Errors of classification can often be found in manuscripts of scientific treatises and some times even in publications

which are focused on act in violation of the principal and the so called combined or complex classification.The causes of the errors are mainly:1 they did not grasp the rules and the methods in classification;2 they considered by mistake that the more types they classfied

the better results they would get

that is actually against the classification principal which starts form complication to simplification

and from disorder to system.This paper describes through examples that three logic regulations must be followed.It also analyses the error in so called “combined classification”

enumerates the methods for multi-factor classification

demostrates the flexible application of verious methods in classification

emphasizes that classification must reflect the essential attributes and properties of the study objects.