Household interview surveys: The surveys in which the primary sampling units are typically geographic regions such as nations or cities. For each such unit sampled, there are additional levels of the subsampling including successively smaller geographic regions, for instance, census districts, neighbourhoods within districts and the households inside neighbourhoods. Individuals within sampled households might also be subsampled. The major purpose of the multistage cluster sampling is to lessen the number of regions to which interviewers should travel.