Impact factor

Apr 21, 2026 - 01:11
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Impact factor

Calculation: concised notation

← Previous revision Revision as of 19:39, 20 April 2026
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==Calculation==
==Calculation==
In any given year, the two-year journal impact factor is the ratio between the number of citations received for publications from the two preceding years, and the total number of "citable items" published in that journal during the two preceding years{{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}}. For example, a 2024 impact factor would be calculated as follows:
In any given year, the two-year impact factor IF^{J}_{year} of {{nowrap|"''Journal J''"}} is the number of citations that all articles published '''by''' ''Journal J'' in the two previous years received '''from''' publications by whatever journal in that given year; normalized by the total number of articles published by ''Journal J'' in the two previous years:{{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}}


IF^{J}_{year} = \frac{Cite_{year}(year-2)+Cite_{year}(year-1)}{Pub^{J}(year-2)+Pub^{J}(year-1)}.
\text{Impact Factor }(2024) = \frac{\text{Citations received in 2024 for articles published in 2022 and 2023}}{\text{Publications in 2022} + \text{Publications in 2023}}


For example, [[Nature (journal)|''Nature'']] had an impact factor of 41.577 in 2017:{{cite book |year=2018 |chapter=Nature |title=2017 Journal Citation Reports |publisher=[[Thomson Reuters]] |edition=Science |series=[[Web of Science]]}}
For example, [[Nature (journal)|''Nature'']] published a total of 1,782 articles in the {{nowrap|2015/2016}} biennium (902 articles in 2015 and 880 in 2016); and 74,090 references listed by articles published (by any journal) in 2017 were sourced from within the group of 1,782 articles that ''Nature'' had published during the previous two years.{{cite book |year=2018 |chapter=Nature |title=2017 Journal Citation Reports |publisher=[[Thomson Reuters]] |edition=Science |series=[[Web of Science]]}} These figures yielded an impact factor of
IF^{Nature}_{2017}=\frac{Cite_{2017}(2015)+Cite_{2017}(2016)}{Pub^{Nature}(2015)+Pub^{Nature}(2016)}
=\frac{74090}{902 + 880}=41.577.


⚫
This means that, on average, its papers published in 2015 and 2016 received roughly 42 citations each in 2017. Impact factors are reported in the subsequent year, when all of the previous year's publications have been processed by the indexing agency.
\text{Impact Factor (2017)} = \frac{\text{Citations received in 2017 for articles published in 2015 and 2016}}{\text{Publications in 2015} + \text{Publications in 2016}} = \frac{74090}{880 + 902} = 41.577.

⚫
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" 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.{{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 }}
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" 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.{{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 }}
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