What is the impact factor, anyway?
Version of Record online: 27 OCT 2011
Copyright © 2011 Wiley Periodicals, Inc.
Volume 25, Issue 3, page 283, April 2012
How to Cite
McKerahan, T. L. and Carmichael, S. W. (2012), What is the impact factor, anyway?. Clin. Anat., 25: 283. doi: 10.1002/ca.21291
- Issue online: 28 FEB 2012
- Version of Record online: 27 OCT 2011
- Manuscript Accepted: 22 SEP 2011
- Manuscript Revised: 21 SEP 2011
- Manuscript Received: 26 AUG 2011
Much has been made about the impact factor of a given journal (including Clinical Anatomy) but relatively few people know what the impact factor is and what it means. It is a metric supplied by Thomson Reuters via their Journal Citation Report® product that ostensibly measures a journal's average citation per article. However, the calculation of that average is subject to limitations that take into account the content and age of the affected articles. The impact factor was originally produced by the Institute for Scientific Information® (ISI), which was taken over by Thomson and is now known by a variety of aliases, including ISI, Thomson ISI, Thomson Scientific and Thomson Reuters, to name a few.
The impact factor is a number that is derived from a simple formula. The numerator is the number of cites a journal receives (from journals indexed by Thomson Reuters) in a specific year to all items that journal published in the preceding 2 years. The denominator is the number of “citable items” published by that journal during the two prior years.
For the impact factors that were released by Thomson Reuters in June 2011, the numerator was restricted to citations received in 2010 to items published in 2008 and 2009, and the denominator was restricted to citable items published in 2008 and 2009. Impact factors are referred to by the citation year; hence, the impact factors released in June 2011 are known as “the 2010 impact factor scores.”
Citable items (also known as “source” or “substantive” items) are the subset of a journal's publications that contribute to the scientific record. These are typically original research articles and review articles. By contrast, publications such as editorials, letters to the editor, book reviews, and memorial articles are not considered citable items and, therefore, are not counted in the denominator of the impact factor equation. Despite their name, noncitable items are occasionally cited; when that happens, the citation is included in the impact factor numerator but is not offset by a presence in the denominator. Such a citation is known as a “free cite.”
Impact factors have become common shorthand for journal quality and are consequently important in a number of ways. One crucial factor is that they help librarians to determine which journals should be maintained in their libraries and which could be dropped. Potential authors also consult impact factor scores to determine where to submit their next manuscript. In fact, some academic institutions require that their faculty publish in journals with an impact factor above 1.0 to be considered for academic advancement. Some people have considered these uses of the impact factor as an abuse because impact factors can be manipulated and potentially misused (Marks, 2003).
Clinical Anatomy's 2010 impact factor numerator (number of citations earned in 2010 to all items published 2008–2009) was 310—including 16 free cites—and its denominator (number of citable items published 2008–2009) was 230, resulting in a score of 1.348. This was an improvement of almost 25% over the journal's 2009 score, and it marked Clinical Anatomy's second consecutive year above the 1.0 milestone.
There are other metrics that are used to quantify the “quality” of a journal. These include, but are not limited to, the Eigenfactor, immediacy index, cited half life, article influence, and source-normalized impact per paper scores. Suffice it to say that the impact of the impact factor is broad as well as deep: it is the main metric used by academics such as the readers of this journal, even if this is not always appropriate.
The authors appreciate the constructive comments of Ms. Dottie Hawthorne, Outreach Librarian, Mayo Clinic Libraries, and of Ms. Jenny Neophytou, Bibliometrics Analyst, John Wiley & Sons.