From the Initiative for Open Citations (I4OC) Blog:
The analyses show that, of 956,050,193 references from journal articles stored at Crossref, 486,041,671 (50.84%) are now in the category “Open”, and are freely available for third parties to download and use for any purpose.
This is a significant milestone for the Initiative for Open Citations (I4OC, https://i4oc.org/), which since early 2017 has been campaigning for scholarly publishers to open their reference lists, and a major gain for the world of open scholarship.
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Crossref has records of 93,184,372 works with DOIs, of which 69,699,144 (74.80%) are journal articles and 23,485,228 (25.20%) are works that are not journal articles (i.e. book chapters, proceedings articles, datasets, etc.).
Crossref stores 1,075,133,743 references from all 93,184,372 works, averaging 11.54 references per work.