Does Open Access Really Increase Impact? A Large-Scale Randomized Analysis

Through this paper, we have analysed the question of Open Access Citation Advantage (OACA) in the Web of Science database. To do so, we compared citation impact (MNCS) of 2,458,378 publications in fully OA journals to that (weighted MNCS) of a control group of non-OA publications (#10,310,842). Similarly, we did the same exercise for OA publications in hybrid journals (#1,024,430) and their control group (#11,533,001), over the period 2010-2020. The results showed that there is no OACA for publications in fully OA journals, and that there is rather a disadvantage. Conversely, the OACA seems to be a reality in hybrid journals, suggesting that a better accessibility in this context tends to improve the impact of publications. Another important finding of our study is that 2016 appears to be a turning point on how an easier accessibility to publications influences their visibility, with a strong and regular downward trend from this year onwards, regardless of the type of journal. Nevertheless, the analysis by discipline showed great discrepancies, both in the initial level of OACA and its evolution over the time. It could still be noticed that the most important falls are for disciplines where the initial level was the higher. Finally, the effect of using an adjusted control group is clearly demonstrated, even if it has no effect in the trends observed