The impact factor is a measure of the frequency with which the average article in a journal has been cited in a particular year or period. It is one of the evaluation tools provided by Thomson Reuters Journal Citation Reports® (JCR®).

The annual Journal Citation Reports impact factor is a ratio between citations and recent citable items published: a journal's impact factor is calculated by dividing the number of current year citations to the source items published in that journal during the previous two years.

Impact factor news

January 2009: Thomson Reuters releases new Journal Citation reports

Using and misusing the impact factor: an interview with bibliometrician David Tempest

Essay: The impact factor

Journal Citation Reports: critical evaluation of the world's leading journals

Citation Impact Center: commentary and discussion on scholarly research evaluation