  A new methodology is proposed for comparing Google Scholar (GS) with other
citation indexes. It focuses on the coverage and citation impact of sources,
indexing speed, and data quality, including the effect of duplicate citation
counts. The method compares GS with Elsevier's Scopus, and is applied to a
limited set of articles published in 12 journals from six subject fields, so
that its findings cannot be generalized to all journals or fields. The study is
exploratory, and hypothesis generating rather than hypothesis-testing. It
confirms findings on source coverage and citation impact obtained in earlier
studies. The ratio of GS over Scopus citation varies across subject fields
between 1.0 and 4.0, while Open Access journals in the sample show higher
ratios than their non-OA counterparts. The linear correlation between GS and
Scopus citation counts at the article level is high: Pearson's R is in the
range of 0.8-0.9. A median Scopus indexing delay of two months compared to GS
is largely though not exclusively due to missing cited references in articles
in press in Scopus. The effect of double citation counts in GS due to multiple
citations with identical or substantially similar meta-data occurs in less than
2 per cent of cases. Pros and cons of article-based and what is termed as
concept-based citation indexes are discussed.
