Thousands of people sign up for online classes they never end up taking

My first massively open online course ended recently, and I just can’t stop asking multiple-choice questions, The Atlantic reports.

Here’s one: Which of the following statements might be true?
1) Two-thirds of those enrolled never showed up
2) More than half of the students earned a passing grade
It’s obviously a trick question since the answer is “both.” The apparent contradiction is entirely dependent on another, perhaps bigger question, one that is often phrased as a challenge—if not to to the idea of MOOCs, to the idea of their value: What good is a class where only 2 percent of the students bother to finish?
Or, to put it a little more quantitatively: What denominator should we use in computing student participation, engagement, and completion in a course like this, when the numerator is going to be the number who passed (in my case, 1,196)?
While there are plenty of ways to answer that, the one I decided to try—in keeping with the modality of a MOOC—was asking the students.
So, halfway through “Understanding Media by Understanding Google,” my Northwestern course on Coursera, that’s what I did.
And though the 302 students who replied didn’t entirely agree, the preponderance of the evidence pointed me to a different answer than any of those I first offered as possibilities. Should it be, I asked them, based on the number of people who watched even one lecture—or all the lectures? How about the number who tried the first quiz? Or should we just stick with that great big enrollment number?