To be considered, the symptoms must have appeared by the age of six to twelve and occur in more than one environment (e.g. at home and at school or work).  The signs must be not appropriate for a child of that age[6][98] and there must be evidence that it is causing social, school or work related problems.  

Most children with ADHD have the combined type. Children with the inattention subtype are less likely to act out or have difficulties getting along with other children. They may sit quietly, but without paying attention resulting in the child's difficulties being overlooked.

Diagnosis of ADHD and Spectrum Disorder

Diagnostic and Statistical Manual DSM IV - As with many other psychiatric disorders, formal diagnosis is made by a qualified professional based on a set number of criteria. In the United States these criteria are defined by the American Psychiatric Association in the Diagnostic and Statistical Manual of Mental Disorders (DSM). Based on the DSM-criteria, there are three sub-types of ADHD: 

  1. ADHD predominantly inattentive type (ADHD-PI) presents with symptoms including being easily distracted, forgetful, daydreaming, disorganization, poor concentration, and difficulty completing tasks. Often people refer to ADHD-PI as "attention deficit disorder" (ADD), however, the latter has not been officially accepted since the 1994 revision of the DSM.

  2. ADHD, predominantly hyperactive-impulsive type presents with excessive fidgetiness and restlessness, hyperactivity, difficulty waiting and remaining seated, immature behavior; destructive behaviors may also be present.

  3. ADHD, combined type is a combination of the two other subtypes.

This subdivision is based on presence of at least six out of nine long-term (lasting at least six months) symptoms of inattention, hyperactivity–impulsivity, or both.