Negative citations in Google Scholar

Google Scholar has become a powerful academic search engine. Google Scholar has much broader coverage of the recent literature than Web of Knowledge and Scopus. Therefore Google Scholar is a primary source for systematic reviews (Gehanno et al., 2013).

Normally, Google Scholar indexes papers within a few days after they appear on the Internet. However, some papers are missed while others are indexed twice (e.g., both the pre-prints and published articles may be indexed). About once or twice per year, Google Scholar performs a major re-crawl of the Internet, taking about 10 days to complete, and in this process finds new citations and corrects errors. Such updates happened on 26 March 2013 and 11 February 2014. Recently, on 16 June 2014, Google Scholar completed a new major update.

The figure below shows the difference in number of citations between June 17 (after the update) and June 13 (before the update) for all 998 scientists having the word “Delft” in their profile title. It can be seen that most scientist must have been delighted because they saw a ‘jump’ in their citation counts. However, 117 out of 998 scientists (11.7%) actually saw a reduction in their citations (i.e., a negative number in the figure). The most highly cited paper ever (Lowry et al., 1951) has seen a dramatic reduction in citations (190,850 on 19 June 2014 vs. 253,684 on 25 May 2014). Unclear what happened here...

Figure. Difference in citations between June 13 and June 17, 2014, for all 998 scientists who have the word 'Delft' in their Google Scholar profile. The differences are sorted in ascending order. The red horizontal line at 0 difference is depicted for reference purposes.

References

Gehanno, J. F., Rollin, L., & Darmoni, S. (2013). Is the coverage of Google Scholar enough to be used alone for systematic reviews. BMC Medical Informatics and Decision Making, 13, 7. http://dx.doi.org/10.1186/1472-6947-13-7

Lowry, O. H., Rosebrough, N. J., Farr, A. L., & Randall, R. J. (1951). Protein measurement with the Folin phenol reagent. The Journal of Biological Chemistry, 193, 265–275.