National and international surveys usually collect data from a sample. Dealing with a sample rather than the whole population is preferable for several reasons.

First, for a census, all members of the population need to be identified. This identification process presents no major difficulty for human populations in some countries, where national databases with the name and address of all, or nearly all, citizens may be available. However, in other countries, it is not possible for the researcher to identify all members or sampling units of the target population, mainly because it would be too time-consuming or because of the nature of the target population.