The accuracy of prediction of mineral processing operation by partial effects summation is analyzed. Methodological and practical aspects of the problem are discussed using laboratory studies of selected comminution and classifying operations as examples. Laboratory experiments show that the effects of examined mineral processing operation depend on simultaneously running processes of classification and comminution. The influence rate of interaction between both processes on the final results is significant, but in some cases it may be neglected. The obtained results have preliminary character and needed a further verification.