The journal Impact Factor is the average number of times articles from the journal published in the past two years have been cited in the JCR year.
The Impact Factor is calculated by dividing the number of citations in the JCR year by the total number of articles published in the two previous years. An Impact Factor of 1.0 means that, on average, the articles published one or two year ago have been cited one time. An Impact Factor of 2.5 means that, on average, the articles published one or two year ago have been cited two and a half times. Citing articles may be from the same journal; most citing articles are from different journals.
For example, the journal PLoS Biology's 2010 impact factor is 12.472.
This was calculated thusly:
Journal Impact Factors can be found in the Thomson Reuters' resource, Journal Citation Reports (JCR).
SCImago Journal and Country Rank is a portal that includes the journals and country scientific indicators developed from the information contained in the Scopus® database.
In addition to the Impact Factor, JCR will also include the following metrics for a journal
From "Price doesn't always buy prestige in open access" - Nature News, Jan. 22. 2013
The graph above utilizes a new tool, called Cost Effectiveness for Open Access Journals It incorporates pricing and prestige information for 657 open-access journals indexed by Thomson Reuters, including 356 that do not charge any fees.The data are plotted to show a journal's Article Influence (AI) score against the fee it charges per article. The AI score is calculated by dividing the Eigenfactor Score of the journal by the number of articles in the journal, normalized so that the average journal has an AI equal to 1. Eigenfactor Scores are like impact factors in that they are based on citations, but they also take into account the source of the citations.
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