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Multilingual Demographic Dictionary, second unified edition, English volume
63
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632
The general term birth rate ^{1} refers to a rate calculated by relating the number of live births observed in a population or sub-population during a given period to the size of the population or sub-population during the period. The rate is usually stated per 1,000, and the most usual period is one year. Where the term birth rate is used without qualification, it is understood to be the crude birth rate ^{2}, and all live births are related to the entire population. The total birth rate ^{3} based on live births and late foetal deaths is sometimes calculated. Legitimate birth rates ^{4} and illegitimate birth rates ^{5} are also calculated with legitimate and illegitimate births in the numerator and the currently married and unmarried female population, respectively, in the denominator. The illegitimacy ratio ^{6}, the number of illegitimate births per 1,000 total births, is more frequently used, however. To compare the fertility of different populations, standardized birth rates ^{7} are often used to eliminate the effect on the birth rate of certain differences in structure of the population (most commonly the age and sex structure). The child-woman ratio ^{8}, most commonly the number of children aged 0 to 4 per 1,000 women of childbearing age, e.g., 15 to 49, is used as an index of fertility when reliable birth statistics are not available.
- 4. The denominator of the legitimate and illegitimate birth rate is sometimes the total population.
- 5. See note 4.
633
The term fertility rate ^{1} is often used when the denominator of the birth rate fraction is restricted to a group of individuals of the same sex in the reproductive ages (620-1). This denominator is commonly the mid-year population in the stated period, but it may also be the number of years lived by the group during the period, or the mean size of the group. Unless otherwise indicated, these rates are female fertility rates ^{2}, and the rates are calculated for groups of women; the number of years lived by a given number of women in an interval is called the number of woman years ^{3}. Male fertility rates ^{4} are computed sometimes in an analogous manner. Fertility rates are generally expressed as births per thousand (implied: individuals of the same category — sex, age, marital status, etc. — cf. 133-4*). Marital fertility rates ^{5} or legitimate fertility rates ^{5} relate the total number of legitimate births (610-3) to the number of currently married women; non-marital fertility rates ^{6} or illegitimate fertility rates ^{6} relate the total number of illegitimate births (610-4) to the number of single, widowed and divorced women. Overall fertility rates ^{7} make no distinction according to the legitimacy (610-1) of the births or the marital status of the parents. The general fertility rate ^{8} relates the total number of births to all women of reproductive age regardless of marital status. Rates based on a narrower age range (usually one-year or five-year age groups) are called age-specific fertility rates ^{9} or age-specific birth rates ^{9}.
- 1. In many expressions used in this and following paragraphs, birth rate is used synonymously with fertility rate.
- 5. Marital fertility or legitimate fertility: the fertility of married persons (see 635-1).
- 6. Non-marital fertility or illegitimate fertility: the fertility of unmarried persons.
634
Order-specific fertility rates ^{1} relate births of a certain order to a number of women, to a number of marriages or to a number of births of the preceding order. Parity-specific fertility rates ^{2} or parity-specific birth rates ^{2} not only restrict the numerator to births of a given order, but also restrict the denominator to the women of the parity (611-6) at risk (134-2), e.g., second order births to one-parity women. Such rates are usually age-specific or duration-specific. In parity-specific birth probabilities ^{3}, the numerator consists of the number of births of order x + 1 occurring during a period, and the denominator consists of the number of women of parity x at the beginning of the same period.
635
When studying marital fertility ^{1} it is possible to arrange the data by marriage cohorts (116-2) of the mothers, and marriage duration-specific fertility rates ^{2} are often computed in preference to age-specific marital fertility rates ^{3}.
636
The term cohort fertility ^{1} refers to the reproductive performance of particular birth or marriage cohorts (116-2). When the age-specific or marriage duration-specific fertility rates are summed from the cohort’s beginning of exposure to risk until some later date, we speak of cumulative fertility ^{2}: age-specific cumulative fertility ^{3★} or marriage duration-specific cumulative fertility ^{3★} when then ending date is that of a womans birthday or marriage (501-4), completed fertility ^{4} or lifetime fertility ^{4} is the cumulative fertility until the date when all members of the cohort have reached the end of the reproductive period. The sum of the products of the fertility rates of the cohort by the probability of survival of the women to successive ages could be called the cumulative net fertility ^{5}, age-specific net cumulative fertility ^{6★} or marriage duration-specific net cumulative fertility ^{6★} and net complete fertility ^{7★} or net lifetime fertility ^{7★} of the cohort.
