Practitioners are certified to perform a medical procedure, such as an orthodontic procedure. Certified practitioners are maintained in a referral directory and classified within tiers based on criteria such as the number of procedures that they have performed. Inquiries are solicited from prospective patients, and referral lists are provided to those patients who request them. The referral lists are obtained from the referral directory with doctors from higher tiers who have performed more procedures receiving preferential inclusion on the referral lists.