Editing Impact factor (section)

==Calculation==
In any given year, the two-year journal impact factor is the ratio between the number of citations received in that year for publications in that journal that were published in the two preceding years and the total number of "citable items" published in that journal during the two preceding years:<ref name="Web of Science Group 2019">{{cite web |title=Web of Science Group |website=Web of Science Group |date=2019-08-05 |url=https://clarivate.com/webofsciencegroup/essays/impact-factor/ |access-date=2020-07-28}}</ref><ref name=garfield/>

<math display=block>\text{IF}_y = \frac{\text{Citations}_y}{\text{Publications}_{y-1} + \text{Publications}_{y-2}}.</math>

For example, [[Nature (journal)|''Nature'']] had an impact factor of 41.577 in 2017:<ref name=WoS>{{cite book |year=2018 |chapter=Nature |title=2017 Journal Citation Reports |publisher=[[Thomson Reuters]] |edition=Science |series=[[Web of Science]]}}</ref>

<math display=block>\text{IF}_{2017} = \frac{\text{Citations}_{2017}}{\text{Publications}_{2016} + \text{Publications}_{2015}} = \frac{74090}{880 + 902} = 41.577.</math>

This means that, on average, its papers published in 2015 and 2016 received roughly 42 citations each in 2017. 2017 impact factors are reported in 2018; they cannot be calculated until all of the 2017 publications have been processed by the indexing agency.

The value of impact factor depends on how to define "citations" and "publications"; the latter are often referred to as "citable items". In current practice, both "citations" and "publications" are defined exclusively by ISI as follows. "Publications" are items that are classed as "article", "review" or "proceedings paper"<ref name="McVeighMann2009"/> in the Web of Science (WoS) database; other items like editorials, corrections, notes, retractions and discussions are excluded. WoS is accessible to all registered users, who can independently verify the number of citable items for a given journal. In contrast, the number of citations is extracted not from the WoS database, but from a dedicated JCR database, which is not accessible to general readers. Hence, the commonly used "JCR Impact Factor" is a proprietary value, which is defined and calculated by ISI and can not be verified by external users.<ref>{{Cite journal |doi=10.1087/20110208 |title=Casting a wide net: The Journal Impact Factor numerator |journal=Learned Publishing |volume=24 |issue=2 |pages=133–137 |year=2011 |vauthors=Hubbard SC, McVeigh ME |s2cid=20172401|doi-access=free }}</ref>

New journals, which are indexed from their first published issue, will receive an impact factor after two years of indexing; in this case, the citations to the year prior to volume 1, and the number of articles published in the year prior to volume&nbsp;1, are known zero values. Journals that are indexed starting with a volume other than the first volume will not get an impact factor until they have been indexed for three years. Occasionally, ''Journal Citation Reports'' assigns an impact factor to new journals with less than two years of indexing, based on partial citation data.<ref>{{cite web |date=24 June 2013 |title=RSC Advances receives its first partial impact factor |url=http://blogs.rsc.org/ra/2013/06/24/rsc-advances-receives-its-first-partial-impact-factor/ |website=RSC Advances Blog |access-date=16 July 2018}}</ref><ref>{{cite news |last1=Konforti |first1=Boyana |title=Our first (partial) impact factor and our continuing (full) story |url=https://crosstalk.cell.com/blog/irst-partial-impact-factor-and-our-continuing-full-story |work=Cell Mentor |date=30 July 2014 }}</ref> The calculation always uses two complete and known years of item counts, but for new titles one of the known counts is zero. Annuals and other irregular publications sometimes publish no items in a particular year, affecting the count. The impact factor relates to a specific time period; it is possible to calculate it for any desired period. For example, the JCR also includes a '''five-year impact factor''', which is calculated by dividing the number of citations to the journal in a given year by the number of articles published in that journal in the previous five years.<ref name=newJCR>{{cite web |url=http://www.thomsonreuters.com/content/press_room/sci/350008 |title=JCR with Eigenfactor |access-date=2009-08-26 |url-status=dead |archive-url=https://web.archive.org/web/20100102180714/http://thomsonreuters.com/content/press_room/sci/350008 |archive-date=2010-01-02}}</ref><ref>{{Cite web |url=https://apa.org/pubs/journals/5-year-impact-factor.aspx |title=ISI 5-Year Impact Factor |website=APA |access-date=2017-11-12}}</ref>