A double dissociation between impact factor and cited half life.

ratio between the number of citations in a given year to any item published in that journal in the previous two years and the number of research items published in the same journal in the same two years. IF, widely acknowledged as the standard measure for scientific quality, places great emphasis on current research, favouring fast moving disciplines, while penalising slower moving topics; the latter would fare much better were the citation interval longer. It is well known within the " journalology " (Lock, 1989) discussion that the Journal Citation Reports (JCR) 2-year period for calculation of IFs does not provide as complete a picture as would a 5-or 10-year period, in fact journal representing slow pacing disciplines, like physiology, have on average a much lower IFs than, for instance, physics. Indeed, when journals are assessed across disciplines, physiology journals fare much better as the number of years considered to calculate IF increases. The usual counter argument is that, precisely for this reason, for all purposes journals should be evaluated within categories and not across categories (e.g., Garfield, 2006). When this is done, the ranking based on 1, 7, or 15 year IF do not differ significantly (Garfield, 1998). However, this is where the problems rest with neuropsychology journals. Currently, neuropsychology journals are not ranked within their own category but are listed together with other disciplines. Neuropsychology is a typical slow-pace and interdisciplinary topic; hence authors may choose to submit their papers to mainstream neuropsychology journals as well as to neuroscience or clinical neurology journals. One way to examine this issue is to compare the IF for neuropsychology and neuroscience and contrast this with the cited half-life (CHL) for these two disciplines. (The CHL is a measure of citation survival measuring the number of years, going back from the current year, that covers 50% of the citations in the current year of the journal – Garfield, 2001). This is what was done by Della Sala and Crawford (2006) using the IF and CHL figures for 2004. The analysis revealed that Neuroscience had a higher IF than neuropsychology but that the position was reversed for CHL. Although these effects were large and statistically significant, it would be useful to examine their robustness. The recent publication of the IF and CHL data for 2005 provides an opportunity to do this. The 2005 figures were used to compare the IFs and CHLs for …