FAQ: What is a journal impact factor and why does it matter?

A journal’s impact factor is used to evaluate the relative importance of a journal in its field.  The higher the impact factor, the more prestigious the journal.  From a researcher’s point of view, ‘high impact’ journals are usually perceived to be the ‘best’ and most career enhancing places to be published.

The impact factor is a statistical measure; a journal with a high impact factor will have received more citations than one with a low impact factor.

A journal’s impact factor is calculated as follows:

2008 impact factor = A/B, where:

• A = the number of times articles published in 2006 and 2007 were cited by indexed journals during 2008.
  (“Indexed journals” are those indexed by the Thomson Reuters Web of Knowledge database)
  
• B = the total number of “citable items” published by that journal in 2006 and 2007.
  (“Citable items” are usually articles, reviews, proceedings, or notes; not editorials or letters to the editor.)

(Wikipedia)

Although widely recognised in academic circles as a measure of journal quality, the use of journal impact factors is controversial.  Some criticisms include:

• impact factors are dependent on citation practices which differ from one discipline to another; it is therefore not appropriate to compare impact factors across disciplines
• citation practices themselves suffer from a number of limitations (see my earlier post for examples)
• impact factors relate to the journal as a whole, not to the individual articles within them, nor to the authors of those articles; a very highly cited paper can increase the impact factor of a journal, hiding the variation in quality of other papers
• the impact factor is sensitive to the definition of “citable items” and to editorial practice in choosing how many and which types of article to publish; it is recognised that certain types of article (e.g. review articles) are more likely to be cited than others (The PLoS Medicine Editors, 2006)

Because of these limitations, HEFCE has announced that impact factors will play no part on the forthcoming REF:

“No sub-panel will make any use of journal impact factors, rankings, lists or the perceived standing of publishers in assessing the quality of research outputs. “ (HEFCE, 2012)

HEFCE’s ambition is to assess all research outputs, irrespective of discipline, on a ‘fair and equal basis’.  In most cases this means through peer review of individual research outputs (although in some areas citation data will provide supplementary evidence).  Whether the reviewers’  perceptions of journal quality, based in part on their prior knowledge of impact factors, has an unplanned role to play in this process remains to be seen.

Image credit: Wiley Asia Blog on Flickr