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: