Journal metrics attempt to quantify the quality or influence of a given journal for purposes of comparison. The idea is that more heavily cited journals must be more prestigious and attract higher quality papers. The Journal Impact Factor, sometimes just called Impact Factor, is the oldest and most widely used measure but its use is often criticized for having a too-short window for analysis (two years) and failure to recognize differing expectations of citation rates among disciplines. Other metrics have sprung up to counter the Impact Factor. Important factors to consider are the time frame of the calculation and whether or not the metric includes all items in a journal (e.g. letters or news) or just the "citable" items. Measures that include all items tend to penalize journals that publish a variety of document types.
Journal Impact Factor = number of citations to a journal in the current Journal Citation Reports (JCR) year to items published in the previous two years, divided by the total number of scholarly items (articles and reviews) published in the journal in the previous two years.
The impact factor for a journal may be found in the Journal Citation Reports database (see link at right), which also provides rankings within subject areas. The JCR is based on journals indexed in the Web of Science Core Collection and comprises two editions, one for sciences and one for social sciences. There is no JCR for humanities. A journal may advertise its impact factor on its website though this claim should be verified in the JCR.
The JCR also offers a five-year impact factor calculation and a variety of other metrics, including Immediacy Index, Cited and Citing Half-Life, and Article Influence Score.
CiteScore = citations to a journal in a given year to documents in that journal from the previous three years, divided by the total number of documents (not just scholarly documents) published in the journal in the previous three years.
CiteScore is based on the Scopus database and can be found in Scopus (see link at right).
Google Scholar Metrics for Journals
h5-index = h5-index is the h-index for articles published in the last 5 complete years. It is the largest number h such that h articles published in 2011-2015 have at least h citations each. So, a publication that had 5 articles but only three had at least 5 citations or more would have an h5 of 3.
h5-median = h5-median for a publication is the median number of citations for the articles that make up its h5-index.
Google Scholar Metrics provides rankings of English-language journals by subject areas and top 100 overall rankings for journals in English and several other languages.
You can search for the scores of individual journals or see rankings within specific subject areas.
SCImago Journal Rank (SJR) = the average number of weighted citations received in the selected year by the documents published in the journal in the previous three years. Citations are weighted based on the importance of the journal determined by a method similar to Google's PageRank algorithm.
The SJR is calculated from the Scopus database and can be found in Scopus or on the publicly available Scopus Journal Metrics and SCImago websites (see links at right). The websites also display journal rankings within subject areas.
Information on how the SJR is calculated is given at http://www.scimagojr.com/SCImagoJournalRank.pdf and https://arxiv.org/pdf/0912.4141&a=bi&pagenumber=1&w=100.pdf.
Source Normalized Impact per Paper (SNIP) = number of citations in the present year to publications in the past three years, normalized to correct for differences between scientific fields.
SNIP is calculated by the Leiden University Centre for Science and Technology Studies (CWTS) and based on the journals in the Scopus database. SNIP values are in Scopus and the Scopus Journal Metrics website as well as at the CWTS website (see links at right), where they are accompanied by stability intervals that show the tendency of the indicator to fluctuate over time. Although SNIP values are given for arts & humanities journals, they tend to be artificially low because of the way the indicator is calculated. The ability to rank journals within disciplines is given in both websites.
More information on how SNIP is calculated is given at http://www.journalindicators.com/methodology.
Eigenfactor Score = a measure of a journal's total importance to the scientific community using a specific year's citations to articles in the previous five years.
Eigenfactor (see link at right) uses the source journals in Journal Citation Reports and an algorithm based on network theory and similar to Google's PageRank. In general, journals that publish more articles have higher scores. Eigenfactor also produces the Normalized Eigenfactor Score, scaled so the average journal has a score of 1 and the Article Influence Score. The Eigenfactor score is also listed in Journal Citation Reports.