The Journal Impact Factor is a ratio devised as a measurement of the average citedness of a journal. It is calculated by dividing the number of citations to a particular publication in a certain year by the number of citable articles published in the journal in the previous two years. Other related measurements may also be calculated, such as the Five-year impact factor (which looks at the previous 5 years), or the Immediacy index (which is the average number of times an article is cited in the year it is published).
Journal Impact Factors are not a measure of the citedness of any particular article in a journal, but rather of the journal as a whole. They also can be influenced by other considerations. For example, a journal that publishes mainly review articles may have a higher impact factor as these are more often cited.
Journal Impact factors are published in the Journal Citation Reports database, which covers both the sciences and social sciences, but not the humanities (though History is included in social sciences).
Once you access Journal Citation Reports you will be asked to select either the Science edition or the Social Sciences Edition. Choose the edition appropriate to your area of interest and click Submit. NOTE: The default setting is to search by a subject grouping of journals. If you want to find information on a specific journal, select the radio button in the middle of the right-hand panel and you will be taken to a search form that allows you to enter the name of an individual journal.
You will be asked to select a subject area before proceeding. Note that it is possible to select more than one subject area.
The result will be an alphabetical listing of all the journals in that particular subject area. A variety of impact factor measures (including the Eigernfactor metric) are shown.
While the default setting is an alphabetical sort, it is possible to change the sort order of the list by using the Sort Again function buttom.
Definition: The journal impact factor measures the importance of a journal by calculating the times it's articles are cited.
How Impact Factor is Calculated: The calculation is based on a two-year period and involves dividing the number of times articles were cited by the number of articles that are citable.
Experts stress that there are limitations in using impact factors to evaluate a scholar's work. There are many reasons cited for not relying on impact factor alone to evaluate the output of a particular individual. Among these are the following:
According to Jim Testa, a researcher for ThomsonReuters Scientific, the most widespread misuse of the Impact Factor is to evaluate the work of an individual author (instead of a journal). "To say that because a researcher is publishing in a certain journal, he or she is more influential or deserves more credit is not necessarily true. There are many other variables to consider." (interview 6/26/2008 in Thomson Reuters blog entry)