FYI, ADD is a term from like, the 70s. It's improper to use that term in a clinical sense, not that you pills care.
ADHD comes in three different subtypes:
Predominantly Inattentive Type (six more or criteria in the Inattentive category (DSM-IV-TR)).
" " Hyperactive-Impulsive Type (six more or criteria in the H-I category).
Combined Type, which is six or more in both.
Plus the child has meet some "overall criteria," like having the issues in more than one setting, and have the issues persist for more than 6 months.
http://www.cdc.gov/ncbddd/adhd/symptom.h tm <------- Actual list of criteria.
People who think ADHD is a myth have been proven wrong through brain imaging studies. While there is no known "cause" (or cure) per se, kids who are correctly diagnosed with the disorder typically have different brain structures and brain activity.