The description of practically all actual and widely used methods of interpretation of results of the analysis of the dissolved gases in oil (DGA) is resulted, and also the new method of identification of a type of defect, algorithm of its application is described, advantages before other methods are specified. The methods of testing the validity of DGA interpretation methods and the results of testing these methods are presented. Comparison of the quality of the determination of various types of defects by the methods considered is given. During testing special attention was paid to solving the problem, which is that different methods of interpretation of the results of the DGA identified a different number of defects type. Quality assessment techniques and test of reliability was conducted on a test set, which consists of 134 cases of damage to transformers. For each case the authors have a detailed description of the defect development process (the results of analyses and testing of the transformer for the prior period, recording of operational documentation), as well as opening during which the cause of the transformer damage was established.