ITATION COUNTS ARE USED TO measure the impact of articles, journals, and researchers and are frequently incorporated in decisions of academic advancement. However, the validity and methods behind the procurement of citation counts have received limited attention. Until relatively recently, Web of Science was the only practical way to obtain citation counts. 1 In the general medical literature, virtually all previous citation analysis studies have used this database exclusively 2-6 and checking the accuracy or validity of these citation counts against another measure was not feasible. Now, however, several other citation databases have become available, including Scopus 7 and Google Scholar, 8 both introduced in 2004. Scopus, like Web of Science, requires a paid subscription, while Google Scholar is free. Each of these databases uses unique methods to record and count citations. The scope of these databases also differ 9-12 in that Web of Science and Scopus claim strong coverage of selected peer-reviewed journals, while Google Scholar might be better able to record citations from books and nontraditional sources, such as Web sites, dissertations, and open-access online journals. Previous studies in some scientific fields, such as computing, biology, physics, and oncology, have shown differences in citation counts among these databases. 9,13,14 To our knowledge, this See also p 1107.

BackgroundAuthor self-citation contributes to the overall citation count of an article and the impact factor of the journal in which it appears. Little is known, however, about the extent of self-citation in the general clinical medicine literature. The objective of this study was to determine the extent and temporal pattern of author self-citation and the article characteristics associated with author self-citation.Methodology/Principal FindingsWe performed a retrospective cohort study of articles published in three high impact general medical journals (JAMA, Lancet, and New England Journal of Medicine) between October 1, 1999 and March 31, 2000. We retrieved the number and percentage of author self-citations received by the article since publication, as of June 2008, from the Scopus citation database. Several article characteristics were extracted by two blinded, independent reviewers for each article in the cohort and analyzed in multivariable linear regression analyses. Since publication, author self-citations accounted for 6.5% (95% confidence interval 6.3–6.7%) of all citations received by the 328 articles in our sample. Self-citation peaked in 2002, declining annually thereafter. Studies with more authors, in cardiovascular medicine or infectious disease, and with smaller sample size were associated with more author self-citations and higher percentage of author self-citation (all p≤0.01).Conclusions/SignificanceApproximately 1 in 15 citations of articles in high-profile general medicine journals are author self-citations. Self-citation peaks within about 2 years of publication and disproportionately affects impact factor. Studies most vulnerable to this effect are those with more authors, small sample size, and in cardiovascular medicine or infectious disease.