K refers to a certain year, Nk- 1+Nk-2 refers to the number of papers published in the previous two years, and Nk- 1 and Nk-2 refers to the number of cited articles in K years. In other words, the impact factor of a journal in 2005 is the total number of cited papers published in 2004 and 2003 divided by the total number of papers published in 2004 and 2003 (citable papers).

The impact factor of a periodical is a quantitative index to characterize its influence. That is to say, the average number of citations of each paper in a journal is actually the ratio of the number of citations of all papers published in a journal in the first two years of a certain year to the number of citations of all source papers published in the journal in the first two years.