Table 1. Open access type definition.

Definitions of types of access used in this analysis.

Type of open access	Definition
Gold OA	Published version is immediate OA. APC charged.
Hybrid OA	Publication is subscription-based. APC can be paid to make individual articles OA.
Bronze OA	Currently free to read on publisher’s site but licence not clear.
Green OA	Only accessible in a reputable repository (i.e., academic social networking sites are not included). Publisher’s version is paywalled.
Closed	Published version is paywalled.
Diamond OA	Published version is immediate OA. Derived from Unpaywall giving the status ‘Gold’ but where DOAJ shows no APC charged.