People who finished the course earn more. Cause?

Question

An online education platform reports that students who complete its coding course earn, on average, 20 percent more a year later than students who enrolled but dropped out. It markets this as the salary boost from finishing the course.

Why is completing-versus-dropping a biased comparison, and what would a cleaner test look like?

Accepted Answer

Finishers are a self-selected group. Completing a demanding self-paced course takes persistence, free time, financial stability, and prior preparation. Those same qualities, grit, resources, background, tend to raise earnings on their own, regardless of the course content. So the people who finish are exactly the people who would probably out-earn the dropouts anyway. Comparing completers to non-completers measures the kind of person who finishes at least as much as the effect of the material. This is self-selection bias. The hidden common cause. A single underlying factor (say, discipline or…

People who finished the course earn more. Cause?

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