Eigenfactor Metrics"A journal's Eigenfactor score is measured as its importance to the scientific community. Scores are scaled so that the sum of all journal scores is 100. In 2006, Nature had the highest score of 1.992." The Eigenfactor score reflects "the influence and prestige of journals." (from Cornell University Library's Guide to Author & Journal Metrics)
Scores are based on data supplied by Thomson-Reuters and are reported six months after they are released in the Thomson-Reuters Journal Citation Reports (a paid subscription service.)