Journal impact factors: a 'bioequivalence' issue?

Aims Journal impact factors (IMFs) are used increasingly by institutions as performance indicators of the quality of 'individual research output'. Alt hough the need for discretion when using the numbers has been emphasized, t here has been little formal analysis of the issues. We therefore investigat ed citation profiles for three clinical pharmacology journals to assess the validity of using IMF as a measure of 'individual research'. Methods We compared the pattern of individual citations for random samples of 120 papers published in Clin Pharmacol Ther (CPT), Br J Clin Pharmacol ( BJCP) and Eur J Clin Pharmacol (EJCP) in 1981, 1991, 1995 and 1996. Using a n analogy between citation-time profiles of papers and concentration-time p rofiles of drugs, it was possible to define 'lag-time', C-max, t(max), t(1/ 2) and AUC(t), and to investigate 'bioequivalence'. Results Citation distributions for individual publications were widely vari able and skewed (skewness = 1.47, 2.16 and 1.37 for CPT, BJCP and EJCP, res pectively). The 90% CI values for the IMF of a publication in each journal (i.e. 90% CI for an observation as opposed to 90% CI for the mean) were 0.2 4-16.94, 0.08-10.3 and 0.09-5.68. Conclusions IMF does not represent the impact of an individual paper. Furth ermore, if the comparison of journals is treated as a bioequivalence issue, the citation data should be log transformed prior to calculating IMF such that they represent the likelihood of citation for the median article. Afte r such transformation, absolute differences between the IMF of clinical pha rmacology journals become much smaller.