<P> Cluster sampling is a 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 random error is smaller when most of the variation in the population is present internally within the groups, and not between the groups . </P> <P> The population within a cluster should ideally be as heterogeneous as possible, but there should be homogeneity between clusters . Each cluster should be a small - scale representation of the total population . The clusters should be mutually exclusive and collectively exhaustive . A random sampling technique is then used on any relevant clusters to choose which clusters to include in the study . In single - stage cluster sampling, all the elements from each of the selected clusters are sampled . In two - stage cluster sampling, a random sampling technique is applied to the elements from each of the selected clusters . </P> <P> The main difference between cluster sampling and stratified sampling is that in cluster sampling the cluster is treated as the sampling unit so sampling is done on a population of clusters (at least in the first stage). In stratified sampling, the sampling is done on elements within each strata . In stratified sampling, a random sample is drawn from each of the strata, whereas in cluster sampling only the selected clusters are sampled . A common motivation of cluster sampling is to reduce costs by increasing sampling efficiency . This contrasts with stratified sampling where the motivation is to increase precision . </P> <P> There is also multistage cluster sampling, where at least two stages are taken in selecting elements from clusters . </P>

The difference between stratified sampling and cluster sampling
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