In general, the allocation of numbers of sample units to various strata so as to maximise some desirable quantity such as precision for fixed cost. Secondarily, allocation of numbers of sample units to individual strata is an optimum allocation for a given size of sample if it affords the smallest value of the variance of the mean value of the characteristic under consideration. Optimum allocation in this sense for unbiased estimators requires that the number of observations from every stratum should be proportional to the standard deviation in the stratum as well as to the stratum number.