How many Wikipedia references are available to read? We measured the proportion of open access sources across languages and topics. – Wikimedia Foundation

"Let’s say you’re planning a trip to a subtropical region and you want to learn about available vaccines for yellow fever. You look up the English Wikipedia article. You’re lucky to find a well-sourced section, with a wealth of references, many of them pointing to information from public health agencies and reputable news articles. Great!

But then you decide to learn more about the scientific research on the treatment of yellow fever. You try to follow three citations, each a review article in a scholarly journal, and find that they’re all behind a paywall and inaccessible to you.

As it turns out, you’re not alone. When following a link to the official version of a scholarly article, Wikipedia readers are twice as likely to hit a paywall than one they can freely read."