One aspect of Pew Research Center’s work is population projections, which represent our best assessment about the growth and composition of the future population, either in the U.S. or elsewhere. We’ve looked at how the racial and ethnic makeup of the U.S population could change over the next several decades, at coming changes in the U.S. age structure, and at what might be the future religious affiliation of the worldwide population.
The standard demographic approach to population projections is called the cohort-component method. It accounts for the age and sex structure of a population (the “cohorts”) and then carries this initial population into the future by applying the three mechanisms (“components”) through which populations change over time:
- New entrants via births (using projections of fertility rates)
- Exits via deaths (using projections of mortality rates)
- And net changes from migration. This includes adding people moving into a country (immigrants, both legal and unauthorized) while subtracting people moving out (emigrants). At the sub-national level, projections also account for people moving between states or regions.
Each of these components requires us to make assumptions about how rates will change in the future. Births and deaths are the largest components, but measurement of immigration is far more complicated because there are multiple channels of entry to and exit from the population and because immigration is directly affected by government policies and other events. For some components, such as legal immigration, the available data are better, and accurate measurement is easier than for others, such as unauthorized or illegal immigration. The cohort-component method has been in existence for more than a century. First suggested by the English economist Edwin Cannan in 1895, then further improved by demographers in the 1930s and ’40s, it has been widely adopted since World War II. It is used by the United Nations Population Division, the U.S. Census Bureau, other national statistical offices and numerous academic and research institutions.