Many of the discrepancies with journal impact factors are eliminated altogether in another ISI database called the Journal Performance Indicators (JPI). This annual compilation now covers the period 1981 to 2004. Unlike JCR, the database links each source item to its own unique citations. Therefore, the impact calculations are more precise. Only citations to the substantive items are counted in the denominator. And it is possible to obtain cumulative impact measures covering longer time spans.
For example, the cumulated impact for JAMA articles published in 1999 was 84.5. This was derived by dividing the 31,257 citations received (from 1999 to 2004) by the 370 articles published in 1999.
(31,257 Citations received 1999-2004) / (370 Articles published in JAMA) = 84.5

In 1999, JAMA published 1905 items of which 680 were letters, and 253 editorials. Citations to these items were not included in the JPI calculation of impact.