Fab Rojas’ response to my last post, reprinted with his permission.

I just read your post about the 10% of students who do nothing in a
college course. They don’t attend, take exams or other appear in any
other capacity. I teach a lot of required classes and I’m the Director
of Undergraduate Studies at my department, so
I have a bit of experience working with these students. 

My observation is that most “no-shows” fall into two categories.
First, many students have drug or alcohol problems. One of the realities
of college these days is that a large minority of students treat it is a
giant party. Unsurprisingly, many of these
students are unwilling or unable to participate in their
courses. Second, many students lack maturity. For whatever reason, they
simply can’t follow through on plans, or deal with challenging classes. 

A small fraction of “no-shows” do have legitimate reasons. A few
genuinely believed that they dropped the course. Others have very
serious personal challenges such as being the victim of sexual assault,
personal illness, or a severe family problem, like
having parents who are getting a divorce. 

The instructors who read the blog may wonder how to distinguish
between these students. Physicians, or the campus health service, will
usually provide a note on letter head verifying illness. Many campuses
have “Student Advocate” offices for students who
are having genuine personal problems. They are usually happy to provide
verification, long as it doesn’t violate confidentiality.

Brief reply from Bryan: Very plausible, but you still usually need student myopia and/or perverse parental incentives to explain why these students fail to officially withdraw from their classes, saving many thousands of dollars.