Saturday, December 22, 2007

Show Me The Data! Show Me The Data! Show Me The Data!

Show Me The Data
Mike Rossner, Heather Van Epps, and Emma Hill
The Journal of Cell Biology, Vol. 179, No. 6, 1091-1092

The integrity of data, and transparency about their acquisition, are vital to science. The impact factor data that are gathered and sold by Thomson Scientific (formerly the Institute of Scientific Information, or ISI) have a strong influence on the scientific community, affecting decisions on where to publish, whom to promote or hire, the success of grant applications, and even salary bonuses. Yet, members of the community seem to have little understanding of how impact factors are determined, and, to our knowledge, no one has independently audited the underlying data to validate their reliability.

Calculations and negotiations
The impact factor for a journal in a particular year is declared to be a measure of the average number of times a paper published in the previous two years was cited during the year in question. For example, the 2006 impact factor is the average number of times a paper published in 2004 or 2005 was cited in 2006. There are, however, some quirks about impact factor calculations that have been pointed out by others, but which we think are worth reiterating here:


3. Some publishers negotiate with Thomson Scientific to change these designations in their favor . The specifics of these negotiations are not available to the public, but one can't help but wonder what has occurred when a journal experiences a sudden jump in impact factor. For example, Current Biology had an impact factor of 7.00 in 2002 and 11.91 in 2003. [snip]

4. Citations to retracted articles are counted in the impact factor calculation. [snip]

5. Because the impact factor calculation is a mean, it can be badly skewed by a "blockbuster" paper. [snip]

When we asked Thomson Scientific if they would consider providing a median calculation in addition to the mean they already publish, they replied, "It's an interesting suggestion...The median...would typically be much lower than the mean. There are other statistical measures to describe the nature of the citation frequency distribution skewness, but the median is probably not the right choice." Perhaps so, but it can't hurt to provide the community with measures other than the mean, which, by Thomson Scientific's own admission, is a poor reflection of the average number of citations gleaned by most papers.

6. There are ways of playing the impact factor game, known very well by all journal editors, but played by only some of them. For example, review articles typically garner many citations, as do genome or other "data-heavy" articles [snip]. When asked if they would be willing to provide a calculation for primary research papers only, Thomson Scientific did not respond.

As journal editors, data integrity means that data presented to the public accurately reflect what was actually observed. [snip]

Thomson Scientific makes its data for individual journals available for purchase. 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 ... . 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 [snip]

Your database or mine?
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). [snip]

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.

Opaque data
It became clear that Thomson Scientific could not or (for some as yet unexplained reason) would not sell us the data used to calculate their published impact factor. If an author is unable to produce original data to verify a figure in one of our papers, we revoke the acceptance of the paper. We hope this account will convince some scientists and funding organizations to revoke their acceptance of impact factors as an accurate representation of the quality—or impact—of a paper published in a given journal.

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.



1 comment:

Ulf-Dietrich Reips said...

Thank you for this very interesting investigation on the Thomson calculations!

In my opinion journal impact is valued too highly, as opposed to *paper impact* and *author impact*. I say it is a greater achievement to have a highly cited paper in a low impact journal than publish a paper in a high impact journal that then never gets cited.

Keep up the good work