Within-unit coverage error refers to the bias, variance, or both, that may result when the respondents who are selected from within a sampling unit, and from whom data are gathered, differ in non-negligible ways from those who were missed from being selected but in theory should have been selected.

Many probability sample surveys are made up of a target population of individuals that belong to one sampling unit. But often these surveys use a two-stage design in which the unit is sampled first, and then a respondent (or more than one) from within the unit is selected to be interviewed. One example is a household or a unit within an organization, from which one person or a subset of persons per sampling unit is surveyed. These ...