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Cluster sampling:
This method involves dividing the population into groups of items known as clusters. A number of clusters are randomly selected from all the clusters rather than individual items of the population. Cluster sampling can be used together with both unrestricted random and stratified sampling, for example 500 to 540, 2015 to 2055 etc. The first item i.e. 500, 2015 is randomly selected from random number tables. The items of selected cluster can either be checked completely or a randomly selected proportion of them can be examined.
The cluster is less effective for a given sample size than unrestricted random and stratified samples as items are not individually selected. However, the time saved can be utilised to have a larger sample to make the sample results more reliable.
As per SA 530, the auditor shall determine a sample size sufficient to reduce sampling risk to an acceptably low level.