Definition: The Journal Impact Factor is published each year by Thomson Reuters. It is a measure of the number of times an average paper in a particular journal is cited during the preceding two years.

For example:

A = the number of times articles published in a specific journal in 2009 and 2010 were cited by journals during 2011.

B = the total number of 'citable items' published by that journal in 2009 and 2010. ('Citable items' are usually articles, reviews, proceedings, etc.; not editorials or letters-to-the-editor.)

2011 impact factor = A/B.

Actual value is intentionally only displayed for the most recent year. Earlier values are available in the Journal Citation Reports from Thomson Reuters.

5-Year Impact Factor

Note: A base of five years may be more appropriate for journals in certain fields because the body of citations may not be large enough to make reasonable comparisons, or it may take longer than two years to publish and distribute leading to a longer period before others cite the work.