Proper measurement of the quality of research requires a thorough understanding of the subject, balanced evaluation of evidence (which may take years to acquire), and ultimately consensus among experts. All in all, a tall order—as shown by the decades which the Nobel Prize Committee may take to recognise achievement and by the controversy which often follows its decisions.

Enter the impact factor, which at first sight is a welcome solution to this conundrum.1 The impact factor has become the global currency for a journal's scientific standing and, by implication, of the papers it publishes. Available at the click of a mouse (http://scientific.thomson.com/isi/) from the Institute of Scientific Information and updated every year, the impact factor has three decimal place precision and an impressive range from close to zero to over 30. Some journals delight in flaunting their impact factors, and when the big …