Impact factors have always been a popular way to measure the influence of academic journals. They have been popularized by ISI, now part of Thomson. RePEc also provides impact factors, and this post is about explaining the differences between the two.
ISI takes a sample of journals and analyzes the citations across those journals. To be eligible, a citations has to appear within two years of the publication of the cited article, the cited article must be printed (not forthcoming, a working paper or a manuscript), and the cited article must be among the analyzed journals (286 in Economics). ISI is currently experimenting with a five year window, in addition to the existing two-year window.
RePEc considers all publications listed in its bibliographic database. Thus, it also considers other publication forms than journal articles: close to 1000 journals and 2600 working paper series. It imposes no time window, citations of any age qualify. In most cases, a citation of a working paper will count towards its published form once the article is included in RePEc, possibly after the original citation (condition: at least one author has both versions in his/her RePEc profile). This implies that working paper series and book series can also have impact factors. RePEc is thus more comprehensive.
However, the pool of citations RePEc is drawing from is different. It relies very much on working papers (who can later be published), as they are typically openly accessible. Some publishers also provide references in the bibliographic metadata, but not all. One implication of this is that RePEc is more current as it includes citations to and from research that is not yet published. As research gets published, this data gets updated. But as references from many journals are missing, RePEc citation data must still be treated as experimental. Whether these omissions matter remain to be seen. After all, impact factors always have to be considered in relative terms, not in absolute terms, and if omissions were not biased, they would not matter.
Another major difference is that RePEc excludes self-citations. This is an important issue as some journals, explicitly or implicitly, encourage authors to cite other articles published within the two year window in the same journal. Thus, just as self-citations are excluded for authors, they are excluded for journals. And this can matter a lot.
Finally, the impact factor is determined by divided the eligible citations by the number of eligible articles. ISI determines itself what articles are eligible for the denominator, and this can even be negotiated with the publisher. In RePEc’s case, if an article (or a working paper) is listed, it counts without adjustment.
RePEc also publishes variations on the “simple” impact factor: recursive impact factors, where every citation counts with the impact factor of the citing publication, this favors impact over numbers; discounted impact factors, where the impact of a citation decays with time (regardless of the age of the cited item; and a combination of the two, discounted recursive impact factors. Finally, there is now also the h-index. All variations have a different story to tell about the publication, and RePEc offers the reader the choice.
Posted by Christian Zimmermann 
The RePEc Blog 