Commentary on Impact Factors in JCB

What goes into a journal's impact factor? It turns out that this is a good question. These impact factors are calculated by Thomsom Scientific and attempt to quantify the import of any particular scientific journal.

But did anyone read this commentary in the December 17th issue of JCB? It's a revealing look as to how impact factors are compiled.

From the article:

With the aim of dissecting the data to determine which topics were being highly cited and which were not, we decided to buy the data for our three journals (The Journal of Experimental Medicine, The Journal of Cell Biology, and The Journal of General Physiology) and for some of our direct competitor journals. Our intention was not to question the integrity of their data.

When we examined the data in the Thomson Scientific database, two things quickly became evident: first, there were numerous incorrect article-type designations. Many articles that we consider "front matter" were included in the denominator. This was true for all the journals we examined. Second, the numbers did not add up. The total number of citations for each journal was substantially fewer than the number published on the Thomson Scientific, Journal Citation Reports (JCR) website (, subscription required). The difference in citation numbers was as high as 19% for a given journal, and the impact factor rankings of several journals were affected when the calculation was done using the purchased data (data not shown due to restrictions of the license agreement with Thomson Scientific).

When queried about the discrepancy, Thomson Scientific explained that they have two separate databases--one for their "Research Group" and one used for the published impact factors (the JCR). We had been sold the database from the "Research Group", which has fewer citations in it because the data have been vetted for erroneous records. "The JCR staff matches citations to journal titles, whereas the Research Services Group matches citations to individual articles", explained a Thomson Scientific representative. "Because some cited references are in error in terms of volume or page number, name of first author, and other data, these are missed by the Research Services Group."

When we requested the database used to calculate the published impact factors (i.e., including the erroneous records), Thomson Scientific sent us a second database. But these data still did not match the published impact factor data. This database appeared to have been assembled in an ad hoc manner to create a facsimile of the published data that might appease us. It did not.

Rossner et al conclude:

Just as scientists would not accept the findings in a scientific paper without seeing the primary data, so should they not rely on Thomson Scientific's impact factor, which is based on hidden data. As more publication and citation data become available to the public through services like PubMed, PubMed Central, and Google Scholar®, we hope that people will begin to develop their own metrics for assessing scientific quality rather than rely on an ill-defined and manifestly unscientific number.

Wow. I urge you to read the rest of the article as Rossner et. al. point out some other troubling issues with impact factors.

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