The professor is talking about clinical psychology. In order to know how behavior patterns can be changed. The clinical psychologist has to know what causes the client to behave the way he or she does. Identifying the cause is called diagnosis. In diagnosis, a psychologist uses two basic tools, interviews and psychological tests. Through interviews and tests, the psychologist tries to classify the problem to see if it falls into any non-categories. A psychologist may also attempt to describe the client's personality in terms of how he or she deals with life, for example, some people like to lead, and some prefer to follow the lead of others. Some people are active and outgoing, while others are quiet and reflective. In a diagnostic interview, the psychologist takes the client's case history. This means learning how the client got along with parents, teachers, and friends, as well as how the person handle difficult situations in the past Psychological testing is the other way that a psychologist tries to diagnose the client’s problems. Clinical psychologists have developed tests that can help them learn about a person's intelligence and personality, as well as tests that show whether a person's behavior or perception is influenced by emotions, disabilities, or other factors. Personality testing is useful in discovering how the client tries to adjust to life. Personality tests can reveal unconscious feelings.