The open access citation premium may depend on the openness and inclusiveness of the indexing database, but the relationship is controversial because it is ambiguous where the open access boundary lies

Do open access (OA) documents benefit from a citation premium in comparison to traditional subscription-based articles? The question has been debated during the last two decades, as OA is gaining momentum and becoming an ever more established option. Without coming to a shared position, the literature on the topic has essentially split into two clusters: on the one hand, the studies that endorse the occurrence of an OA citation advantage; on the other hand, the works suggesting that it has a negligible extent, or is due to other confounding factors. The primary aim of this study is not to bring new evidence in favor or against the citation premium supposedly characterizing OA articles. Instead, this work is meant to test a specific hypothesis connected with the OA citation advantage, namely, that OA papers may benefit from higher citations in open indexing databases (e.g., Google Scholar) rather than in selective indexing engines (i.e., Scopus and Web of Science). The empirical findings, although conflicting, show that the hypothesis above is not misplaced since a few confirmatory results are achieved. However, the hypothesized relationship remains controversial, also because of an uncertain boundary between OA and paywall articles.