The average number of times articles from a journal published in the past two years have been cited in the Journal Citations Report (JCR) year.
When to Use It
Journal impact: Though not a strict mathematical average, the Journal Impact Factor provides a functional approximation of the mean citation rate per citable item.
How It's Calculated
(Total number of citations from JCR year to items in “year -2” + citations from JCR year to items in “year -1”) ÷ (total number of citable items in “year -2” + citable items in “year -1”) = Journal Impact Factor
The Journal Impact Factor takes into account the outbound cited references from any of the five journal and proceedings indexes in Web of Science (Web of Science):
- Science Citation Index Expanded (SCIE)
- Social Sciences Citation Index (SSCI)
- Arts & Humanities Citation Index
- Conference Proceedings Citation Index, Science edition
- Conference Proceedings Citation Index, Social Science and Humanities edition
- For each title in SCIE or SSCI (only these two indexes get JIFs), the citations it earns (among the outbound citations measured), are collected and summed.
This collection and summation takes into account the year of publication for (a) the outbound citation (=JCR year) and for (b) the item that has been cited. The Journal Impact Factor is restricted to a two-year window of interest for cited item publication year: one year prior to the JCR year (= year -1) and two years prior to the JCR year (= year -2).
For example, in the 2015 JCR, each Journal Impact Factor will measure the citations earned by a publication where the citing year is 2015, and the cited year is either 2013 or 2014.
Also, because the Journal Impact Factor is ultimately a ratio of citations earned (in the given window) to citable items published (in the same window) by a publication, a count must be made of all the items published (and of the subset deemed to be “citable”) in that publication during that window. In the ratio, the number of citations earned is the numerator. The number of citable items is the denominator.
The value of the denominator is restricted to the same window of time as the numerator (i.e., year -1 and year -2). Any item assigned the document type “Article” or “Review” (in Web of Science) is included in the denominator. An item with any other document type is excluded from the denominator.
The citing works may be articles published in the same journal. However, most citing works are from different journals, proceedings, or books indexed in Web of Science.
The venue for measuring these data points is the Web of Science production database. This database is constantly ingesting new data, and old data are regularly corrected or updated. This degree of flux makes producing a metric like the JIF difficult because the data inputs are liable to change from minute to minute. As a result, the JCR team fixes a date (usually in the spring of the year following the JCR year) when they take an indelible “snapshot” of the database. This is JCR extraction, and it is from this extract that all JCR metrics are calculated.
How to Interpret It
A Journal Impact Factor of 1.0 means that, on average, the articles published one or two years ago have been cited one time. A Journal Impact Factor of 2.5 means that, on average, the articles published one or two years ago have been cited 2.5 times.
- Journal Impact Factor Without Self Cites