Journal metrics based on citations are developed by different publishers and organisations, and are based on data from either Web of Science (Clarivate) or Scopus (Elsevier)
The metrics are based on the relationship between citations to a journal and the number of articles in that journal over a set period. Some of the calculations try to take account of differences in citation practices within different fields (AIS, SNIP, SJR), while others can only be used to compare journals within a certain field (JIF, IPP).
In recent years, a movement has begun which, via a number of recommendations, aims to reduce the use of journal metrics as indicators of research impact (San Francisco Declaration on Research Assessment (DORA)).
Journal Impact Factor is based on citations from Web of Science. You can find JIF via Journal Citation Reports.
“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. The citing works may be articles published in the same journal. However, most citing works are from different journals, proceedings, or books indexed by Web of Science.”
(http://admin-apps.webofknowledge.com/JCR/help/h_impfact.htm)
Source Normalized Impact per Paper is based on citations from Scopus. You can find SNIP in Scopus and http://www.journalindicators.com/.
“The source normalized impact per publication, calculated as the number of citations given in the present year to publications in the past three years divided by the total number of publications in the past three years. The difference with IPP is that in the case of SNIP citations are normalized in order to correct for differences in citation practices between scientific fields. Essentially, the longer the reference list of a citing publication, the lower the value of a citation originating from that publication. A detailed explanation is offered in our scientific paper.”
(http://www.journalindicators.com/methodology)
Article Influence scores are based on citations from Web of Science. You can find AIS via Journal Citation Reports.
“The Article Influence determines the average influence of a journal's articles over the first five years after publication. 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. This measure is roughly analogous to the 5-Year Journal Impact Factor in that it is a ratio of a journal’s citation influence to the size of the journal’s article contribution over a period of five years.”
“The mean Article Influence Score is 1.00. A score greater than 1.00 indicates that each article in the journal has above-average influence. A score less than 1.00 indicates that each article in the journal has below-average influence. “
“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.“
(http://admin-apps.webofknowledge.com/JCR/help/h_eigenfact.htm)
Read more about Eigenfactor project.
Scimago Journal rank is based on citations from Scopus. You can find SJR in Scopus and at www.scimagojr.com/
”It expresses the average number of weighted citations received in the selected year by the documents published in the selected journal in the three previous years, --i.e. weighted citations received in year X to documents published in the journal in years X-1, X-2 and X-3.” (http://www.scimagojr.com/help.php)
