The theoretical dependence of the separation efficiency on the physical data of the screen and the duration of the screening operation as well as on the relationship between particle size and slot dimensions is discussed on the basis of a phenomenological approach, the problem being solved with a deterministic and probability model, The statistical exponents of the particle distribution according to size and the statistical precision of the manufacture of screen holes ("slots") was taken into account. Difficulties in particle separation and screening, despite small differences in size, are defined. The application of the results obtained to the selection of the appropriate screen and to the construction of the screening network of the separation machine is discussed,