The Impact Factor [IF] of scientific journals is a measure published by Thomson Reuters and represents the average number of citations of articles in a particular journal during a given year.
For example, an IF of a journal in 2008 is the product of the division: the number of times that all items published in that journal in 2006 and 2007 were cited by indexed publications during 2008 in the total number of "citable items" published by that journal in 2006 and 2007.
The calculations are made solely on the journals cataloged in the database of the Institute for Scientific Information (ISI) Web of Knowledge.
The IF is not, and was never intended to be, a direct measure of the quality and importance of the journal. Furthermore, the IF of journals which publish a specific researcher’s work cannot serve as a measure of the quality or importance of his/her work.
The IF was created to serve as a tool for librarians in deciding which scientific journals to stock in their libraries. Today, with the availability of all journals online, this need has decreased significantly.
The IF of a journal can be influenced by many factors. First, the number of articles or "citable items" that constitutes the denominator of the equation for the calculation is crucial. There are types of articles, such as methods articles and review articles that tend to be cited more often and raise the total number of citations counted for the journal. In addition, a journal can have a high IF based on a few heavily-cited articles and its IF does not necessarily reflect the number of citations the author could expect if choosing to publish in the journal.
Without going into the subject of IF manipulation , a high IF reflects a broad readership, which is not suitable for every article. Many articles focus on a narrow subject that is of interest to a limited readership and therefore the IF can never be a sole consideration for choosing a target journal for ones’ manuscript. Proper use of the IF is assessing it against the IFs of other journals within a specific research area and including it within a wider array of considerations, which I will detail in the next post.