- 4. Before the end of the reproductive period, the terms incomplete fertility or fertility to date are employed to show that the cohort’s cumulative fertility may be expected to increase.
637
Censuses and surveys may provide information on fertility when they include questions on the number of children born to enumerated women or couples, either during the current marriage ^{1}, or overall. The mean number of children ever born per woman ^{2} or average parity ^{2} can be computed. The number of children per couple is sometimes called average family size ^{3}. It is also possible to calculate the mean number of births per marriage ^{4}. Special attention is paid to marriages of completed fertility ^{5}, those in which the wife had reached the end of the reproductive years before the marriage was dissolved. The final parity ^{6} or completed parity ^{6}, i.e., the mean number of children per woman past the childbearing age, is not very different from completed fertility (636-4). The tabulation of final parity or completed fertility by number of children serves to compute series of parity progression ratios ^{7}; these are fractions whose denominator is the number of women with n children, and whose numerator is the number of women with n + 1 children. Special studies yield information on family formation ^{8} and the family life cycle ^{8}. Among those, frequency of premarital conceptions ^{9★}, birth intervals (612-1) and the age at the birth of the last child ^{10} for women of completed fertility are of particular interest.
638
Fertility histories ^{1} or reproductive histories ^{1} are accounts obtained for individual women of the important events in their reproductive lives, such as marriages, pregnancies, births, infant deaths, etc., and their dates. Fertility histories are often obtained retrospectively from surveys. Family forms ^{1} are used in historical demography (102-1), where they are established for a married couple and its children by family reconstitution ^{2} on the basis of vital records (211-3). A woman’s pregnancy history ^{3} or pregnancy record ^{3} contains detailed information about her pregnancies including the date when each began and ended, and the outcome of the pregnancy. Such detailed records on the timing of fertility have been used for various purposes. For example, they can provide information on natural fertility ^{4}, i.e., fertility in the absence of family limitation (624-4). They are also used to estimate fecundability ^{5}, the probability of conceiving per menstrual cycle (622-2). A distinction is made between natural fecundability ^{6}, in the absence of contraception, and residual fecundability ^{7} in the opposite instance. The term effective fecundability ^{8} designates a fecundability that is reckoned in terms of conceptions that result in live births only. The conception rate ^{9} during the period of exposure to risk often estimated using the Pearl index ^{10★} (613-1) is used to measure the effectiveness of contraception during periods of contraceptive use.
- 1. Birth histories are usually limited to live births.
- 6. When used alone, the word fecundability stands for natural fecundability.
639
A summary index of period fertility ^{1}, i.e., the fertility of a particular year or period, computed by the summation of the series of age-specific fertility rates constituting the fertility schedule ^{2} and representing a synthetic measure of fertility ^{3}, is the total fertility rate ^{4} or total fertility ^{4}. Other summary period indices can be obtained, such as the total marital fertility rate ^{5}, the summation of marriage duration-specific fertility rates, and the order-specific total fertility rate ^{6}, the summation of age-specific fertility rates order by order. The ratio of births to marriages ^{7} is computed by relating the number of births of a given year, either to the marriages of the year, or to a weighted average of the marriages of the current and of the preceding years.
- 2. Also, fertility distribution or fertility function.
- 4. This is not a rate in the meaning of (133-4). Total fertility for a given year represents the number of children that would be bom per 1,000 women if they experienced no mortality and were subject to the age-specific fertility rates observed for that year. The period gross reproduction rate (see 711-4) which is derived by multiplying the total fertility rate by the proportion of female births, has often been used in the past, but the total fertility rate is preferred at present as the summary index of period fertility.
- 5. Or total marital fertility. The term is also used to describe the sum of the age-specific marital fertility rates above age 20.
640
Where induced abortion (604-2) has been legalized, it is possible to compile statistics on legal abortions (604-4). The abortion rate ^{1} is a measure of the frequency of abortion in a population during a given period, usually a year. Abortions may be related to the total population or to the number of women in the reproductive ages and may be specific for age, parity or any other characteristic. The abortion ratio ^{2} is a measure of the frequency of abortions in relation to the number of live births (601-4) during the same period. The life-time abortion rate ^{3} is the sum of age-specific abortion rates ^{4★} and is a synthetic measure of abortion per woman or per 1000 women. These rates are the ratio of the number of abortions reported at each age to years lived by all women of the same age. If women can be classified according to their marital status, abortion rates by age and marital status ^{5★} can be obtained. It is often also relevant to divide the number of abortions by the number of conceptions and so to calculate the probability that a pregnancy results in an abortion by age and marital status ^{6★}.
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