Article Influence Scores can be accessed freely at eigenfactor.org or through a subscription to Journal Citation Reports (JCR).
Article Influence scores of a journal can vary between eigenfactor.org and JCR, even for the same year. This may be because the eigenfactor metrics take into account some other sources, such as dissertation and newspaper citations.
The Article Influence Score came out of the Metrics Eigenfactor Project in 2008, a bibliometric research project conducted by Professor Carl Bergstrom and his laboratory at University of Washington.
The Article Influence Score calculates measures the relative importance of the journal on a per-article basis. It is the journal's Eigenfactor Score divided by the fraction of articles published by the journal. That fraction is normalized so that the sum total of articles from all journals is 1.
The mean Article Influence Score is 1.00. A score greater than 1.00 indicates that each article in the journal has above-average influence. A score less than 1.00 indicates that each article in the journal has below-average influence.
In 2006, the top journal by Article Influence score is Annual Reviews of Immunology, with an article influence of 27.454. This means that the average article in that journal has twenty seven times the influence of the mean journal in the JCR.
Article Influence uses Thomson Reuters (ISI Web of Knowledge) citation data.