The Eigenfactor score of a journal is an estimate of the percentage of time that library users spend with that journal. An Eigenfactor is based on the number of cited references, with citations from highly ranked journals weighted to make a larger contribution than those from poorly ranked journals.
The Eigenfactor Score measures the number of times articles from the journal published in the past five years have been cited in the JCR year.
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. In 2011, the journal Nature had 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.
Eigenfactor.org and JCR - Journal Citation Reports from Thomson Web of Science both display Eigenfactors for indexed journals.