Population forecasts are generally quite accurate. Country-level forecasts, such as those made by the United Nations and the World Bank, miss the mark by about 6 percent on average.
Using the “cohort-component” method. In almost every serious population forecast, demographers first slice a given population into tranches, such as women ages 20 to 24 and men ages 65 to 69. Then they issue separate forecasts for each.There's been remarkably little change in most demographers' techniques over the past century, so accuracy is mostly a function of how good (and how current) the underlying fertility, mortality, and migration data are.
A study from 1983 found that while neither U.N. predictions for developed countries nor the Census Bureau's predictions for the U.S. population had improved, those in developing countries had become more accurate—likely because of more-detailed recordkeeping.
A 2001 study found little change in the U.N.'s overall forecasting accuracy since 1950s; if anything, accuracy peaked in the 1970s and late 1980s.