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—Dr Morfesis highlights an important methodological issue: the problem of self-selection and the bias it may introduce into the results of a retrospective analysis. We share his concern and deliberately drew attention to the problem in the introduction and the Comment sections of our study.The general question that his communication poses is whether the existence of this type of bias invalidates any conclusions derived from retrospective analysis. We do not believe this to be the case. All studies concerning the effect of prenatal care on birth outcome suffer, to greater or lesser degree, from self-selection bias. The only definitive method of eliminating this source of bias is to conduct an intervention in which women are randomly assigned to receive varying amounts of prenatal care, an unethical proposition.As in many fields of scientific inquiry, the existence of potential sources of bias does not, in and of itself,
Racine AD, Joyce TJ, Anderson R. Association Between Newborn Birth Weight and Prenatal Care for Cocaine Users-Reply. JAMA. 1994;271(15):1161. doi:10.1001/jama.1994.03510390031024
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