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 2007 "impact factor," as enumerated in
the current edition of
Journal
Citation Reports®. The 2007 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 27-year period
(right-hand column).