For the colleges overall, students had the best completion rates with 40% online courses. For institutions with higher completion rates the optimum is 60% online. Most surprisingly, students at institutions with a lower overall completion rates should still do 10% online courses.
The authors don't present an explanation as to why a mix of face-to-face and online courses improves outcomes. It may be that the highly scaffolded nature of online courses helps struggling students. However, it may be just that students with conflicting demands on their time, an online course is the only way to keep studying. Some of my ICT Sustainability students have been in this category, where they were not able to be on campus and so their options were are an online course, or no-completion of their degree.
What the study doesn't say is what proportion of the learning for the face-to-face courses is actually online. An Australian higher education course labeled "face-to-face" is likely to have a significant proportion of online learning. This supports my rule of thumb is that about 80% of Higher Education can be done online.
SHEA, Peter; BIDJERANO, Temi. Online Course Enrollment in Community College and Degree Completion: The Tipping Point. The International Review of Research in Open and Distributed Learning, [S.l.], v. 19, n. 2, may. 2018. ISSN 1492-3831. Available at: <http://www.irrodl.org/index.php/irrodl/article/view/3460/4568>. Date accessed: 05 May. 2018. doi:http://dx.doi.org/10.19173/irrodl.v19i2.3460.