What Dr. Sheffield is doing is testing the validity of the psychological measurement of pathological gambling. The validity testing that he is engaging in is called criterion validity. Criterion-based validity is a type of validity where you test the constructed measurement’s validity against a criterion that has been proven to have a relationship with the measured variable.
In this case, Dr. Sheffield’s testing of his measurement on people who are in gambling anonymous vs. people who are in alcoholics anonymous and the subsequent result shows that his measurement can be used to measure the pathological gambling variable since high scorers for his measurement can be found only in the people who are in the gambling anonymous group.