Currently I am reading with a great deal of interest the Arxiv preprint of the review by Ludo Waltman (2015) from CWTS on bibliometric indicators. In this post I want to provide a brief comment on his section 5.1 where he discusses the role document types in bibliometric analyses. Ludo mainly reviews and comments on the inclusion or exclusion of certain document types in bibliometric analyses, he does not touch upon the subject of discrepancies between databases. I want to argue that he could take his review a step further in this area.
Web of Science and Scopus, differ quite a bit from each other on how they assign document types. If you don’t realize that this discrepancy exists, you can draw wrong conclusions when bibliometric analyses between these databases are studied.
This blogpost is a quick illustration that this is an issue that should be adressed in a review like this. To illustrate my argument I looked to the document types assigned to Nature publications from 2014 in Web of Science and Scopus. The following tables gives an overview of the results:
|Document Types Assigned to 2014 Nature Publications in Web of Science and Scopus|
|WoS Document type||# Publications||Scopus Document type||# Publications|
|Article in Press||56|
In the first place Scopus yields for the year 2014 a few more publications for Nature than Web of Science does. The difference can be explained by the articles in press that are still present in the Scopus search results. This probably still requires maintenance from Scopus and should be corrected.
More importantly WoS assigns 833 publications as “editorial material” whereas Scopus assigns only 244 publications as “editorial”. It is a well known tactic from journals such as Nature to assign articles as editorial material, since this practice artificially boosts their impact factor. I have had many arguments with professors whose invited news and views items (most often very well cited!) were not included in bibliometric analyses since they were assigned to “editorial material” category and therefore not included in the analysis.
“Letters”, “corrections” or “errata” are in the same order of size between Scopus and Web of Science. “News Items” are a category of publications in Web of Science, but not in Scopus. They are probably listed as “note” in Scopus. Some of the “short surveys” in Scopus turn up in Web of Science as “news item”. But all these categories probably don’t affect bibliometric analyses too much.
The discrepancy in “reviews” between Web of Science and Scopus however is important. And large as well. Web of Science assigns 34 articles as a “review”, whereas Scopus counts 51 “reviews” in the same journal over the same period. Reviews are included in bibliometric analyses, and since the attract relatively more citations than oridinary articles, special baselines are construed for this document type. But comparisons between these databases are foremost affected by differences in document assignation between these databases.
The differences in editorial material, articles and reviews between Web of Science and Scopus are most likely to the affect outcomes of comparisons in bibliometric analyses between these two databases. But I am not sure about the size of this effect. I would love to see some more quatitative studies in the bibliometrics arena to investigate this issue.
Waltman, Ludo (2015). A review of the literature on citation impact indicators. http://arxiv.org/abs/1507.02099