As the authors point out, doctoral programs have a high non-completion rate (40% and 70%). The dropout rate for on-line students (not based on a campus) is even higher (another 10% to 20%). The authors examined if a "private doctoral workspace" would improve this.
The term "private doctoral workspace" seems to be a misnomer, perhaps "individual doctoral workspace" would be better, as the idea is that this can be used for the student to communicate with their supervisors.
The authors looked at the literature on doctoral programs in the North America which have a coursework component followed by research and contrasted this with UK, Europe and Australia, where there is typically no coursework. They also looked at the more recent Australian Professional Doctorate which includes fieldwork and applied research. The authors do not appear to have been able to find any studies as to the relative success of the different forms of program in terms of completion rate.
One curious aspect of this study is that the authors do not appear to have considered a role for student-to-student interaction as a way to reduce isolation of doctoral students. The assumption is that a doctoral student works alone carrying out research under the supervision of a panel of supervisors, so only need to communicate with the supervisors. However, researches rarely work alone and so it would seem to make sense for students to learn to work with their future colleagues.
If the aim is to reduce isolation then relying on one supervisor, or a small panel, does not seem a good idea. A batter approach, I suggest, would be for their to be a team of supervisors looking after a group of doctoral students.
Athabasca University's MEd capstone provides an example of an on-line program which has students in a team with multiple supervisors. The students complete an e-portfolio in stages, with peer and supervisor support.
Ames, C. C., Berman, R., & Casteel, A. (2018). A Preliminary Examination of Doctoral Student Retention Factors in Private Online Workspaces. International Journal of Doctoral Studies, 13, 079-107. URL http://ijds.org/Volume13/IJDSv13p079-107Ames4060.pdf