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  • Title: A note on Brass's model for the distribution of births in human populations.
    Author: Bhattacharya BN, Nath DC.
    Journal: Janasamkhya; 1987 Dec; 5(2):103-9. PubMed ID: 12281337.
    Abstract:
    Brass's model for the distribution of births considered only births to fertile women, making it inapplicable for countries with low fertility or samples of women who differ markedly in terms of fecundability. The authors show that a simple extension of a modified Poisson distribution, considering females to comprise 2 groups with respect to fecundability (high versus low ability to conceive), describes satisfactorily the data provided by Brass. Brass had noted that the major discrepancies between the observed and expected distribution of births in low-fertility countries reflect the practice of contraception and variations in the rate of childbearing during the reproductive period. The analysis indicated that the proportion of subfecund females is 70-90% in developed countries, reflecting high contraceptive usage. In contrast, in India, the data show that only 14% of fertile females belong to the low-risk group and about 4% remain sterile throughout the reproductive period. The revised model was applied to data from a Rural Development and Population Growth survey conducted in Varanasi, India, in 1978. Only women who were married when under 20 years of age and did not practice contraception were included in the test of the model. The observed (expected) number of females in each number of births category was as follows: 0, 41 (41); 1, 25 (24.3); 2, 50 (51.8); 3, 96 (93.5); 4, 182 (183.8); 5, 261 (272.3); 2, 221 (221.1); 7, 86 (79.6); 8, 13 (9.4); and 9 and over, 2 (0.2).
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