From the comments to yesterday's post:
ISI/Web of Science (WoS) is " better than Google Scholar by an order of magnitude."
"..my citations are definitely higher on Scopus than on WoS."
"Web of Science has a few errors in my records, though not nearly as bad as Google Scholar.."
"I prefer Google Scholar.. My prediction is that WOS will decline in popularity over time unless it makes drastic changes."
OK, so let's do the numbers.
I compared citation data in Google Scholar and Web of Science for 25 of my publications. (I did not search in Scopus).
I looked at a range of publications in terms of publication date, my place in the authorship order, and type of publication. For 18 of the 25 publications, Web of Science counted more citations, so I definitely like WoS better. For these 18, Google Scholar's citation count ranged between 0-92% of the citations in WoS; the average was 62%.
For 3 of the 25, Google Scholar counted the same number as WoS, and for 4 others Google Scholar counted more citations, although typically only slightly more than WoS (84-92%). There aren't enough data for me to conclude anything systematic based on these small numbers, but I was intrigued by the fact that 2 of the publications that had a higher citation count in GS than in WoS were in topics outside my primary research field.
The publications for which Google Scholar did a significantly worse job of finding citations than WoS --i.e., finding <40% of the citations listed in WoS -- were typically in my oldest publications and in my most recent publications, although there is one paper published in 2002 in a mainstream journal for which GS found <40% of the citations listed in WoS.
These results are not surprising; it is not news that these sites are not perfect at counting citations.
These databases are very useful for doing literature searches, and should be used primarily for this purpose rather than as key data in decision-making about jobs, promotions, and awards. Nevertheless, I have been on committees in which various members exclusively used one or the other of these sites for looking up the publication records of applicants/nominees, and I have seen citation numbers listed in many CVs and in letters of recommendation (typically without reference to which citation index was used to determine those numbers).
To some extent, this is OK. A very high number of citations is impressive, whether it is 250 or 320. For some of my papers with more modest numbers of citations, though, I might as well just make up a number between 5 and 50 than rely on the count in either Google Scholar or Web of Science.
Even so, for my field (or subfield) of the physical sciences, Web of Science is definitely "better" at counting citations for most publications. For those of you who prefer GS to WoS, perhaps you could leave a comment indicating your field. Are there particular fields for which GS is better at finding citations?