"Our estimates show that at least 114 million English-language scholarly documents are accessible on the web, of which Google Scholar has nearly 100 million. Of these, we estimate that at least 27 million (24%) are freely available since they do not require a subscription or payment of any kind. "

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0093949

Ob diese auf die Auswertung von Google Scholar und MSN gestützten Ergebnisse methodisch haltbar sind?