  We report a new optimal resolution for the statistical stratification problem
under proportional sampling allocation among strata. Consider a finite
population of N units, a random sample of n units selected from this population
and a number L of strata. Thus, we have to define which units belong to each
stratum so as to minimize the variance of a total estimator for one desired
variable of interest in each stratum,and consequently reduce the overall
variance for such quantity. In order to solve this problem, an exact algorithm
based on the concept of minimal path in a graph is proposed and assessed.
Computational results using real data from IBGE (Brazilian Central Statistical
Office) are provided.
