The tool for building ratings of scientific journals is one of the popular services of bibliographic databases. The task of building a rating is usually divided into two main subtasks: determining the reference group of journals and calculating the rating indicator for journals of this reference group. Practice shows that for the correct comparison of journals, a necessary condition is to limit the reference group to exclusively journals of a certain subject. In the case of methodological errors made at the stage of selecting a reference group, the values of the journal index in the rating may differ greatly from the expected ones.

For example, in the ranking of journals in the Russian Science Citation Index (RSCI) according to the two-year impact factor in the thematic area “Mathematics”, classical fundamental mathematical journals, contrary to expectations, do not reach the first positions of the rating. The first positions were taken by journals for which mathematics is not the dominant profile discipline. Analysis of statistical data on the subject of published articles and citations in journals that occupy leading positions in the RSCI rating shows that the multidisciplinary nature of these journals significantly influenced the rating indicators.

The noted misunderstanding leads to the idea that in this case, not all the articles of the journal should have been involved in the calculation of the rating, but only those related to this thematic area. At the same time, the existing scheme of thematic classification of directions also raises questions. The "bottom-up" classification, which is gaining popularity and works on a representative array of articles, seems to be more promising. Here thematic clusters are isolated on the basis of the concept of proximity of articles, interpreted as the proximity of their bibliographic links. And further, the thematic affiliation of the article is not assigned by the volitional decision of the author or the editorial board, but is strictly formally calculated on the basis of its bibliographic list.