"An Introduction to Open Educational Resources" by Abbey Elder is licensed under a CC BY 4.0 International license: https://creativecommons.org/licenses/...
Additional Research on OER Efficacy is available form the Open Education Group:
“The foundation of Open Education is Open Educational Resources (OER), which are teaching, learning, and research resources that are free of cost and access barriers, and which also carry legal permission for open use. Generally, this permission is granted by use of an open license (for example, Creative Commons licenses) which allows anyone to freely use, adapt and share the resource—anytime, anywhere. “Open” permissions are typically defined in terms of the “5R’s”: users are free to Retain, Reuse, Revise, Remix and Redistribute these educational materials.” - SPARC
There are a lot of things out there that are free, but that does not make them OER. Free does not equal open. To be considered OER, the material must have an open license. Open licenses enable what are called the 5R permissions: