EN
In this paper the optimization of the number of the strata in a situation where the researcher has a previously known stratification of the population is presented. Usage of Neyman optimal allocation is assumed. It is also assumed that the researcher has the information about the number of elements, the mean value and the variance of the characteristic under study in each strata. Under these assumptions the condition of efficiency (in terms of reduction of the variance of the estimator) is defined. This condition is used for construction of the strata-merging algorithm.