How to measure infoproducts effectiveness

Effectiveness is a function of client results. During the purchase process the seller promise to the customer to eliminate a certain problem with his infoproduct. Does this result come into fruition? 100 * (n of buyers that complete the course) / (n of buyers that complete the course and eliminates the problem) = Effectiveness Why don’t count people that don’t complete the course? It’s their responsibility. Anyway the seller has a market incentive to facilitate completion. Those who eliminate the problem, will buy again from him. The others won’t.