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 2006 "impact factor," as
enumerated in the current edition of
Journal Citation Reports®. The 2006
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
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 26-year period (right-hand column).