Impactitis: new cures for an old disease
Although impact factors for journals have been around for quite a while, there is little doubt that they are increasingly perceived as important for measuring the quality of journals and of research, both by editors and authors, sometimes to the point of obsession. In today's rapidly changing world of scientific publication, we would like to review the impact factor concept, and critically evaluate its importance, sometimes with our tongues in our cheeks! Having concluded that the impact factor is an imperfect factor, we explore some potential alternatives.
How are impact factors calculated?
The impact factor was invented by Eugene Garfield as a simple method for comparing journals, regardless of their size.1 The impact factor of a journal is calculated as the number of citations in a certain year to papers in the same journal in the two years before (numerator), divided by the number of “source” (citable) items published in that journal in the same two years (denominator). To give an example, the impact factor for 2001 will be calculated from the total number of citations in 2001 to papers published in the years 2000 and 1999, divided by the total number of citable items in the years 2000 and 1999. “Citable” is important here, because not all items published in a journal are considered to be citable by ISI, The Institute for Scientific Information (www.isi.com), which is responsible for compiling the impact factors. Research papers and short reports will be counted as citable items, but editorials, reviews, and letters to the editor are usually not. However, this is not completely predictable because ISI is independent and known to have a mind of its own, and may therefore not follow the allocation of papers to certain categories by an individual journal. To give an example, we traditionally call review papers “leaders”,2–8 which …
