National Children's Study, Worcester County, Massachusetts: Capturing Health-Relevant Environmental and Sociodemographic Variability

Academic journal article Environmental Health Perspectives

Vulnerability-Based Spatial Sampling Stratification for the National Children's Study, Worcester County, Massachusetts: Capturing Health-Relevant Environmental and Sociodemographic Variability

Article excerpt

BACKGROUND: The National Children's Study is the most ambitious study ever attempted in the United States to assess how environmental factors impact child health and development. It aims to follow 100,000 children from gestation until 21 years of age. Success requires breaking new interdisciplinary ground, starting with how to select the sample of > 1,000 children in each of 105 study sites; no standardized protocol exists for stratification of the target population by factoring in the diverse environments it inhabits. Worcester County, Massachusetts, like other sites, stratifies according to local conditions and local knowledge, subject to probability sampling rules.

OBJECTIVES: We answer the following questions: How do we divide Worcester County into viable strata that represent its health-relevant environmental and sociodemographic heterogeneity, subject to sampling rules? What potential does our approach have to inform stratification at other sites?

RESULTS: We developed a multivariable, vulnerability-based method for spatial sampling consisting of two descriptive indices: a hazards/stressors exposure index (comprising three proxy variables), and an adaptive capacity/sociodemographic character index (five variables). Multivariable, health-relevant stratification at the start of the study may improve detection power for environment-child health associations down the line. Eighteen strata capture countywide heterogeneity in the indices and have optimal relative homogeneity within each. They achieve comparable expected birth counts and conform to local concepts of space.

CONCLUSION: The approach offers moderate to high potential to inform other sites, limited by inter-site differences in data availability, geodemographics, and technical capacity. Energetic community engagement from the start promotes local stratification coherence, plus vital researcher-community trust and co-ownership for sustainability.

KEY WORDS: environmental health, GIS, National Children's Study, stratified sampling, vulnerability. Environ Health Perspect 118:1318-1325 (2010). doi:10.1289/ehp.0901315 [Online 8 March 2010]

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The National Children's Study (NCS) is the most ambitious study ever attempted in the United States, and arguably the world, to assess how environmental factors affect child health and development. To complete the project, 100,000 children from across the United States--the largest pregnancy cohort in the history of the country--will be followed from gestation until 21 years of age. The ultimate goal of the study is to improve the health and well-being of children (NCS 2010). Its appropriately broad definition of environment encompasses the following:

* Natural and human-made environmental factors

* Biological and chemical factors

* Physical surroundings

* Social factors

* Cultural and family influences and differences

* Geographic locations.

Population health disparities are viewed as a function of gradients in environmental exposures; health care access, utilization, or quality; and health status (Carter-Pokras and Baquet 2002; Gibbons 2005). The NCS will combine direct measures of environmental media and biological specimens with indirect questionnaire measures (e.g., health care access, utilization, and quality; health status, age, sex, income, education) and extant demographic, geographic, and environmental data (Needham et al. 2005). The NCS represents a strategic opportunity to synthesize lessons and methods from epidemiology, clinical science, and risk assessment (Gilliland et al. 2005; Trasande et al. 2009).

To reach NCS goals, new interdisciplinary methods are needed, including methods to select a sample representative of children nationwide, a "unique and demanding challenge" (Scheidt et al. 2009). As a first sampling stage, NCS investigators chose 105 study sites based on geographic and demographic characteristics, including 10 pilot Vanguard Centers and 95 other sites to be implemented in three waves (waves 1, 2, and 3). …

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