Optimal estimation of population variance using equilibrated stratified sampling from infinite populations

A class of unbiased estimators of the population variance is proposed when the coefficient of variation is known for each stratum. The technique of equilibrated stratification is used. It is shown that, with optimum allocation, the variance of the suggested estimator for sufficiently large sample size is less than that of the classical minimum variance unbiased estimator under distribution-free setting. Other classes of estimators are also given with similar or better practical properties.