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. During the 1980s, about 94 percent of the world's population was covered in a national census. The population characteristics that censuses record also are subject to error. Residents may lie about their income or forget the exact ages of some household members.