What is Journal Impact Factor?

The impact factor, often abbreviated IF, is a measure of the citations to science and social science journals. It is frequently used as a proxy for the importance of a journal to its field. The Impact factor was devised by Eugene Garfield, the founder of the Institute for Scientific Information, now part of Thomson, a large worldwide US-based publisher. Impact factors are calculated each year by Thomson Scientific for those journals which it indexes, and the factors and indices are published in Journal Citation Reports. The publication of each year covered occurs in the summer of the following year.For example impact factors for 2008 will be published in the summer of 2009.

Calculation

The impact factor of a journal is calculated based on a two-year period. It can be viewed as the average number of citations in a year given to those papers in a journal that were published during the two preceding years. For example, the 2003 impact factor of a journal would be calculated as follows:

A = the number of times articles published in 2001-2 were cited in indexed journals during 2003
B = the number of "citable items" (usually articles, reviews, proceedings or notes; not editorials and letters-to-the-Editor) published in 2001-2

2003 impact factor = A/B

(note that the 2003 impact factor was actually published in 2004, because it could not be calculated until all of the 2003 publications had been received.)

A convenient way of thinking about it is that a journal that is cited once, on average, for each article published has an IF of 1 in the expression above.

There are some nuances to this: ISI excludes certain article types (so-called 'front-matter' such as news items, correspondence, and errata) from the denominator. Thomson Scientific does not have a fixed rule for which types of articles are considered "citable" and which front-matter.

New journals, that are indexed from their first published issue, will receive an Impact Factor after the completion of two years' indexing; in this case, the citations to the year prior to Volume 1, and the number of articles published in the year prior to Volume 1 are known zero values. Journals that are indexed starting with a volume other than the first volume will not have an Impact Factor published until three complete data-years are known; annuals and other irregular publications, will sometimes publish no items in a particular year, affecting the count. The impact factor is for a specific time period; it is possible to calculate the impact factor for any desired period, for which the web site gives instructions. Journal Citation Reports includes a table of the relative rank of journals by Impact factor, in each specific science discipline, such as organic chemistry or psychiatry.