Although Impact Factors are based on cites to articles published in the
previous two years, average citation rates can be calculated using older
or longer time periods.
A base of five years may be more appropriate for journals in certain
fields because the body of citations may not be large enough to make reasonable
comparisons, publication schedules may be consistently late, or it may
take longer than two years to disseminate and respond to published works. The 5year Impact Factor is already calculated for JCR years 2007 and later. You may calculate the 5year Impact Factor using an earlier JCR year as a starting point by following these steps:

Find the journal for which you want to calculate a fiveyear Impact Factor.
Go to the Journal page.

Click the Impact Factor Trend button.
Scroll down the page to find the number of articles published in the
past five years. Add up the numbers to find the total. For example,
if you start out in JCR Science Edition 2003, you need to find the total
number of articles published in 2002, 2001, 2000, 1999, and 1998.

Click the Return to Journal. On the Journal page, click the Cited
Journal Data button to go to the Cited Journal Table.

Look in the All Journals row at the top of the table. Skip the first
two columns (All Yrs and JCR year). Add up the numbers in the next
five columns. For example, if the JCR year is 2003, you want find
the total of the numbers found in columns 2002, 2001, 2000, 1999,
and 1998.

Divide the total number of citations found in step 5 by the total
number of articles found in step 3. This is the fiveyear Impact Factor.

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