Interesting descriptive comparison of Google Scholar (GS), Microsoft Academic, Scopus, Dimensions, Web of Science, and OpenCitations citation data
https://rdcu.be/dtAQu
Reference sample:
top-10 most cited documents in GS in 2006 across 252 subjects.
In the joint set of citations, none of the non-GS databases covers more than 47% (most substantially less!) of citations in Humanities, Arts, Literature, Social Sciences, Business, Economics, Management.


