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County-level hurricane exposure and birth rates: application of difference-in-differences analysis for confounding
Shannon C Grabich1, Whitney R Robinson1, Stephanie M Engel1
1Department of Epidemiology, Gillings School of Global Public Health, University of North Carolina at Chapel Hill, 135 Dauer Drive 2101 McGavran-Greenberg Hall, CB #7435, Chapel Hill, NC 27599 USA.
Emerging Themes in Epidemiology
|December 25, 2015
Summary
The difference-in-differences method revealed no significant impact of hurricane exposure on live birth rates. This approach helps control for unmeasured factors in environmental health studies.
Area of Science:
- Environmental Epidemiology
- Public Health Research
- Biostatistics
Background:
- Ecological studies on natural disaster health effects face confounding from unmeasured population differences.
- Aggregated data analyses are common but susceptible to bias.
- Hurricane exposure assessment is crucial for understanding environmental impacts.
Purpose of the Study:
- To demonstrate the difference-in-differences method for controlling time-invariant confounders in environmental health research.
- To assess the impact of hurricane exposure on live birth rates in Florida.
- To compare difference-in-differences estimates with traditional regression models.
Main Methods:
- Utilized Florida live birth data conceived in 2003 (pre-hurricane) and post-hurricane 2004.
- Defined hurricane exposure using wind speed (≥74 mph) and a 60 km spatial buffer.
- Quantified live birth rate differences (RD) and 95% confidence intervals (CI).
- Compared difference-in-differences estimates with general linear models adjusted for covariates.
Main Results:
- Difference-in-differences analysis showed null associations between hurricane exposure and live birth rates across metrics.
- For Hurricane Ivan (60 km buffer), RD was -0.02 births/1000 individuals (-0.51, 0.47).
- General linear models indicated a positive association (2.80 births/1000 individuals (1.94, 3.67)) for Hurricane Ivan, but results varied.
Conclusions:
- Ecological studies on environmental exposures and health are prone to confounding.
- The difference-in-differences method offers an accessible approach to control for time-invariant confounders.
- This study highlights the importance of robust methodologies in environmental epidemiology.
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