Nat makes a great point. There are courses which I've chosen not to do the exercises on but which I just watched the lectures for (or plan to, so have kept enrolled even though course is finished and I've "failed").
If I pick two MOOCs of the eight to continue with, saying I have a 25% completion rate is nonsense.
I'm currently "enrolled" in eight courses on edX alone that start in the next couple of months.
I have no intention of doing all eight. I'd struggle to manage three at a time.
But the nature of MOOCs lets me try them all before I narrow down the two or three I want to focus on.
I frequently drop out of MOOCs because:
MOOCs let me evaluate this after I've enrolled (and been counted as a enrollment statistic).
No one would measure "book completion" by how many people who "Looked Inside" a book on Amazon read the entire book.
Enrolling in a MOOC isn't like enrolling in a college course, it's like looking at the course catalog and thinking a course might be interesting.
Critics of MOOCs on this basis, however, miss the point.
MOOCs register students at a much higher point in the funnel than most other course delivery approaches.
A lot has been made of MOOC Completion Rates (and how low they are).
I've completed 10 MOOCs and probably started four or five times that. For a given course, the average completion rates are much lower: often low single digit percentages.