According to U.S. Census Bureau projections, the U.S. population could reach 394 million by 2050. But we should remember that these projections are based on educated guesses about future trends in fertility, mortality, and migration. The role of these processes in population change can be depicted in what is termed the balancing equation. The births minus the deaths can also be shown as natural increase. The immigration total includes an estimated net immigration of 225,000 illegal immigrants.
Demographers use a variety of rates, ratios, and other measures to study population.
But these measures are only as accurate as the data from which they are calculated. There were 248,709,873 residents in the United States during 1990, for example, and 1,730,000 births in Iran. Counts come from population censuses, vital registration systems, national registers, and surveys. Their accuracy varies greatly by country and even by regions within countries. The population census forms the cornerstone of demographic analysis.