What are Impact Factors?
Impact Factors give the average number of citations received by articles in a particular journal; essentially, the average number of times that articles in a particular journal are referenced by other articles.
The Impact Factors, published annually in the Journal Citation Reports (JCR), are defined as follows:
Number of citations (references) received in the Impact Factor year to articles published in the two previous years, divided by the number of articles published in these two years.
Therefore, the 2014 JCR Impact Factors (released in 2015) were calculated as follows:
#Citations received in 2014 to articles published in 2012 and 2013 in Journal X
#Articles published in 2012 and 2013 in Journal X
For more information see: Impact Factors Back to Basics
Other JCR metrics
In 2007 Thomson Reuters added three new metrics to the JCR: the 5-year Impact Factor, the Eigenfactor, and the Article Influence Score. The JCR also includes Total Citation counts, the Immediacy Index, and other metrics.
5-year Impact Factor
This is simply a modified version of the Impact Factor that uses five years’ worth of data rather than two in the calculation. For example, in the 2014 JCR the calculation is:
Citations in 2012 to content published in 2009–2013
Number of articles and reviews published in 2009–2013
The 5-year Impact Factor suffers from many of the same issues as the traditional 2-year Impact Factors, but is more stable year on year for smaller titles as there are simply a larger number of articles and citations included in the calculation.
A journal must be covered by Thomson Reuters for five years or from volume 1 before it will receive a 5-year Impact Factor score.
Eigenfactor and Article Influence Score
Eigenfactors and Article Influence (AI) Scores were created by Carl T. Bergstrom at the University of Washington, and have been included since the 2007 JCR. These scores do not have a simple calculation but instead borrow their methodology from network theory and are similar to Google’s initial PageRank system.
Eigenfactors weight the value of a citation from a journal based on how many citations that journal received. Effectively, a citation from a journal such as Science (IF 31.201) is given more weight than a citation from Naval Architect (IF 0.005). Self-citations are not included as part of the calculation and to adjust for subject areas the citations are further weighted by the length of the reference list that they are from. The calculation uses an iterative process to find stable values for the Eigenfactor. These are then normalized so the sum for the entire JCR is 100. No adjustment for journal size is made.
Article Influence Scores are calculated by dividing the Eigenfactor by the proportion of articles covered by the JCR in the previous five years that were published in that particular journal. These are then normalized so that the average journal in the JCR has a score of 1. Like 5-year Impact Factors, journals have not been given an Article Influence Score unless they have been covered by Thomson for at least five years or from Volume 1.
This is the simplest metric included in the JCR and is simply the total number of citations recorded to the current journal title in that year. If a journal changes titles, the citations to historic versions are not included.
This is a measure of how quickly content in the journal is being cited. For example, in the 2014 JCR the calculation is:
Citations in 2014 to content published in 2014
Number of articles and reviews published in 2014
The Immediacy Index is highly variable between subject area and journal type and can be highly distorted by self-citations.
Impact Factor without self-citations
The Impact Factor without self-citations is an adjusted Impact Factor that excludes all the citations that come from the journal itself.
For more information:
Optimizing citations to your journal