The IF is calculated as follows:
IF = the number of citations in year 3 to articles published in years 1 and 2, divided by the number of citable articles published in years 1 and 2.
(Note that citations are in any of the journals indexed by Thomson Reuters. Note that articles published and citable articles are from the journal being given the IF.).
i.e. IF = citations in 2012 to articles published in 2010 and 2011 ÷ citable articles published during 2010 and 2011.
So if 100 citations were made (in all the journals indexed) during 2012 to the 50 articles published (by journal X) during 2010 and 2011.
IF = 100/50 = 2.
The IF of journal X is 2.
Impact factor
IF = the number of citations in year 3 to articles published in years 1 and 2, divided by the number of citable articles published in years 1 and 2.
(Note that citations are in any of the journals indexed by Thomson Reuters. Note that articles published and citable articles are from the journal being given the IF.).
i.e. IF = citations in 2012 to articles published in 2010 and 2011 ÷ citable articles published during 2010 and 2011.
So if 100 citations were made (in all the journals indexed) during 2012 to the 50 articles published (by journal X) during 2010 and 2011.
IF = 100/50 = 2.
The IF of journal X is 2.
Impact factor
