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).