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Last Edited: 9/30/2010

Population Fertility: Measurement and Limitations

How is fertility measured?

To assess fertility of large groups of people, researchers frequently consider rates of childbearing.  It is acknowledged that these rates are determined by social custom, economic circumstances, and personal relationships in addition to physiology.  Therefore childbearing rates are considered to be only a rough proxy of the reproductive capacity of populations. 


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How are rates of fertility calculated?

When calculating a measure of fertility, researchers incorporate the fact that fertility varies over the course of women’s lifetimes.  They therefore construct a measure based on the age-specific birth rates observed in a given year.  The total fertility rate (TFR) is therefore the average number of births to a hypothetical cohort of 1,000 women if they experienced these age-specific birth rates during a given year.  

While this measure appears abstract, it allows one to compare populations with different proportions of women of different ages.  As a benchmark, one can remember that a population is considered to be maintaining a constant size if its TFR is equal to 2,000, while it is considered to be expanding or contracting if the TRF is greater or less than this number, respectively.

Due to the statistical properties of these numbers, TFR values based on small populations are subject to uncertainty. The degree of uncertainty for any given rate is represented by its confidence intervals. Researchers utilizing these data may wish to know that we used the method of Tiwari, et al1 for the calculation of confidence intervals applicable to standardized rates.


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Where do these data come from?

In the US, states are responsible for issuing birth certificates and recording and maintaining the data included in them.  Birth certificates are considered one type of vital record (others include deaths, fetal deaths, and marriage).  In California, the Office of Health Information and Research (OHIR) is responsible for stewardship and distribution of vital statistics data and provides written reports and data tables analyzing these data. Since several of the important functions of the CEHTP include the analysis and processing of these records, we maintain our own databases consisting of records produced by OHIR and then subject to further processing, most notably regarding address and other geographic information fields.

Birth certificates provide the numerator portion of the calculation for TFR.  For the denominator, we employ population counts provided by the US Census.


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The fertility measure is influenced by social and demographic choices for reproduction, cultural practices, and the use of contraception and infertility treatments. These factors all may lead to variations in overall fertility across populations and geographic locations and that make assessments of the environmental effects on fertility difficult.

Calculation of this measure assumes one has an accurate count the numbers of women of different ages in a population.  This is most true every decade when the US Census is conducted; in other years the number of women in the population is known with less precision and can be affected by geographic shifts in population and housing.


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1.    Tiwari R, Clegg L, Zou Z. Efficient Interval Estimation for Age-Adjusted Cancer Rates. Statistical Methods in Medical Research. 2006;15:547-569.