What is an Impact Factor?

Impact factor is a measure of the frequency with which the "average article" published in a given scholarly journal has been cited in a particular year or period and is often used to measure or describe the importance of a particular journal to its field.Clarivate Analytics ranks, evaluates, and compares journals within subject categories and publishes the results in Journal Citation Reports. The new rankings come out in the Spring, for the previous year's journals.  Three years worth of data are required to calculate a Journal Impact Factor.

The formula to determine the 2019 impact factor for a journal would be calculated as follows:

A = the number of times articles published in the journal during 2017-18 were cited by other journals during 2019
B = the number of articles or reviews that were published in the journal during 2017-18

2019 Impact factor for a journal = A/B

Impact factors have a huge, but controversial, influence on the way published scientific research is perceived and evaluated. Numerous criticisms have been made of the system:

1. Journal impact factors depend on the research field: high impact factors are likely in journals covering large areas of basic research and less likely in more subject-specific journals.
2. Although Journal Citation Reports includes some non-English journals, the index is heavily skewed toward English-language journals, leaving out important international sources.
3. Researchers may be more likely to pursue fashionable topics that have a higher likelihood of being published in a high-impact journal than to follow important avenues that may not be as popular.
4. Impact factors mean little if they are not contextualized to rank within a quartile across the subject categories in which a journal is classed. A journal could have an impact factor higher than 10 but not be within the top quartile of the journals within that category (discipline).