  Despite citation counts from Google Scholar (GS), Web of Science (WoS), and
Scopus being widely consulted by researchers and sometimes used in research
evaluations, there is no recent or systematic evidence about the differences
between them. In response, this paper investigates 2,448,055 citations to 2,299
English-language highly-cited documents from 252 GS subject categories
published in 2006, comparing GS, the WoS Core Collection, and Scopus. GS
consistently found the largest percentage of citations across all areas
(93%-96%), far ahead of Scopus (35%-77%) and WoS (27%-73%). GS found nearly all
the WoS (95%) and Scopus (92%) citations. Most citations found only by GS were
from non-journal sources (48%-65%), 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and unpublished materials.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. Despite
the many unique GS citing sources, Spearman correlations between citation
counts in GS and WoS or Scopus are high (0.78-0.99). They are lower in the
Humanities, and lower between GS and WoS than between GS and Scopus. The
results suggest that in all areas GS citation data is essentially a superset of
WoS and Scopus, with substantial extra coverage.
