Dr. Sheffield is a clinical psychologist who specializes in treating pathological gambling. Pathological gambling is defined as being unable to resist impulses to gamble. Bothered by not having a good measure that he can give to clients to determine whether they are suffering from this condition, he creates a new measure of pathological gambling. The measure has 15 questions, and it takes 20 minutes to complete. Dr. Sheffield wants to establish the discriminant validity of his pathological gambling measure. He gives his measure and three others to a group of 100 people. Which of the following provides the best evidence for discriminant validity?