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PLoS One. 2016 Apr 22;11(4):e0154199. doi: 10.1371/journal.pone.0154199. eCollection 2016.

Reasons for Journal Impact Factor Changes: Influence of Changing Source Items.

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Laboratory for Tumour Biology and Experimental Therapies (TREAT), Institute for Physiology and Pathophysiology, Paracelsus Medical University, Strubergasse 21, 5020, Salzburg, Austria.
Department of Internal Medicine I, Paracelsus Medical University / Salzburger Landeskliniken, Müllner Hauptstrasse 48, 5020, Salzburg, Austria.
Research Office, Paracelsus Medical University, Strubergasse 21, 5020, Salzburg, Austria.


Both the concept and the application of the impact factor (IF) have been subject to widespread critique, including concerns over its potential manipulation. This study provides a systematic analysis of significant journal Impact Factor changes, based on the relative contribution of either one or both variables of the IF equation (i.e. citations / articles as the numerator / denominator of the quotient). A cohort of JCR-listed journals which faced the most dramatic absolute IF changes between 2013 and 2014 (ΔIF ≥ 3.0, n = 49) was analyzed for the causes resulting in IF changes that theses journals have experienced in the last five years. Along with the variation by number of articles and citations, this analysis includes the relative change of both variables compared to each other and offers a classification of `valid`and `invalid`scenarios of IF variation in terms of the intended goal of the IF to measure journal quality. The sample cohort features a considerable incidence of IF increases (18%) which are qualified as `invalid`according to this classification because the IF increase is merely based on a favorably changing number of articles (denominator). The results of this analysis point out the potentially delusive effect of IF increases gained through effective shrinkage of publication output. Therefore, careful consideration of the details of the IF equation and possible implementation of control mechanisms versus the volatile factor of number of articles may help to improve the expressiveness of this metric.

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