Journal Impact Factor
Journal Impact Factor
The Journal Impact Factor is defined as all citations to the journal in the current JCR year to items published in the previous two years, divided by the total number of scholarly items (these comprise articles, reviews, and proceedings papers) published in the journal in the previous two years.
Though not a strict mathematical average, the Journal Impact Factor provides a functional approximation of the mean citation rate per citable item. 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 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 in Web of Science.
The Journal Impact Factor takes into account the outbound cited references from any of the five journal and proceedings indexes in Web of Science (WoS):
- 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 WoS) is included in the denominator. An item with any other document type is excluded from the denominator.
The venue for measuring these data points is the WoS 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.
The Journal Impact Factor has a simple formula:
Journal Impact Factor = (citations from JCR year to items in “year -2” + citations from JCR year to items in “year -1”)/ (citable items in “year -2” + citable items in “year -1”)
The Journal Impact Factor is a publication-level metric. It does not apply to individual papers or subgroups of papers that appeared in the publication. Additionally, it does not apply to authors of papers, research groups, institutions, or universities.