Completion rate
Completion rate is the percentage of enrolled students who finish a course, calculated by dividing completions by total enrolments. In open online courses it tends to be very low.
It is the metric that best predicts referrals, testimonials and repeat purchases. Students who finish buy again; students who drop out rarely do.
The levers that move it most are structural: short lessons, a clear goal per module, reminders at the right moment, and some form of closure such as a certificate or final project.