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The SPARC open access group monitors studies tracking the citation advantage of open access (AO) articles and has found that out of 70 OA studies, 46 found a citation advantage for articles available via open access.
Research by Ottaviani (2016) published in PLoS ONE, discusses the flaws in a number of studies that suggested that OA articles attract more citations, but still found an open access citation advantage as high as 19%.
Ottaviani, J. (2016). The post-embargo open access citation advantage: It exists (probably), its modest (usually), and the rich get richer (of course). PLoS ONE, 11(8), e0159614. doi:10.1371/journal.pone.0159614