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back to Introduction | preface | Index
chapters | General concepts index 1 | The treatment and processing of population statistics index 2 | Distribution and classification of the population index 3 | Mortality and morbidity index 4 | Nuptiality index 5 | Fertility index 6 | Population growth and replacement index 7 | Migration index 8 | Economic and social aspects of demography index 9
section | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 20 | 21 | 22 | 23 | 30 | 31 | 32 | 33 | 34 | 35 | 40 | 41 | 42 | 43 | 50 | 51 | 52 | 60 | 61 | 62 | 63 | 70 | 71 | 72 | 80 | 81 | 90 | 91 | 92 | 93



There are a number of measures which are used to calculate the frequency of births. The simplest is the birth rate1, more accurately the crude birth rate1, which is the fraction, the numerator of which is the total number of births in a population during a given period and the denominator is the total number of person-years lived by the population during that period. The latter expression is generally approximated by the size of the population at the mid-point of the period multiplied by the length of the period in years. The rate is usually stated per 1,000. Like other rates in which the population at the mid-point of the period is used as the denominator of the fraction this is sometimes called the central birth rate1. Where the term "birth rate" is used without qualification, the live birth rate2 is generally meant and only live births (601-4) appear in the denominator. The total birth rate3 based on live births and late foetal deaths is sometimes calculated. Legitimate birth rates4 and illegitimate birth rates5 with legitimate and illegitimate births respectively in the denominator are computed and the illegitimacy ratio6, the number of illegitimate births per 1,000 total births, is frequently used. These rates and ratios are generally based on live births. To compare the fertility of different populations, standardized birth rates7 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 ratio8, most commonly the number of children aged 0 to 4 per 1,000 women of childbearing age, i.e., 15 or 20 to 44 or 49, is used as an index of fertility when reliable birth statistics are not available.

  • 3. The stillbirth ratio or late foetal death ratio is obtained if the number of stillbirths or late foetal deaths is divided by the number of live births {of. 411-4 for late foetal death rate or stillbirth rate.)


The term fertility rate1 is often used when the denominator of the birth rate fraction is restricted to females (or males) of reproductive age. Female fertility rates2, which relate births to women of childbearing age, are more common but male fertility rates3 are also computed sometimes. Legitimate fertility rates4 relate the total number of legitimate births (610-3) to the number of currently married women (or men); illegitimate fertility rates5 relate the total number of illegitimate (610-4) births to the total number of single, widowed and divorced women (or men). Fertility rates without distinction of marital status (515-1) (called in French taux de fécondité générale) make use of legitimate and illegitimate births combined. General fertility rates7 usually relate all births to all women of reproductive age regardless of marital status and in such cases are the same as female fertility rates (631-2). Rates based on a narrower age range (usually one year or five years) are called age-specific fertility rates8 or age-specific birth rates8. Here again, births arc usually taken to mean live births.


The term cohort fertility1 refers to the reproductive performance of particular birth or marriage cohorts. (Cf. 116-2 for "cohort"). When birth cohorts are considered, the term generation fertility1 is also used. The cohorts used are more commonly female cohorts. When total reproductive performance from the beginning of the exposure to the risk until some later date is being considered, we speak of cumulative fertility2. Completed fertility3 or lifetime fertility3 is the cumulative fertility of a cohort at the time when all its living members (in the case of a birth cohort) or its youngest living members (in the case of a marriage cohort) reach the end of the reproductive period. Before this time the term incomplete fertility4 is employed to show that the cohort’s cumulative fertility may be expected to increase.

  • 3. In computing cumulative birth rates and completed birth rates for birth or marriage cohorts of women, the denominator is usually the number of members of the cohort who are living at a specified age or date and the numerator is restricted to the births i,o these women before that age or dale, i. e., the members of the cohort who died previously and the birLhs to these women are excluded.


The fertility of marriage1 or marital fertility1 may be studied by means of duration-specific rates2 which are the ratios of births within a particular segment of married life to the total number of person-years lived by the women (or men) within that segment. The index of current marriage fertility3 and the average number of children per marriage4 may be computed by summing the duration-specific rates for a given period. The results thus obtained refer to the current fertility of a hypothetical marriage cohort and not to the completed fertility (632-3) of an actual marriage cohort.


