Aliakbar Akbaritabar and Stephan Stahlschmidt have self-archived "Merits and Limits: Applying Open Data to Monitor Open Access Publications in Bibliometric Databases."
Here's an excerpt:
Identifying and monitoring Open Access (OA) publications might seem a trivial task while practical efforts prove otherwise. Contradictory information arise often depending on metadata employed. We strive to assign OA status to publications in Web of Science (WOS) and Scopus while complementing it with different sources of OA information to resolve contradicting cases. We linked publications from WOS and Scopus via DOIs and ISSNs to Unpaywall, Crossref, DOAJ and ROAD. Only about 50% of articles and reviews from WOS and Scopus could be matched via a DOI to Unpaywall. Matching with Crossref brought 56 distinct licences, which define in many cases the legally binding access status of publications. But only 44% of publications hold only a single licence on Crossref, while more than 50% have no licence information submitted to Crossref. Contrasting OA information from Crossref licences with Unpaywall we found contradictory cases overall amounting to more than 25%, which might be partially explained by (ex-)including green OA.