I will give Alzheimer's as an example. You may use the word mild when the person has some memory loss and begins to withdraw socially. DAT is moderate when the person wanders out of the house and does not know how to get home or where he lives. You may use the word severe to describe DAT when the person becomes nonverbal, has to be fed through a tube, and has long periods of unconsciousness. Our text has a beautiful section on the danger of labeling.
Diagnosis can be extremely difficult and psychiatric diagnoses are notorious for being unreliable.
Some symptoms, like hearing voices, will always get one a diagnosis of schizophrenia. But other behaviors are more elusive. We will find out this semester that females outnumber males when we look at depression or anxiety, but males outnumber females as substance abusers. Our text discusses unstructured versus structured interviews, and the structured interview does yield more reliable data for making a diagnosis. However, if they do not ask the right questions, their guts can be wrong.