This approach relies on estimates generated from a previous study or alternatively by the ability to gauge relative differences in variation among strata, such as expecting one stratum to have 1.5 times the variation as another stratum. Optimal Allocation Both allocation approaches above are special cases of the optimal allocation strategy which estimates the population mean or total with the lowest variance for a given sample size in stratified random
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sampling. The number of samples selected from each stratum is proportional to the size, variation, as well as the cost (ci) of sampling in each stratum. More sampling effort is allocated to larger and more variable strata, and less to strata that are more costly to sample