A new preprint on arxiv.org, “Google Scholar, Web Of Science, and Scopus: A Systematic Comparison of Citations in 252 Subject Categories”.

Google Scholar’s… “citation data is essentially a superset of WoS and Scopus, with substantial extra coverage.” This is partly because “Google Scholar is able to pick up citations… “from non-journal sources … including theses, books, conference papers, and unpublished materials … Many were non-English (19%-38%), and they tended to be much less cited than citing sources that were also in Scopus or WoS.”

However, there are also warnings in the conclusions section, especially that in Google Scholar… “some of the citations [come] from Master’s theses”. Also note that Google Scholar’s citation counts were found to be “lower in the Humanities”.