The above table compares the citation impact of journals in a given field as measured over three different time spans. The left-hand column ranks journals based on their 2000 “impact factor,” as enumerated in the current edition of the ISI Journal Citation Reports®. The 2000 impact factor is calculated by taking the number of all current citations to source items published in a journal over the previous two years and dividing by the number of articles published in the journal during the same period—in other words, a ratio between citations and recent citable items published. The rankings in the next two columns show impact over longer time spans, based on figures from the ISI Journal Performance Indicators. In these columns, total citations to a journal’s published papers are divided by the total number of papers that the journal published, producing a citations-per-paper impact score over a five-year period (middle column) and a 20-year period (right-hand column).

source: http://www.in-cites.com/research/2001/oct_15_2001-2.html