Can you please explain why add is considered a subtype of adhd?

DSM-IV defines both: Psychologists and psychiatrists gather together to define mental disorders. The definition allows researchers to try different solutions to solving the same problem. Research encourages new definitions which are compiled in a book known as the dsm. Today, we are close to the 5th edition. The third edition - dsm-3 had two disorders add & hyperactivity. The current edition dsm-iv combined them.

Not a subtype: You have it reversed, due to confusing set of initials. 75% of people with ADD are inattentive. Only 25% are hyperactive. So ADHD is really a subtype of ADD.