Segments is a term for sample units in area probability sampling (a specifie kind of cluster sampling). Most often, segments are sample units in the second stage of area probability sampling and are more formally referred to as secondary sampling units (SSUs).

As second stage sample units, segments are neighborhoods or blocks (either census-defined or practically defined by field workers) within the selected primary sampling units, which are often counties or whole metropolitan areas). Occasionally, segments can refer to the first-stage sample units or to larger areas than neighborhoods or blocks, such as entire census tracts or even counties, but this entry describes them in their more common usage as second-stage sample units.

Individual units (often housing units, or sometimes clusters of housing units) within the selected ...