Of the MOOCs I've participated in, the student surveys have never asked the most relevant question I can think of with respect to completion rates: "do you (or did you) intend to complete the course?"
Very good point. Of course, for a completely fair comparison you need to ask the same question (before the course starts) to the traditional students. I expect it can't be much more than a percent or so, but it depends on whether the university penalizes students for signing up for a course they don't finish (in my years of college, the CS department didn't seem to care very much. It was very different for other departments, though).
Of course, without penalties, you will get students that start out a semester following more courses than they can finish, only to decide later on which one to drop. I'm not entirely sure how that translates to MOOCs, though.
That's a big point - I have joined both courses that matter to me and courses that seem fun or relate to my side interests, where I just want to watch a few of the lectures.
In the first part, I have 4/5 completion (and I'll finish the missing one when it will be run next time); but for the second part, naturally, I have 0 out of 20+ courses.
Anybody here ever been asked that?