Reproduction rates: if the majority of births are those of boys the population is bound to decrease while the reverse will be the case if the majority of births are girls. Similarly, if mortality is ignored a correct idea of the rate of growth of population cannot be formed because it is possible a number of female children may die before reaching the child bearing age. For measuring the rate of growth of population we calculate the reproduction rates. Reproduction rates are of two types:
1. Gross reproduction rate, and
2. Net reproduction rate
Gross reproduction rate: gross reproduction rate measures the rate at which a new born female would, on an average, add to the total female population, if they remained alive and experienced the age specific fertility rate till the end of the child bearing period. It is the sum of fertility rate till the end of the child bearing period. It is the sum of the age specific fertility rates calculated from female births for each single year of age. It shows the rate at which mothers would be replaced by daughters and the old generations by the new if no mother died or migrated before reaching the upper limits of the child bearing age, i.e. 49 years. Another underlying assumption is that the same fertility rate continued to be in operation. If the gross reproduction rate of a population is exactly 1, it indicates that the sex under consideration is exactly replacing itself; if it is less than 1, the population would decline, no matter how the death rate may be. The gross reproduction rate is computed by the following formula:
G.R.R. = (number of female births/total number of births × total fertility rate
G.R.R. = no. of female children born to 1000 women/1000
The G.R.R. is used as a measure of the fertility in a population. It is useful for comparing fertility in different areas or in the same area at different time periods. The G.R.R. could in theory range from 0 to about 5.
Gross reproduction rate has an advantage over the total fertility rate because in its computation we take into account only the female babies who are the future mothers whereas in the total fertility rate we include both male and female babies that are born.
An important limitation of the gross reproduction rate is that it ignores the current mortality. All the girls born do not survive till they reach the child bearing age. Hence the gross reproduction rate is misleading in that it inflates the number of potential mothers. This defect is removed by computing the net reproduction rate.
Net reproduction rate: though gross reproduction rate gives an idea about the growth of population, it includes the effect of the mortality on the birth rate. The rate estimates the average number of daughters that would be produced by women throughout their lifetime if they were exposed at each to the fertility and mortality rates on which the calculation is based. If thus indicates the rate at which the number of female births would eventually grow per generation if the same fertility and mortality rates remained in operation. A net reproduction rate of 1 indicates that on the basis of the current fertility and female fertility, the present female generation is exactly maintaining itself. Both fertility and mortality are taken into account while calculating net reproduction rate. In its calculation it is assumed that 1000 mothers give birth to a certain number of girls of whom a percentage dies in infancy and certain percentage do not marry.
The net reproduction rate is obtained by multiplying the female specific fertility rate of each age by the population of female survivors to that age in a life table and adding up the products. An allowance is thus made for mortality.
Symbolically
N.R.R. = Σ(no. of female births × survival rate)/100
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