The Eigenfactor 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.
Like the Impact Factor, the Eigenfactor Score is essentially a ratio of number of citations to total number of articles. However, unlike the Impact Factor, the Eigenfactor Score:
Eigenfactor scores are scaled so that the sum of the Eigenfactor scores of all journals listed in Thomson's Journal Citation Reports (JCR) is 100.
The Eigenfactor uses Thomson Reuters Web of Science citation data.
In 2011, the journal Nature has the highest Eigenfactor score, with a score of 1.65524. The top thousand journals, as ranked by Eigenfactor score, all have Eigenfactor scores above 0.01.
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.
Article Influence Scores can be accessed freely at eigenfactor.org or through a subscription to Journal Citation Reports (JCR).
In 2011, the top journal by Article Influence score is Reviews of Modern Physics, with an article influence of 28.9002. This means that the average article in that journal has twenty eight times the influence of the mean journal in the JCR.