Created by ISI (the Institute for Scientific Information, now known as Thomson Reuters), the Impact Factor is the most widely-recognized method for attempting to gauge a journal's rank/importance. It is particularly well-known in the Sciences and Social Sciences.
The impact factor is based on two figures: the number of citations to a given journal over the previous two years (A) and the number of research articles and review articles published by that journal over the same two-year period (B), so: A/B = Impact Factor
Note that there is a discrepancy between the type of content counted for each factor: A = any type of content (including letters, news items, etc.), but B = research or review articles only, making the Impact Factor not a true average.
Impact factor topics on this page:
Find the Impact Factor for your journal using the various Thomson Reuters products listed below. Impact factors are also usually listed on individual journal publisher pages.
From the Thomson Reuters web page:
Informed and careful use of these impact data is essential. Users may be tempted to jump to ill-formed conclusions based on impact factor statistics unless several caveats are considered.
When considering the use of the impact factor (IF), keep these aspects in mind:
Impact factors have often been used in advancement and tenure decision-making. Many recognize that this is a coarse tool for such important decisions, and that a multitude of factors should be taken into account in these deliberations.
Using Impact Factors within a given discipline should only be done with great care, as described above. Using impact factor data to compare journals across disciplines is even more problematic. Here are some of the reasons:
Access Journal Citation Reports, then select either the Science or Social Sciences Edition and Year.
2. Search by Subject Category and choose Sort by Impact Factor.
3. Here are the results for a search in the Behavioral Sciences category (shown in two parts due to the width of the display):
You'll see that additional metrics, such as the Eigenfactor and Article Influence scores, are also captured in this display.
From the Journal Citation Reports Help pages:
The mean Article Influence Score is 1.00. A score greater than 1.00 indicates that each article in the journal has above-average influence. A score less than 1.00 indicates that each article in the journal has below-average influence.