Is Your Loyalty Program Really Worth It?

Dear Dr. pROfIt: Our company has a loyalty program: customers get five points for every $100 they spend. Once a customer acquires 30 points, he/she receives a free gift and a $20 gift certificate. Customers have to enroll in the program in order to receive the benefits. Our CEO wants me to measure the impact of the loyalty program on sales, and thinks I should simply compare the annual spend between loyalty customers and nonloyalty customers. Is this the right approach?