"The Impact Factor is a very useful tool for evaluation of journals, but it must be used discretely. Considerations include the amount of review or other types of material published in a journal, variations between disciplines, and item-by-item impact." --- Eugene Garfield
How scientific research is evaluated, recognized, and perceived affects the lives of scientists, the distribution of research funds, and the entire process of doing science. When Eugene Garfield, founder of the Institute for Scientific Information (ISI), created the Science Citation Index (SCI) in the 60s, he could not have imagined that it would be used for purposes it had never been designed for. There are many reasons why the SCI and the Journal Impact Factor (IF) derived from it attracted so much attention.
Authors of scientific articles wanted to know how often their works have been cited, because they are often evaluated, hired, promoted, and funded on the basis of their citation counts and the quality (the IF) of the journals, in which they have published their papers.
Funding agencies are placing an increased weight on IF of journals in which the grant applicants have published their articles.
Editors are trying to understand how IF can be influenced so that could increase their journal's ranking
Publishers of scientific journals are using IFs to conduct market research and promote their journals.
Librarians are often making their decisions about which journals to drop or add on the basis of their IFs
The IFs of journals are published in Journal Citation Reports (JCR). The IF of a particular journal is the ratio between the number of times articles published in the journal were cited, divided by the number of citable articles* published in this journal during the same period of time. For example, the IF of a journal for the year 2010 is calculated as follows: ISI (now Thomson-Reuters) counts the number of citations made in 2010 to papers published in this journal during the previous two years (2008 and 2009) and divides this number by the number of citable articles published in the same two-year period. JCR for a particular year reports data from the previous year. For example, JCR for 2011 reports the IFs for the year 2010.
* The way the term "citable articles" is defined has caused a lot of criticism. When the IF is calculated, the numerator includes all citations to the journal, whereas the denominator, instead of also including all articles published in the journal, excludes editorials, letters, and comments. This has allowed editors to manipulate the IF of their journals by publishing review articles as editorials, which are usually often cited. So the citations to these reviews are counted in the nominator but are excluded from the denominator, which increases this ratio and thus - the IF.
There are three additional ranking scores offered within the Journal Citation Reports: the Eigenfactor Score (EF), Normalized Eigenfactor (NEF), and Article Influence Score (AI).
The Eigenfactor (EF) score is intended to measure a journal's total importance. The scores are scaled so that the rankings of all journals listed within their category will equal 100. The Eigenfactor Score calculation is based on the number of times articles from the journal published in the past five years have been cited in the JCR year, but it also considers which journals have contributed these citations so that highly cited journals will influence the network more than lesser cited journals. References from one article in a journal to another article from the same journal are removed, so that Eigenfactor Scores are not influenced by journal self-citation. If a journal has an EF of 1 then it has 1% of the total influence of all the indexed publications for a given year. That would be a very high score. The size of a journal will influence its score, with larger journals having higher scores.
The Normalized EF (NEF) score rescales the (EF) score so that it is easier to interpret. In this score the average journal has a score of 1, so that a journal with a NEF of 3 has three times the influence of a journal with a NEF of 1.
The Article Influence Score (AI) measures the average influence of the articles in a journal over 5 years. It is calculated by dividing a journal’s Eigenfactor Score by the number of articles in the journal, normalized as a fraction of all articles in all publications. The Article Influence Score is considered comparable to the JCR Impact Factor. This score has also been normalized so that the mean AI score is 1.0. A score greater than 1.0 indicates that each article in the journal has above-average influence. Scores below 1.0 signify that each article in the journal has below average-influence.
The Eigenfactor, Normalized Eigenfactor, and Article Influence Score may also be found at the Eigenfactor.org website, in addition to the JCR.
The SCImago Journal Rank (SJR) is a indicator which uses data, from 1996 onward, contained in Scopus and a platform based on the algorithm Google PageRank. It is a size-independent metric for journals that also takes into account how close citations are to the original journal (how well it is reaching related journals) and includes country-specific data. The indicator provides a measure of the visibility, impact, prestige of the journals in Scopus. The interface used to search for an SJR is very simple and easy to use.
For more information on how it is calculated see this article on SJR2.
CiteScore is Elsevier's answer the JCR. It is defined as the number of citations received by a journal in one year to documents published in the three previous years, divided by the number of documents indexed in Scopus published in those same three years.
CiteScore for 2015 counts the citations received in 2015 to documents published in 2012, 2013 or 2014, and divides this by the number of documents published in 2012, 2013 and 2014.
