An element, or a cluster of elements, considered for selection in some stage of sampling. For a sample with only one stage of selection, the sampling units are the same as the elements. In multi-stage samples (e.g., enumeration areas, then households within selected enumeration areas, and finally adults within selected households), different sampling units exist, while only the last is an element. The term ‘primary sampling units’ (PSUs) refers to the sampling units chosen in the first stage of selection. The term ‘secondary sampling units’ (SSUs) refers to sampling units within the PSUs that are chosen in the second stage of selection.
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