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Falagas, ME, Pitsouni EI, Malietzis, GA, and Pappas, G, 2008, 'Comparison of PubMed, Scopus, Web of Science, and Google Scholar: strengths and weaknesses', The FASEB Journal, vol 22, pp. 338-342.
There are several measures of author impact. They include
The h-index is a popular metric. It was developed by J.E. Hirsch, a physicist at the University of California, San Diego and published in Proceedings of the National Academy of Sciences of the United States of America 102 (46): 16569-16572 November 15 2005.
(see link below)
Wikipedia describes the h-index as follows
a scholar with an index of h has published h papers each of which has been cited in other papers at least h times. Thus, the h-index reflects both the number of publications and the number of citations per publication. The index is designed to improve upon simpler measures such as the total number of citations or publications. The index works properly only for comparing scientists working in the same field; citation conventions differ widely among different fields.
The g-index is a weighted version of the h-index. It was proposed in 2005 by Leo Egghe. The g-index is available for indexed articles using Publish or Perish software, from data in Google Scholar.
Egghe, L, 2006, Theory and practise of the g-index, Scientometrics, vol. 69, no 1, pp. 131–152. doi:10.1007/s11192-006-0144-7
Factors affecting citation rates include