Journal Impact Factor



  • Seeks to measure the influence a journal has in its field.
  • Uses "bibliometric analysis" of journals indexed in the ISI database. More specifically, it measures how often scholars and researchers have cited articles in a particular journal in the most recent two years.
  • Simply put, the higher the number, the better the journal's impact factor. The better the journal's impact factor, the more influence it is supposed to have in its field.

Formula for calculation

Numerator: Number of cites in the current year to anything in that journal in the past two years.
Denominator: Number of "citable" articles published in the past two years.


  • Impact factor was first proposed by Eugene Garfield (who is a chemist, librarian, and linguist by training) in a 1955 article in the journal Science.
  • Garfield saw impact factor as a way to "eliminate the uncritical citation of fraudulent, incomplete, or obsolete data by making it possible for the conscientious scholar to be aware of criticisms of earlier papers."
  • For Garfield's reflections on the impact factor over fifty years after its invention, see his article in JAMA entitled, "History and Meaning of the Journal Impact Factor".

Finding Impact Factors in Journal Citation Reports

Getting Started

  • Go to Journal Citation Reports (often called JCR) at the ISI Web of Knowledge Spotlight on Web of Knowledge page.
  • Choose to view either the impact factor of a specific journal or a comparison of journals in a particular subject area.

JCR splash page


Change the Sorted By option from Journal Title to Impact Factor
Example: In the subject area of Obstetrics and Gynecology, the highest impact factor for 2006 belongs to Human Reproduction Update at 6.793.

JCR journal summary list screenshot

Click on the journal title to see the numbers and formulas JCR used to calculate that number:

Impact factor calculations

Science Gateway

Also check out the Science Gateway. Here, you can find lists of high-impact journals in various subject areas, from Acoustics to Veterinary Sciences.

Strengths & Weaknesses


  • Impact Factor gives researchers a quantitative measure of journals' influence and impact.
  • Impact Factor is a simple metric, and provides a consistent way of comparing journals.
  • Ranking is consistent within fields of study.


  • Impact factor gets misused alarmingly often. Though it was only intended to measure a journal's influence, people sometimes use it to judge the influence of the researchers themselves.
  • Even though impact factor is supposed to measure a journal's prestige and value, it can be affected by a number of things unrelated to a journal's quality.
    • Examples: Journals self-citing, publication timing, and types of articles published.
  • Since one cannot calculate impact factor with less than two years of data, new journals are left out of impact factor lists.
  • The impact factor only includes journals indexed by Thomson Reuters Scientific.
  • Thomson Reuters Scientific, which updates impact factors every year, does not share their criteria for what constitutes a "citable paper," which is part of the impact factor equation's denominator.
  • The numbers vary quite a bit by subject area.
    • Example: The journal with the highest Impact Factor in the field of medicine from 2006 is the New England Journal of Medicine, at 51.296. In contrast, the highest Impact Factor in the field of nursing in 2006 belongs to Birth: Issues in Perinatal Care at 2.058. As you can see, Impact Factor is not to be used to compare journals in different fields of study.


Reading Factor

  • Measures electronic journal use by people in a given field.
  • Unlike the impact factor, it measures use by information seekers, as opposed to use by published researchers.
  • Seeks not to measure a journal's quality, but to measure how much interest the journal generates within a given group of users.

H-Index, also known as the Hirsch Index

  • Seeks to measure the output of researchers.
    • Note the difference between this goal and the goal of the Impact Factor: I.F. measures the influence of scholarly journals, while the H-Index measures the influence of the researchers themselves.
  • The H-Index was first proposed by J.E. Hirsch, a physicist at the University of California at San Diego.

Immediacy Index

  • Like the impact factor, except that its formula only includes the number of cites in the most recent year, instead of incorporating the previous two years.
  • Available through Web of Knowledge: Once you have found a title in the Journal Citation Reports, click on it. A few headings down, you will see the journal's Immediacy Index, along with the formula for calculation.


  • Short for "Reliability-Based Citation Impact Factor."
  • Seeks to quantify a journal's effectiveness.
  • Incorporates citation data over the lifespan of the journal, instead of the journal's recent performance history.

Further Reading

Arizona State University's Citation Research LibGuide.

Darmoni, Stefan J. Reading factor: A new bibliometric criterion for managing digital libraries. Journal of the Medical Library Association 90(3) 323-327. July 2002.

Editorial Policy Committee of the World Association of Medical Editors. "WAME Policy Statements." Accessed 10/7/07.

Garfield, E. "Citation Indexes for Science: A New Dimension in Documentation through Association of Ideas". Science 122(3159). July 15, 1955. Accessed 10/7/2007.

Garfield, Eugene. "The History and Meaning of the Journal Impact Factor." Journal of the American Medical Association 295(1) 90-93, January, 2006.

Gugliotta, Guy. "The Genius Index: One Scientist's Crusade to Rewrite Reputation Rules." Wired 17(6) 92-, May 22, 2009. Accessed 6/15/2009.

Impact Factor Game. PLoS Medicine Vol. 3, No. 6, e291

Kuo, Way, and Jason Rupe. "R-Impact: Reliability-Based Citation Impact Factor." IEEE Transactions on Reliability, Sept. 2007, 56(3), p366-367.

Martyn, Christopher. "Advice to a new editor." British Medical Journal, 17 March 2007, 334 (7593), p. 586.


Page last updated: June 18, 2012
Questions or suggestions? Contact Ann Combs