1. Learning on demand: Online education in the United States, 2009;Allen,2009
2. Class differences: Online education in the United States, 2010;Allen,2010
3. Changing course: Ten years of tracking online education in the United States;Allen,2013
4. Getting the mix right again: an updated and theoretical rationale for interaction;Anderson;International Review of Research in Open and Distance Learning,2003
5. Learning environments;Anderson,1974