A Correction Regarding Bibliographic Impact Factors
Hernán, Miguel A.
My recent editorial discussed the appropriateness of Thomson Reuters bibliographic impact factor (BIF) as a measure of a journal's scientific impact.1 As Porta did before,2 I warned that “a journal's BIF can be increased simply by publishing commentaries and other nonsubstantive articles that cross-cite themselves.” In fact, I calculated that Epidemiology's recent series of editorials, commentaries, and letters3–13 on the BIF had resulted in 28 “free” citations that will increase the numerator, but not the denominator, of Epidemiology's BIF.
Though my argument was generally valid, it turns out that my calculations were incorrect. As George Davey-Smith kindly reminded me (personal communication), citations to an article during the calendar year in which the article was published are not used in the calculation of the BIF. Thus the correct number of free citations resulting from Epidemiology's series on the BIF is not 28 but 31. This number includes 10 citations from the above paragraph. Publishing corrections may be embarrassing, but it increases a journal's BIF.
1. Hernán MA. Impact factor: a call to reason. Epidemiology. 2009;20:317–318.
2. Porta M. The bibliographic “impact factor” of the Institute for Scientific Information: how relevant is it really for public health journals? J Epidemiol Community Health. 1996;50:606–610.
3. Hernán MA. Epidemiologists (of all people) should question journal impact factors (with discussion). Epidemiology. 2008;19:366–368.
4. Szklo M. Impact factor: good reasons for concern. Epidemiology. 2008;19:369.
5. Porta M, Alvarez-Dardet C. How come scientists uncritically adopt and embody Thomson's bibliographic impact factor? Epidemiology. 2008;19:370–371.
6. Rothenberg R. The impact factor follies. Epidemiology. 2008;19:372.
7. Wilcox AJ. Rise and fall of the Thomson impact factor. Epidemiology. 2008;19:373–374.
8. Castelnuovo G. More on impact factors. Epidemiology. 2008;19:762–763.
9. Giuliani F, de Petris MP. More on impact factors. Epidemiology. 2008;19:763.
10. Kogevinas M. More on impact factors. Epidemiology. 2008;19:876.
11. Davey-Smith G, Shah E. More on impact factors. Epidemiology. 2008;19:876–877.
12. von Elm E. More on impact factors. Epidemiology. 2008;19:877–878.
13. Colditz IG, Colditz G. What are we BIF-fing about? Science needs impact metrics. Epidemiology. 2009;20:462.
© 2009 Lippincott Williams & Wilkins, Inc.