  This article uses Google Scholar (GS) as a source of data to analyse Open
Access (OA) levels across all countries and fields of research. All articles
and reviews with a DOI and published in 2009 or 2014 and covered by the three
main citation indexes in the Web of Science (2,269,022 documents) were selected
for study. The links to freely available versions of these documents displayed
in GS were collected. To differentiate between more reliable (sustainable and
legal) forms of access and less reliable ones, the data extracted from GS was
combined with information available in DOAJ, CrossRef, OpenDOAR, and ROAR. This
allowed us to distinguish the percentage of documents in our sample that are
made OA by the publisher (23.1%, including Gold, Hybrid, Delayed, and Bronze
OA) from those available as Green OA (17.6%), and those available from other
sources (40.6%, mainly due to ResearchGate). The data shows an overall free
availability of 54.6%, with important differences at the country and subject
category levels. The data extracted from GS yielded very similar results to
those found by other studies that analysed similar samples of documents, but
employed different methods to find evidence of OA, thus suggesting a relative
consistency among methods.
