Google Scholar has several metrics which measure the impact of authors and researchers, journal titles, and articles. To calculate author metrics, they operate off the h-index, or Hirsch index. This is an author-level metric that attempts to measure both productivity and citation impact of a scholar's publications. It's based on the set of the researcher's most cited papers and the number of citations that they have received in other publications. "It is defined as the highest number of publications of a scientist that received h or more citations each while the other publications have not more than h citations each" (Schreiber 2008). The h-index reflects both the number of publications and the number of citations per publication. Researchers in different fields cannot be compared with this metric; comparison only works with scientists researching in the same field.
Web of Science, Google Scholar, and Scopus use this measurement for calculating an author's impact. A researcher's h-index can vary in these different databases because databases index different journals over different amounts of time. For example, Google Scholar only considers work published after 2013 in their calculations, while Scopus goes back to 1996.
We are not sure exactly how Google Scholar indexes items, but you can read more about how Google defines its metrics.
For more information on publishing metrics, check out our guide on Scholarly Publishing.
Schreiber, M. 2018. “An Empirical Investigation of the G-Index for 26 Physicists in Comparison with the h-Index, the 4-Index, and the R-Index.” Journal of the American Society for Information Science and Technology 59 (9): 1513–22. Accessed October 23. doi:10.1002/asi.20856.
Google Scholar is a very popular search engine for finding many academic resources, but it is not without fault. There are potentially important problems with using it as a citation analysis tool for a few reasons. Google Scholar, like many electronic resources, pulls citation information from metadata. This means there isn't always a person typing in the exact author, publication title, year of publication, or other citation details. Due to being born online, Google Scholar also doesn't always index older print materials. Sometimes this information is incorrect in Google Scholar, so we advise researchers to double check citation information with a Library database.