An author's impact on their field or discipline has traditionally been meausred using the number of times their academic publications are cited by other researchers. There are numerous algorithms that account for such things as the recency of the publication, or poorly or highly cited papers. While citation metrics may reflect the impact of research in a field, there are many potential biases with these measurements and they should be used with care. For a critique of author impact factors, see this article in the Chronicle of Higher Education.
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