Fertility and its Measurements

Fertility and its Measurements

Fertility refers to births occurring to the women who are at child bearing age. A woman at child bearing age is defined as the age of the female who can give birth to a child. In other words, it is the reproductive age of woman.

Fertility rates are the quantitative characteristics, which are used to measure the rate of growth of the population due to births during a specified period, usually a year. The fertility rates are expressed per thousand women who are at child bearing age.

As like mortality rates, there are several fertility rates. Among them, the following are the basic fertility rates discussed here

(i) Crude Birth Rate

(ii) Specific Fertility Rate

(iii) General Fertility Rate

(i) Crude Birth Rate

Crude Birth Rate (CBR ) of a region or a community relates the number of live births to size of the population of the region or the community. This quantity can be computed using the formula

where

B_t denotes the number of live births occurred in a given region/community during the period t, and

P_t denotes the population size of the given region/community during the period t.

Example 8.10

The number of children born in a city during a period was 15,628 and the total population of the city in that period was 80,00,000. Find the crude birth rate of the city.

Solution:

The Crude Birth Rate can be calculated using the formula

CBR = [ B_t / P_t ] ×1000

The CBR of the city is

CBR = [15628/8000000] ×1000

= 1.95 per thousand.

Example 8.11

People living in a town are grouped according to their age into nine groups. Details about the number of live birts are also grouped according to the age group of women. These information are presented in the following table:

Calculate crude birth rate of the town.

Solution:

The total number of persons in the town during the specified period can be calculated from the given information as

P_t = 1,89,000

and the total number of live births as

B_t = 4,130.

Hence, the Crude Birth Rate of the town is

CBR = [4130/189000] ×1000

= 21.85 per thousand.

CBR is the simplest fertility rate, which provides a comprehensive idea about the population growth of any region/community. It is easy to compute for any given region.

However, CBR does not take into account of the age and gender distribution of the population or specifically the number of women at the child bearing age. This is a crude measure, since it includes the sections of the population who are not in the reproductive age group. It means that P_t includes the sections of the population who are not exposed to the risk of producing children, in particular, male and also the female beyond the reproductive age. CBR also assumes that all the women at child bearing age have the same reproductive capacity irrespective of their age, which is totally unrealistic.

(ii) General Fertility Rate

General Fertility Rate (GFR) of a region or a community relates the number of live births to the number of women in the reproductive age.

i.e., GFR = Number of live births / Number of women at child bearing age ×1000

This quantity can be computed using the formula

Where

P_tⁱ denotes the number of women in the reproductive age i years in the given region/ community during the period t, i = a₁ to a₂.

In India, generally, a₁ = 15 years and a₂ = 49 years.

GFR overcomes the disadvantage of CBR considering only the women population at the child bearing age group, since the denominator in the above formula represents the entire women population at the reproductive age group. GFR expresses the increase in the women population at the child bearing age through live births.

However, GFR does not express the age composition of women population at the reproductive age group. Hence, two different regions/communities cannot be compared with respect to age of women using GFR.

Example 8.12

Women, at child bearing age, of a district are grouped into seven age groups. The number of women lived during a calendar year in the district and the number of live births recorded during the same period are as follows:

Calculate the general fertility rate of the district.

Solution:

The total number of women, at child bearing age in the district during the study period can be calculated from the given information as

and the total number of live births as

B_t = 7,630.

Hence, the general fertility rate of the district is

GFR = [7630/163000] ×1000

= 46.81 per thousand.

(iii) Specific Fertility Rate (SFR)

It is a well known fact that fertility is affected by several factors such as age, marriage, migration, region etc. But, both CBR and GFR do not take into account of this fact. In this respect, Specific Fertility Rate (SFR) is defined as

SFR = [ Number of live births to the women population in the reproductive age groups of specific section in a given period  / Total number of women in the reproductive age groups of the specific section in the given period ] × 1000

SFR can be calculated separately for various age groups of females who are at child bearing age such as 15-20, 20-25, and so on. The SFR computed with respect to different reproductive age of women is known as the Age Specific Fertility Rate (ASFR), which can be calculated using the formula

where

B_t ( x , x + n) denotes the number of live births to the women in the reproductive age group (x,x+n) during the period t in the given region, and

P_t ( x , x + n) denotes the number of women in the reproductive age group (x,x+n) during the period t in the given region.

Grouping of women with respect to their age is essential, since capacity of women to give birth to child varies over the age of women. Thus, ASFR enables to compare the fertility rates of two or more different regions with respect to specific age groups. Moreover, ASFR can be considered as a probability value.

Example 8.13

The female population, at reproductive age, of a country is grouped into six age groups. The number of women in each group and the number of live births given by them are given the following table.

Calculate the general fertility rate and the age specific fertility rates of the country.

Solution:

Total number of women in the country at reproductive age during the period of ‘t’ is

and the total number of live births of the country during the same period is

B_t = 52,852

Therefore, the general fertility rate of the country is

GFR= [52852/564070] ×1000

= 93.69 per thousand.

The age specific fertility rates of the country can be calculated for each age group using the formula

where

B_t ( x , x + n) denotes the number of live births to the women in the reproductive age group (x,x+n) during the period t in the given region, and

P_t ( x , x + n) denotes the number of women in the reproductive age group (x,x+n) during the period t in the given region.

The ASFRs are calculated from the given information and are presented in the following table:

It is can be observed, with respect to ASFRs, that the women in the country falling in the age group of 20-25 years have given relatively more live births. The women at the age of 40-45 years have reproduced less number of live births.

Example 8.14

The following is the data regarding the size of female population in a country at reproductive age and the live births during a period.

Calculate the general fertility rate and the age specific fertility rates of the country.

Solution:

Size of the women population of the country at reproductive age during the period ‘t’, is

and the total number of live births occurred during the same period in the country is

Bt = 1,11,652.

Hence, the general fertility rate of the country during the period is

GFR = [111652/1163040] × 1000 = 96.00

The age specific fertility rate of the country during the same period is calculated for each reproductive age group and the rates are presented in the following table:

It can be observed, with respect to ASFR, that the women population of the country falling in the age group of 20-25 years have given relatively more live births. The women at the age of 40-45 years have reproduced less number of live births.