Over the years, several Impact Factor versions based on Thomson Reuters data have been developed: 5 year Impact factor (number of annual citations divided by the number of journal publications in the five years prior) and Eigenfactor (represents an average score in which citations from highly ranked journals have a higher weight than those from poorly ranked journals).

A journal impact factor for a specific year (2015 for instance) is calculated by dividing the number of citable publications in a journal in the previous two years (2013-2014) by the total number of citations of these publications in the specific year (2015). An impact factor of 3 represents 3 citations in average for each citable publication that was published in a journal two years prior to the specific year. Please note that the true Impact Factor for a specific  year (2015) is actually only published in the following year (2016) – an Impact Factor cannot be calculated before the end of 2015 and before all the journals that were published during 2015 have been updated in bibliographic database.