Crossref as a new source of citation data: A comparison with Web of Science and Scopus

"The Initiative for Open Citations (I4OC) encourages scholarly publishers to make the references found in their journals and books openly available through Crossref. With a few exceptions (most notably the American Chemical Society, Elsevier, IEEE, and Wolters Kluwer Health), almost all large publishers support the initiative. So far, this support has resulted in approximately half of all references deposited in Crossref being openly available, yielding about half a billion open references."