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 2008 "impact factor," as enumerated in
the current edition of
Journal Citations Report®.
The 2008 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
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 28-year period (right-hand column).