Impact Factors

ISI Impact Factor

What is it: The original impact factor, developed by ISI, is a measure of the frequency with which the “average article” has been cited in a particular year. It is based on the most recent two years of citations. It is only supplied for journals indexed by ISI in the Web of Science [MIT only].

Sample 2006 Impact Factor Calculation:
Cites to recent articles (11388) divided by the Number of recent articles (3788) = 3.047

Availability: through subscription to ISI’s Journal Citation Reports, provided by the MIT Libraries.


What is it: The eigenfactor ranks journals as Google ranks websites; includes the journal price in its calculation; bridges discipline gaps; adjusts for citation pattern differences across disciplines (which ISI does not); assesses impact based on 5 years of data (as compared with ISI’s 2); and is free.

Availability: through, a non-commercial academic research project sponsored by the Bergstrom lab in the Department of Biology at the University of Washington, which aims to develop novel methods for evaluating the influence of scholarly periodicals and for mapping the structure of academic research.

See: The Eigenfactor Metrics: A Network Approach to Assessing Scholarly Journals [PDF]

Hirsch’s H index

What is it: Suggested by Jorge E. Hirsch, a physicist at UCSD, in 2005, its goal is to quantify scientific productivity and apparent scientific impact of a scientist/author. It can be applied to journal, author, or group, and assesses quality as well as quantity.

In comparison with the ISI Impact factor, The h index corrects for highly cited papers not in highly cited journals.

The h-index is based on a scientist’s most cited papers and the number of citations to their papers in other people’s publications. It is defined as:
A scientist has index h if h of his/her Np papers have at least h citations each, and the other (Np-h) papers have no more than h citations each.

Availability: Can be manually determined using free Internet databases, such as Google Scholar. One open source tool that calculates the h index using Google Scholar is “Publish or Perish.”

Also available via subscription-based databases such as Scopus (not available at MIT) and the Web of Science (available at MIT for the MIT community). To view the h index via Web of Science, search for a journal name (or author) and then click on “create citation report.”

More Information

  • Access to these impact factors is demonstrated in a video presentation “Publishing Smart.”
  • A Critique of the ISI Impact factor appears in “Show me the Data” by Mike Rossner, Heather Van Epps, and Emma Hill in The Journal of Cell Biology, Vol. 179, No. 6, 1091-1092.