Editing Cluster sampling (section)

{{Short description|Sampling methodology in statistics}}
[[File:Cluster sampling.PNG|thumb|upright=1.3|Cluster sampling. A group of twelve people are divided into pairs, and two pairs are then selected at random.]] 
In [[statistics]], '''cluster sampling''' is a [[sampling (statistics)|sampling]] plan used when mutually homogeneous yet internally heterogeneous groupings are evident in a [[statistical population]].  It is often used in [[marketing research]]. 

In this sampling plan, the total population is divided into these groups (known as clusters) and a [[simple random sample]] of the groups is selected. The elements in each cluster are then sampled. If all elements in each sampled cluster are sampled, then this is referred to as a "one-stage" cluster sampling plan. If a simple random subsample of elements is selected within each of these groups, this is referred to as a "two-stage" cluster sampling plan. A common motivation for cluster sampling is to reduce the total number of interviews and costs given the desired accuracy. For a fixed sample size, the expected [[Observational error|random error]] is smaller when most of the variation in the population is present internally within the groups, and not between the groups.