Sir

I would like to draw attention to a substantial drawback in publishing supporting scientific data online, in supplements to the printed research paper, usually because of space limitations. Unfortunately, the additional citations in this supplementary information are invisible to those services that rely on citations as a measure of the 'quality' of journals or of individual scientists, using them to determine impact factor, h-index or Scimago journal ranking, for example.

This becomes obvious when looking under the article heading for any citation that is referenced only in the supplement, using search engines such as PubMed, Scopus, Web of Science or Google Scholar. None will indicate that the particular reference is cited in the paper's supplement. This omission will affect ranking calculations, particularly for journals that post details of experimental methods in their supplements.

Like it or not, ranking of scientific achievement by citation-based methods is an important part of the scientific system, and journals should make all their citations accessible to those who need accurate numbers. The solution to this problem seems quite simple: the citations in the supplement have to be incorporated into the reference section of the main text by the authors.