What are Journal Impact Factors, and how are they calculated?
The Journal Impact Factor (JIF) is a way to measure the relative impact of a particular journal within its field, based on the average number of times an article published will be cited in the near future. The data is based on the journals and citing sources (journals, proceedings, books) in the Web of Science Core Collection, with impact factors available for more than 11,500 journals in the Science and Social Science Citation Indexes.
The impact factors are released annually in the Journal Citation Reports and calculated like this:
If a journal published 2,000 articles and reviews (citable items) in 2015 and 2016, and those articles and reviews (plus anything else the journal published those two years) get cited 20,000 times in the articles, conference proceedings and books in 2017, then the 2017 impact factor for that journal is 10 If the journal also publishes pieces like letters, those do not get counted in the citable items, but citations to those pieces will get counted in the numerator..
Where can I find Journal Impact Factors?
The impact factors are published in Journal Citation Reports (JCR). There's one report for the sciences, and another for the social sciences. There is no JCR for the arts and humanities journals indexed in Web of Science. You can also access JCR from the Web of Science interface.
In JCR, you can search for a particular title, or "Browse By Journal" to bring up a ranked list of journals in one or more categories to compare impact factors. Some disciplines are divided into multiple categories (7 for Chemistry, 11 for Psychology, etc.) and many journals are assigned to multiple categories. The subject lists may also help you identify journals in your field for potential article submissions.
What data can I get from JCR?
For each journal, you'll get:
Full list of indicators:
Many of these indicators are also available (and sortable) at the subject category level. Use customize to set up which ones you want to see.
Considerations when using journal impact factors, and how not to use them
Journal Impact Factors tell you nothing about the impact of the author or particular article, and they should not be used as a measure of a researcher's impact. They should not even be the sole factor when evaluating the quality of the journal.
Impact factors vary widely across disciplines and should not be compared with each other. Here are the impact factors for journals that are the highest in their given category. However, they may be ranked lower in other assigned categories.
There are several factors that affect impact factors:
Impact factors can be subject to manipulation.
What about other journal-level metrics?
They exist, but are less well-known. Eigenfactor and Article Influence scores are also listed in Journal Citation Reports. The same caveats apply: do not compare journals across disciplines, and that journal rankings should not be used to evaluate the impact of articles or researchers.