What is a journal "impact factor"?

Journals use the numerical "Impact Factor" as an indirect quantitative measure of a journal's importance in the medical field and scientific literature. Thompson Scientific calculates the impact factor scores annually. This score provides journals with bragging rights, especially when it comes to marketing. Be aware that there are ways to manipulate the numbers a little and thus brings the true value of this score into question.

How is the impact factor calculated?
The impact factor is a calculation of how frequent a journal's articles are cited in a 2-year period. As an example, the 2009 impact factor for a journal would be:

Impact Factor = A / B

A = Number of times 2007-08 articles are cited from a given journal
B = Number of total "citable items" published in given journal during 2007-08

The ambiguous issue is how the denominator of "citable items" is determined. Basically articles which qualify as potentially citable items include original research, reviews, proceedings, and notes. These do not include such items as editorials, coresspondences, and errata. Sometimes it's unclear which articles don't qualify. The more articles that you exclude, the smaller your denominator and thus the higher (and better) the impact factor.

Below are impact factors of several journals, relevant to those interested in publishing in EM and medical education. In addition to impact factors, you should also consider the journal's general focus when deciding where to submit your manuscript. If you read through several back-issues, you will get a sense of each journal's "flavor":