It's hard to make an easy and still deeply and thorough evaluation of subject coverage in Scopus, Web of Science and Google Scholar due to a lot of reasons. Especially because the databases in question do not use established thesauri. Though I made a small comparison between these multidisciplinary databases and PubMed.

I chose three MeSH terms (two of them with subheadings included) with three words included. I limited my search to 1996, mainly because Scopus subject coverage before 1996 is selective. The MeSH-terms were:

Hormone replacement therapy
Antifreeze proteins toxicity
Neonatal screening ethics

Result from PubMed searching MeSH database:

Result from Scopus searching field keywords:

Result if broading the search to title, abstracts and keywords.

Result from Web of Science when searching Topic in General search which include title, abstracts and Keywords (author keywords and keywords plus):

As for results in Google Scholar they are more hard to evaluate, because Google Scholar indexes significant parts of the fulltext. It's possible to limit to title search but not abstracts for example. A lot of the material Google Scholar indexes is retrieved from the open web and other material is Journal articles references (and fulltext) from publishers. Google Scholar has not integrated any thesauri for the article references, however. Instead they have 7 subject areas available for limiting in advanced search. As viewed in this screenshot one of the 7 subject areas is Medicine, Pharmacology and Veterinary Medicine. I made a limit to that subject area and timespan 1996-.

2310 hits are definitely more that the others but as you see the second hit is definitely of high relevance but the others have indexed the word ethics in the fulltext where the word ethics is part of a ethics committee and not necessarily relevant.

Screen shot of search on antifreeze proteins toxicity shows 60 hits:

Not all of these hits are relevant and some are hits from books.

Screen shot of search on hormone replacement therapy shows 26.200 hits:

Conclusion: It's not recommendable to use Web of Science, Scopus or Google Scholar when doing exhausitve, specific searches when all possible important records of current science have to be found. This is due to the fact that thesauri and controlled vocabulary are not integrated at all or not properly.

Broadening a subject search in Scopus from searching Keywords to searching Title, abstracts and keywords gives a higher recall but not in all cases relevant records. To broaden a search both Scopus and Google Scholar is recommended but not Web of Science which indexes less material from 1996.

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