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. These are the source of the counts used to calculate fertility, mortality, marriage, and divorce rates. But in countries in which mothers give birth at home, or where many residents are illiterate, a large proportion of vital events are never recorded. A few countries have comprehensive registration systems, or national population registers, that track individuals from birth to death and record changes in their residence or marital status.
Surveys often provide estimates of demographic events where registration systems are inadequate.
They also aid in developing estimates of population size during the long interval between censuses. Surveys usually collect data for a sample group within a specific geographic area. In the United States, a monthly national survey is used to track the unemployment rate as well as many demographic indicators. But surveys suffer from many of the same accuracy problems as censuses and registration systems, and their data are subject to varying degrees of error. Demographers have developed statistical techniques to overcome some of the shortcomings of the basic data with which they work.