Journal-level metrics attempt to quantify a journal's impact by analyzing the citations arising from the articles it publishes.
Advantages: Journal metrics can give a sense of which journals are popular and/or respected within a specific field.
Disadvantages: These metrics effectively average the impact of a journal's articles and authors, so they hide variations among articles and authors. Journal metrics also are not generalizable across disciplines.
The Impact Factor (IF) is calculated by dividing the number of citations in a year by the total number of articles published in the two previous years. For example, an Impact Factor of 1.0 means that, on average, the articles published in a given journal 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.
The Journal Citation Indicator (JCI) is field-normalized and takes into account the differences of citation patterns across all subjects. For example, JCI can be used to compare an oncology journal to a physics journal. The JCI is fairly easy to interpret: the baseline impact for all journals is 1. If the JCI for a journal is 2, that journal would be twice as impactful compared to all other journals. A score of below 1 would indicate that the journal is less impactful than other journals.
The Eigenfactor, like the Impact Factor, starts with the citation data from Journal Citation Reports but has a more complicated algorithm. Journals are considered to be more influential if they are cited often by other influential journals. For example, citations from Nature or Cell are valued more highly than citations from journals with a narrower readership. Eigenfactor scores are also adjusted for differences in citation patterns across disciplines. They rely on data from five years, as compared to two for the Impact Factor. Eigenfactor scores are scaled so that the sum of the Eigenfactor scores of all journals listed in Journal Citation Reports (JCR) is 100. In 2012, the journal Nature had the highest Eigenfactor score, with a score of 1.56539. In 2020, Nature fell to #4 with a score of 1.21714. PloS One had the highest Eigenfactor with a score of 1.38933.