A fertility table1 (cf. 153-1) or fertility schedule1 commonly shows fertility rates classified by age or by duration of marriage (or by both) and often shows these rates by order of birth (611-1). Such tables are sometimes said to be tables of fertility functions2. Some fertility tables show the actual birth experience of marriage cohorts (116-2) as they proceed through life, and contain the rates mentioned in para. 632. Other fertility tables show the experience of different age groups (or cohorts) in, a given year or a short period of years. With the latter tables the sum of all age-specific fertility rates is called the total fertility rate3 and represents the number of children, that would be born per 1,000 females (or males) if they experienced no mortality and were subject to the age-specific fertility rates of a specified fertility table. The female gross reproduction rate (711-4) which is obtained by multiplying the total fertility rate by the proportion of births which are female, is used frequently. The proportion of female births is the complement of the masculinity proportion at birth4. The sex ratio at birth5, which is sometimes called the secondary sex ratio5 to distinguish it from the sex ratio at conception6 or primary sex ratio7 is generally expressed as the ratio of the number of live boys born to the number of live girls born. It would be desirable to know the masculinity proportion at conception6 but as yet few data are available regarding sex in the case of foetal deaths (410-6*) during the first week or month after conception (602-1).


The distribution of women by the number of births they have had is called a parity distribution1 or sometimes a prolificacy distribution1 or family size distribution1. The last term is ambiguous because relatives other than children are included in some studies of family size2 while other studies do not consider all children but only surviving children3 or dependent children4. In the study of fertility of marriage, attention is often devoted to marriages of completed fertility5 i.e., to marriages in which the wife has reached the end of her reproductive period.


The term birth order rate1 is commonly used when the numerator of the birth rate (630-1) is restricted to births of a given order (cf. 611-1), e.g. general fertility rates (631-7) are often sub-divided by birth order. Parity-specific birthrates2, parity-specific fertility rates2 or select issue rates2 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 (631-8) or duration-specific (633-2). Occasionally parity-specific birth rates are also specific for both age and duration. Parity-specific birth probabilities3 are the same as parity-specific birth rates except that the denominator consists of the number of women at the beginning of the period who have had a birth of the next lower order to the one considered in the numerator. A related index of fertility is the parity-progression ratio4 which is the fraction whose denominator is the number of women of parity in a population whose fertility is complete and whose numerator is the number of women of parity n + 1 in the same population. These ratios reflect all prior childbearing and may also be computed for women of a specified age or cohort (116-2). The foregoing ratios could be, but in practice generally are not, computed for men.


Couples who do not practise contraception during the period studied are called non-contracepting couples1. The fertility of non-contracepting couples2 is sometimes studied and the probability of conception (602-1) in a menstrual cycle (622-5) has been called their fecundability3. A woman’s pregnancy history4 or pregnancy record4 contains detailed information about her pregnancies, including the dates when each began and ended. If the record also provides data of other important periods of non-exposure to the risk of conception (e.g. prolonged absence on the part of the husband), a conception rate5 or pregnancy rate5 may be computed to show the number of conceptions per unit of time among women during periods of exposure to the risk of conception6. Pregnancy rates are often age-specific (631-8) or duration-specific (633-2) and may be computed separately for contraceptors and non-contraceptors or for periods with and without the use of contraceptives. They are generally stated per 100 months of exposure. The months of exposure per conception7 obtained by reversing the previous procedure gives another index. It is obtained by dividing the months of exposure by the number of conceptions. A comparison between the pregnancy rates of non-contraceptors and contraceptors of equal fecundity will yield a measure of the efficacy of contraception8, efficiency of contraception8 or contraceptive effectiveness8.

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back to Introduction | preface | Index
chapters | General concepts index 1 | The treatment and processing of population statistics index 2 | Distribution and classification of the population index 3 | Mortality and morbidity index 4 | Nuptiality index 5 | Fertility index 6 | Population growth and replacement index 7 | Migration index 8 | Economic and social aspects of demography index 9
section | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 20 | 21 | 22 | 23 | 30 | 31 | 32 | 33 | 34 | 35 | 40 | 41 | 42 | 43 | 50 | 51 | 52 | 60 | 61 | 62 | 63 | 70 | 71 | 72 | 80 | 81 | 90 | 91 | 92 | 93