How we rate: is impact factor the most important measure?
First, the good news
The International Journal of Clinical Practice now has an impact factor (IF) of 2.427, a figure that has gone up for the 5th year in a row. This is a large improvement from the IF of 2.007 reported in 2008. The International Journal of Clinical Practice is now ranked 30th out of the top 150 journals in the General and Internal Medicine category. Below is a graphic (courtesy of Tiffany Brake) showing the IF for 2008 through 2012.
How is the IF calculated?
Eugene Garfield proposed the concept in a landmark 1955 Science article , and has written profusely on the topic [2-4]. The IF of a journal is the average number of citations received per paper published in that journal during the two preceding years. For example, if a journal has an IF of 2.427 in 2012, then its papers published in 2010 and 2011 received 2.427 citations each, on average, in 2012. The formula used is as follows:
These ‘citable items’ are usually research articles or reviews. Excluded from ‘citable items’ are editorials and letters to the editor. IFs for 2012 were published in June 2013, allowing for the processing time required to capture all citations contained in indexed journals in 2012.
Is the IF important?
The higher the IF, the more influential that journal is thought to be. An assumption is that journal articles that are often cited by other journal articles are more important than journal articles that are not cited as often. Publishing in high IF journals can be important for academic departments at medical schools when evaluating faculty for promotion and tenure. Libraries may look at journal IF when deciding which subscriptions to renew. There may even be financial incentives to publish in high IF journals, as described by Shao and Shen  where, at Zhejiang University in China, first authors get a monetary award for publishing in indexed journals, with rewards being substantially greater for higher IF journals.
The journal impact factor measures citations, not downloads; however both are important
Caveats regarding the IF
Publishing in a high IF journal will not guarantee that an article will garner many citations . There is wide variability in citation rates for individual articles. Moreover, a highly cited paper in a specialty journal may not necessarily be well known to practicing clinicians.
In addition, there are several ways to ‘game’ the IF. Publishing many review articles, which are generally cited more often than most research reports, will increase a journal's IF. Recalling that the IF is calculated by dividing the numerator of all citations by the denominator of ‘citable items,’ it is possible to reduce the number of ‘citable items’, which can thus increase the IF. This is made possible because items which are not considered to be ‘citable’ and thus not included in the denominator can still contribute to the numerator when cited. Some of the most prestigious journals with high IFs devote many pages to editorials, news stories and letters to the editor. Another tactic is to publish the ‘gems’ early in the year so that more time is available to accumulate citations; the ‘duds’ would be relegated to the December issue.
Perhaps a more ‘real-world’ measure of the importance of a journal and its content is the metric of how many times it is accessed and its articles downloaded from the publisher's internet site. This is already calculated and made available publically by publishers such as the Public Library of Science (PLoS) and Dove Medical Press. Additional metrics reported include the number of mentions on social media such as Facebook and Twitter. Stay tuned.
In the past 36 months, Leslie Citrome has engaged in collaborative research with, or received consulting or speaking fees, from: Alexza, Alkermes, AstraZeneca, Avanir, Bristol-Myers Squibb, Eli Lilly, Envivo, Forest, Genentech, Janssen, Lundbeck, Merck, Mylan, Novartis, Noven, Otsuka, Pfizer, Reckitt Benckiser, Shire, Sunovion and Valeant.