What is Abnormality?

With respect to a research study to answer the DB prompt question, you defined abnormality as that which has a deleterious impact on functioning, and then selected a case study approach. Just so I understand fully, let me ask you this… if you were to observe an individual, Bob, engaging in Behavior X, Behavior Y, and Behavior Z, and you had ample time to also interview Bob (be as thorough as needed) >> how would you decide which of those behaviors (if any) were abnormal?