Self-Selection Bias

There are 80 students in one of my classes. I passed back an exam today:

Mean score for the 63 students present in class today: 78.2
Mean score for the 17 students absent from class today: 63.3

What a surprise. Many of those who weren't there today will ask how they could do better. Uh, start by showing up and we'll go from there. I like this too from Hypothetical Bias. [Sorry. I needed to get that out. Back to work...]