If you work in academia, you’re bound to be familiar with Impact Factors. You’ll probably know that “good journals” have an Impact Factor, and you may know that “really good journals” have a high Impact Factor. But do you know how Impact Factors are calculated? Or how journals are ranked? In short, do you know what an Impact Factor actually is?
If not, don’t worry. Consider this your Impact Factor 101.
An Impact Factor (IF) is, in essence, a fairly simple sum. A journal’s 2021 IF is calculated using citations received by that journal in 2021 for articles published in the previous two years (2019 + 2020), divided by the number of articles the journal published in those two years.
So, if in 2021 a journal received 13 citations to articles published in 2019 and 17 citations to articles published in 2020 (a total of 30) and published 35 articles over those two years; it would have a 2021 IF of 0.857.
The reason that only the previous two years are taken into consideration is simply to level the playing field. If you took into consideration citations to articles from a journal’s full history, then it would be extremely biased towards older journals who naturally have a far larger number of articles to choose from.
At the end of each year, the citations for each journal are counted and put into the Journal Citation Reports (JCR) which are published the following summer.
Which articles count towards an IF?
You have written an article published in a journal which is included in the JCR and, let’s say, it’s been published this year. Any citations your article receives next year or the year after will contribute towards the journal’s IF for those years.
Not everything published by a journal counts as “Article Content”, only Research Articles, Review Articles, Short Reports, etc. Editorials, Book Reviews, Letters to the Editor, etc, are not classed as Article Content so won’t be counted in the number of articles published. Any citations they receive the two years following publication will, however, be included in the citation count.
Alright, so what is a “good” or “high” Impact Factor?
The simple answer to that is the bigger the IF (i.e. the higher the number), the better it is. But, of course, it’s not quite as simple as that.
Different disciplines are likely to receive different levels of citations. For example, a scientific journal publishing up to the minute research in a fast-moving field is likely to receive more citations within the “IF window” (those two years after an article is published) than a humanities journal in a field which moves somewhat slower. An IF of 1.000 might be brilliant in one discipline, but be pretty poor in another.
To allow for this, journals are split into categories within the JCR and ranked within those. Therefore, rather than looking at their IF alone, to find the best journals within your field you should find the most relevant categories and see which journals rank highest within those.
For more information on the Journal Citation Reports (and Impact Factors, naturally), check out Clarivate’s website here.