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  • Title: Timing the second birth: fecundability models for selected race and age groups in Hawaii.
    Author: Swanson DA.
    Journal: Janasamkhya; 1986 Dec; 4(2):81-113. PubMed ID: 12268733.
    Abstract:
    This article tests exponential models for fecundability for use in predicting the timing of the second birth in a non-contracepting population, using data from Hawaii for reference. The paper begins with an extensive review of the literature on mathematical models for birth intervals. The most common problem with prior studies is that the reference data are often biased with uncontrolled parameters such as maternal age, abortion history, lactation practice and pregnancy planning. Pertinent bias in the present data is evaluated. The principal tool used here is an exponential distribution, maximum-likelihood method, involving partial differential equations. The model was constructed by subjecting it to 3 steps: 1) it was tested for exponentiality with a Gail-Gastwirth test that does not depend upon an unknown parameter; 2) the reciprocal of the mean second birth interval was calculated for each group that passed the first test; 3) the estimated frequencies were tested against observed by the chi-squared goodness of fit test. The results are tabulated as separate racial groups, combined races, and age groups. Fecundabilities varied widely by age within all races. For all races, rates were .07045, .03423 and .02777 for ages 20-24, 25-29, and 30-34. Thus the results suggest that fecundability rates are largely determined by age related factors associated with coital frequency, rather than by racial or physiological variations. Other factors potentially influencing fecundability in actual data or models are discussed, such as period effects, the "intendedness" of pregnancy, types of non-live birth intervals, lactation, induced and spontaneous abortion, and differences in pre- and post-demographic transition populations. Finally appendices are added discussing cohort size bias, presenting data set documentation and evaluating the length of pregnancy term and post-partum sterility in the data used here.
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