Monday, July 26, 2011
Researchers supported by the National Institutes of Health have developed a new statistical technique to forecast changes in fertility rates. The new method mathematically compensates for uncertainty and is expected to allow governments to plan more reliably for the infrastructure and social services needed to accommodate large-scale population changes.
Conventional methods for predicting a country’s fertility rate are based on two considerations. The first is an average figure for the number of times a woman gives birth during a lifetime. The second consideration is an estimation of how this number changes as a woman grows older. In addition, to account for the possibility of deviations, analysts have added and subtracted .5 children to the average rate predicted, creating a range of predictions. However, previous methods could not calculate how likely it was that such a variation would actually occur.
The new method is based on the idea that the transition from high to low fertility rates follows a similar pattern in all countries. The method uses a statistical formula to take into account historical fertility estimates and the likelihood of future trends. So the new method predicts future fertility rates in the country of interest based on historical fertility rates for that country as well as the experiences of the remaining 195 countries in the world.
In the article, the authors explained that estimates calculated with the new method have a 1 in 10 chance that the target country’s fertility rate will be greater than the rate that is actually observed, and a 1 in 10 chance that it will be less than the rate that is observed.
“More accurate forecasts of fertility trends will allow officials to better plan for a country’s municipal, economic and social needs,” said the program official for the study, Michael Spittel, Ph.D., formerly of the Demographic and Behavioral Sciences branch at the NICHD, the NIH institute that funded the study, and now at the NIH’s Office of Behavioral and Social Science Research.
Lead author Leontine Alkema, at the National University of Singapore, collaborated with Adrian E. Raftery, of the University of Washington, Seattle; Samuel J. Clark, of the University of Washington, South Africa’s University of Witwatersrand and the INDEPTH research network; and Patrick Gerland, Francois Pelletier and Thomas Buettner, of the United Nations.
Their findings appear in the journal Demography.
Dr. Raftery noted that, 200 years ago, fertility rates were high throughout the world. On average, each woman gave birth to five or more children in her lifetime. After the Industrial Revolution, this rate started to fall in Europe. This trend spread to other regions of the world in the 20th and 21st centuries.
The authors argue that the transition from high to low fertility rates generally takes place in three phases. The first phase is marked by stable, high fertility—typically a lifetime birth rate of six or seven children for each woman. The second phase marks the transition from high to low fertility, beginning when a country’s fertility rate falls to five children per woman and ending when it reaches or falls below the replacement level rate of 2.1 children. The replacement level is the number of births per woman at which the country maintains its population, without growing or shrinking. In the third phase, a country’s fertility rate recovers to slightly above 2.1 children.
According to the study authors, in the last five years 20 countries have entered this final phase.
Using their new method, the researchers forecast fertility trends for the world’s countries through the year 2100. They noted it is more difficult to forecast fertility trends in countries with relatively high fertility rates, because there is more uncertainty about the future pace of changes in fertility rates.
The researchers used fertility rate estimates between 1950 and 2010 to perform two tests of their method. They first used estimates from 1950 to 1980 to make predictions and compared their results to the actual estimates recorded for the period from 1980 to 2010. The second test used 1950 to 1995 estimates to predict the rates between 1995 and 2010.
Calculations derived from the new formula revealed an unexpected trend among countries where fertility rates have fallen below the replacement level. According to conventional wisdom, once fertility rates fell below the replacement level they would remain stable, varying within only a narrow range.
However, the researchers found fertility rates are likely to fall significantly below replacement level, as low as 1.5 births, and then begin to increase slowly. The authors noted that the United States is the only country in which the lifetime fertility rate has fallen below replacement level and later recovered. After the U.S. rate fell to a low of 1.75 births in the mid 1970s, it rose and is at 2.0 births today.
The researchers also found that in several cases, projections derived from the new calculations differed significantly from current projections. For example, the researchers concluded current prediction models may overestimate the speed at which fertility rates will fall in many high-fertility countries, including the majority of countries in sub-Saharan Africa.
“The ability to forecast population trends is vital for planning infrastructure,” said Dr. Raftery. “We want to do better than give planners a best guess. With our projections, they can increase the certainty that they will be able to accommodate everyone.”
In addition to fertility, rates of migration and mortality are the major factors that influence population growth. Next, the researchers plan to develop statistical methods that take mortality and migration rates into account to more accurately forecast population trends.
The NICHD sponsors research on development, before and after birth; maternal, child, and family health; reproductive biology and population issues; and medical rehabilitation. For more information, visit the Institute’s Web site at http://www.nichd.nih.gov/.
NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit http://www.nih.gov.