CLUSTER SAMPLING:
In cluster sampling, individual units are not selected as sample but are grouped together and are selected group-wise for inclusion in the sample Thus, groups are selected on random basis as sample For example, the total universe will be divided into number of groups. Each group contains equal number of items. The sample will be selected in groups only. Similarly, if one family is selected as sample, the information will be collected from each member of the family.Such selection of sample in group form is called cluster sampling.
For example, if a survey is to be undertaken in a city to collect data from individual households, then, selection of households from all over the city would involve a considerable amount of fieldwork and consequently, would cost more. Instead, a few localities are first chosen. Then, all the households in these localities are covered in the sample. Apart from reduction in cost, such a cluster sample would be desirable in the absence of a suitable sampling frame for the whole population. If, on the other hand, a sample of individual households from the entire city is to be chosen, it will be necessary to first undertake the listing of all households. In view of the non-availability of a satisfactory sampling frame, in the case of cluster sampling, such a listing could be confined to only a few localities that are to be entirely covered in the sample.
A few points regarding cluster sampling may be noted here. First, “whether or not a particular aggregate of units should be called a cluster” will depend on the circumstances of each case. In foregoing example, localities were taken as clusters and households as individual units. In another case, the households may be taken as a cluster and the members of the households as individual units. Second, it is not necessary that clusters should always be natural aggregates such as locality constituencies, schools or classes. Artificial clusters may be formed, as is generally done in area sampling where grids may be determined on the maps. Third, several levels of clusters may be used in any one sample design. Thus, in a city survey, localities or wards, streets and households may be selected in which case localities or wards are the clusters at the first level and streets at the second level and households would be the units.
Cluster sampling method is less costly as the expenditure on traveling of interviewers is minimized. It is useful when the researcher desires to study the characteristics of certain individuals or items of identical nature.
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