ClusterSampling
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:StratifiedSampling -- MultistageSampling -- SimpleRandomSampling
A geographically dispersed population can be very expensive to survey with personal interviews due to extravagant travel costs.
Greater economy than random sampling can be achieved by including several respondents within a local area. It is usually necessary to increase the total sample size to achieve equivalent precision in the estimators, but the savings in cost may make that feasible.