EXPLANATION OF IMPACT FACTORS AS A MEASURE
The Institute of Scientific Information (ISI),
a commercial company based in Philadelphia, collates information
on citations of articles in several thousand of the world's major
science journals. Each year, they produce a Science Citation Index.
This lists each journal covered and ranks it according to average
numbers of citations to its articles. In year x, it takes account
only of articles cited during year x and published during years
x-1 and x-2. Thus, it assesses only articles published and cited
during a specific window of time. An article that is so ground-breaking
that it is cited in its year of publication will not be included
(although it is likely to continue to feature well during the
relevant two years). Equally, an article whose worth is only recognised
later, or which continues to be cited for many years, will not
necessarily have full influence on the impact factor.