In probability sampling, every observation in the population from which the sample is drawn has a known probability of being selected into the sample; when that probability is the same for every observation in the population, the sample is an equal probability sample or EPSEM sample (equal probability of selection method; see Kish, 1965, for details).

EPSEM samples have certain desirable properties; for example, the simple formulas for computing means, standard deviations, etc., can be applied to estimate the respective parameters in the population. In STATISTICA, you can create EPSEM samples by using the Random Sampling options, which also support Stratified Random Sampling.