WASHINGTON — The Food and Drug Administration announced on Monday that it had approved the first brain wave test to help diagnose attention deficit hyperactivity disorder in children.

The test uses an electroencephalogram, or EEG, with sensors attached to a child’s head and hooked by wires to a computer to measure brain waves. It traces different types of electrical impulses given off by nerve cells in the brain and records how many times those impulses are given off each second.

The test takes 15 to 20 minutes, and measures two kinds of brain waves — theta and beta. Certain combinations of those waves tend to be more prevalent in children with A.D.H.D., the Food and Drug Administration said in a news release.

The disorder is one of the most common behavioral disorders in children. About 9 percent of adolescents have A.D.H.D. and the average age of diagnosis is 7, the drug agency said, citing the American Psychiatric Association. Children who have it tend to be hyperactive, impulsive and exhibit behavioral problems.