American Journal of Epidemiology Advance Access originally published online on December 7, 2005
American Journal of Epidemiology 2006 163(4):393-394; doi:10.1093/aje/kwj043
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Letter to the Editor |
THE AUTHORS REPLY
1 Department of Health Services, School of Public Health, University of California at Los Angeles, Los Angeles, CA 90095
2 Center for Health Policy and Research, School of Public Health, University of California at Los Angeles, Los Angeles, CA 90095
3 Department of Epidemiology, School of Public Health, University of California at Los Angeles, Los Angeles, CA 90095
4 Department of Environmental Health Sciences, School of Public Health, University of California at Los Angeles, Los Angeles, CA 90095
e-mail: nponce{at}ucla.edu
We thank Drs. Shukunami, Nishijima, and Kotsuji for their comments (1
) on our article (2) and agree that making exclusions can lead to selection bias, a potentially serious threat to the validity of our inferences. However, the specific exclusions that they note have little impact on our preterm birth risk estimates and would not change our study's principal finding that traffic-related air pollution exposure disproportionately affects lower socioeconomic status neighborhoods in the winter (2).
First, we address their concern on exclusions of extreme values of birth weight. "Extremely low birth weight" has been defined as a birth weight of less than 1,000 g (3). In our case-control sample drawn from Los Angeles County, California, 1994–1996 births, infants born weighing less than 500 g comprised 0.4 percent of the overall sample and 1 percent of the preterm births. Excluding very heavy babies weighing greater than 5,000 g also has virtually no impact on our study, since these babies were only 0.1 percent of the overall case-control sample. Perhaps including this detailed information on the inconsequential proportion of excluded extreme values of birth weight in our article would have allayed their concerns earlier.
With regard to the exclusion of cesarean sections, we explained in our article that cesarean section births may be scheduled or might occur on an emergency basis for specific medical reasons (e.g., preeclampsia, diabetes, problems with baby). Scheduled cesarean sections are unlikely to be related to exposures that occur prior to birth, such as air pollution, while emergency cesarean sections very well may be. Unfortunately, it is difficult to distinguish these two types of cesarean sections in our data. However, given that cesarean sections comprise approximately 9 percent of preterm births prior to their exclusion, as part of our research we did estimate our models with the inclusion of singleton cesarean section deliveries. Our traffic-related air pollution (distance-weighted traffic density) results remained robust (table 1). In line with the important conceptual considerations enumerated here and in the article, we maintained the cesarean section exclusions for publication.
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We thank the Journal for the opportunity to respond to these concerns.
ACKNOWLEDGMENTS
Conflict of interest: none declared.
References
- Shukunami K, Nishijima K, Kotsuji F. Re: "Preterm birth: the interaction of traffic-related air pollution with economic hardship in Los Angeles neighborhoods." (Letter). Am J Epidemiol (DOI: 10.1093/aje/kwi042).
- Ponce NA, Hoggatt KJ, Wilhelm M, et al. Preterm birth: the interaction of traffic-related air pollution with economic hardship in Los Angeles neighborhoods. Am J Epidemiol 2005;162:140–8.
[Abstract/Free Full Text] - Vohr BR, Wright LL, Poole WK, et al. Neurodevelopmental outcomes of extremely low birth weight infants <32 weeks' gestation between 1993 and 1998. Pediatrics 2005;116:635–43.
[Abstract/Free Full Text]
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