Methods for Population Planning was a course I took in graduate school largely because there was no final exam. The class, taken for somewhat lazy reasons, ended up being a formative one in my understanding of how population data are collected and manipulated. By learning to use modeling software, I realized how changing one variable a tiny bit could alter a country’s entire population projection drastically. Our syllabus was full of readings on this very subject—the validity and reliability of the data that is first collected (to the best of field workers’ ability) and then analyzed and projected by demographers in statistics labs.
The data upon which projections are based are collected either through censuses, Demographic and Health Surveys, or similar tools for gathering population information. These data are then “smoothed” to account for common errors in reporting, and survey responses are extrapolated from the sample taken to the wider population. Finally, assumptions (basically, educated guesses) are made about future fertility and mortality, based on past trends, and computer models are set to generate the projections that are shared with policymakers the world over.
A number of different agencies calculate population projections based on the assumptions they consider most likely to pan out over time. The UN probabilistic projections for 2050 look a bit scattershot because there are many overlapping lines depicting different scenarios. The critical takeaway, though, is that there is a 95-percent chance that the population in 2050 will land somewhere between 9.0 and 10.1 billion people.
These “probabilistic projections” are a bit different from the better-known UN low, medium, and high projections, which are more arbitrary (the low and high projections are obtained by adding or subtracting half a child from the total fertility rate on which the medium projection is based for each country). Hania Zlotnik, the former head of the UN Population Division and the current Secretary of the Population Connection Board of Directors, explains these projections in detail beginning on page 22. She groups countries based on their levels of fertility and describes the various projections for each, highlighting the necessity of increasing funding for voluntary family planning if population stabilization is to be achieved by the end of the century.
My hope is that this issue of Population Connection magazine illuminates just how fickle population projections are, and how much bearing our actions today will have on the future population of the world. As Dr. Zlotnik states in her article, “The longer a rapid decline in fertility is delayed, the higher the eventual population size that the high-fertility countries will reach.” Put another way, the more children people have today, the more adults there will be in the future who will have their own children, and so on. Populations grow exponentially, and the larger the base, the longer it will take to slow the momentum created by rapid population growth today.
World population has grown by more than 810 million—two-and-a-half times the current size of the United States—since I first learned how to create population projections in 2005. The vast majority—95 percent—of that growth occurred in less developed countries, where fertility rates and levels of unmet need for family planning are the greatest. Our mission is clear: We must make a real investment in family planning, and soon.