The impact factor of a journal reflects the frequency with which the journal's articles are cited in the scientific literature. It is derived by dividing the number of citations in year 3 to any items published in the journal in years 1 and 2 by the number of substantive articles published in that journal in years 1 and 2
The 2011 impact factor of a journal would be calculated as follows:
A = the number of times articles published in 2009 and 2010 were cited by indexed journals during 2011
B = the total number of "citable items" published by that journal in 2009 and 2010. ("Citable items" are usually articles, reviews, proceedings, or notes; not editorials or Letters-to-the-Editor.)
2011 impact factor = A/B
(Note that 2011 impact factors are actually published in 2012; they cannot be calculated until all of the 2011 publications have been processed by the indexing agency.)
New journals, which are indexed from their first published issue, will receive an impact factor after two years of 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 get an impact factor until they have been indexed for three years. Annuals and other irregular publications will sometimes publish no items in a particular year, affecting the count.