Over the years, Thomson Reuters employees have answered thousands of questions about the Journal Impact Factor, how it is calculated, what does it mean, how are the data prepared. We are continuing to broaden our efforts to maintain open and cooperative communication with scholars, publishers and librarians to support the proper use of the Journal Impact Factor. The September 9, 2009 issue of JAMA-Journal of the American Medical Association contains a Commentary prepared by members of Thomson Reuters Journal Citation Reports team. This writing describes in detail the determination of citable items (the denominator of the Journal Impact Factor calculation), and how the distinction between citable and non-citable items affects the Journal Impact Factor.
The first 150 words of the article can be viewed at the following link. Access to the full text is available to JAMA subscribers.
UPDATE: JAMA-Archives has made this item freely available - subscription is no longer required.