There's no single screening test that tells exactly who has ADHD and who doesn't. In fact, ADHD symptoms may differ slightly from one person to the next. The diagnosis of ADHD is based on three common types of behavior:

The first ADHD symptom is inattention. An inattentive person quickly gets bored with tasks that aren't highly enjoyable. This lack of focus makes it hard to think ahead or to learn something new. According to The National Institute of Mental Health (NIMH), signs of inattention include:

• Easily getting distracted by irrelevant sights and sounds
• Failing to pay attention to details and making careless mistakes
• Rarely following instructions carefully and completely
• Losing or forgetting things

The second ADHD symptom is hyperactivity. Hyperactive people can't sit still. They fidget and squirm or even roam about the room. They may try to do several things at once, accomplishing none.

The third ADHD symptom is impulsivity. Impulsive people don't think before they act. They may blurt out inappropriate comments or rush into the street without looking. It's hard for them to take their turn or to wait for things they want. According to the NIMH, signs of hyperactivity and impulsivity are:

• Feeling restless, often fidgeting with hands or feet, or squirming
• Running, climbing, or leaving a seat, in situations where sitting or quiet behavior is expected
• Blurting out answers before hearing the whole question
• Having difficulty waiting in line or for a turn

Not every inattentive, hyperactive, or impulsive child or adult has ADHD. Nearly all kids -- and grownups -- act these ways from time to time. But people with ADHD consistently exhibit these behaviors over a period of time.

To be considered ADHD, these behaviors must appear before age 7 and go on for at least six months. However, the symptoms may not be identified by teachers and parents and the diagnosis may not be made until later in childhood.