Unlike stratified sampling, where the available information about all units in the target population allows researchers to partition sampling units into groups (strata) that are relevant to a given study, there are situations in which the population (in particular, the sampling frame) can only identify pre-determined groups or clusters of sampling units. Conducive to such situations, a cluster sample can be denned as a simple random sample in which the primary sampling units consist of clusters. As such, effective clusters are those that are heterogeneous within and homogenous across, which is a situation that reverses when developing effective strata. In area probability sampling, particularly when face-to-face data collection is considered, cluster samples are often used to reduce the amount of geographic dispersion of the sample ...
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