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Presentation by Sue Ranney of Revolution Analytics at JSM 2012, San Diego CA, Aug 1 2012.
The Center for Disease Control and Prevention recently issued a report, widely cited in the popular press, on the increased incidence of multiple births in the United States over the last 30 years. Twin birth rates were extracted from annual birth data by a variety of mother's characteristics in order to examine this trend. Our research extends this analysis by applying multivariate analysis to individual-level data obtained from public-use data sets on all births in the United States from 1985 to 2009. We combine the data into a single, multi-year data file (an .xdf file easily accessed by R) containing over 100-million birth records. To analyze the relationship between parental characteristics and multiple birth pregnancies, we first change the unit of observation from the baby to the pregnancy in order to remove replicated observations of parents of multiples. Then, estimating a logistic regression on all of the remaining observations, we show that the trends in increased multiple births are more strongly associated with the age of father than the age of mother, and that controlling for ages, the relative incidence of multiple births for black mothers has been declining.