Differences in citation records across international databases reflect variations in their coverage of the scientific literature, rather than inaccuracies (D. Shotton Nature 502, 295–297; 2013).

Google Scholar, for example, indicates the number of citations that have appeared in almost any online scientific material. Thomson Reuters' Web of Science records only the number of citations in selected high-impact journals. Both numbers are accurate on the basis of each source's indexing criteria, so each should be interpreted individually.