How many kids in the world have ADHD? According to a recent study in the journal Pediatrics the answer is about 7 percent. But a study in the Journal of Child Psychology and Psychiatry earlier this year found that only 3.4 percent of kids have ADHD. And the Centers for Disease Control claims it’s as many as 11 percent in the U.S. Estimates of how many kids have ADHD vary widely, says Sheldon Horowitz, director of LD resources at the National Center for Learning Disabilities. That’s very confusing for parents. There’s a simple reason for why the numbers vary, however. These studies are based on research that uses many different ways to determine who has ADHD. “The incidence of ADHD is very much influenced by the way in which it was identified,” Horowitz says. The latest research looked at more than 170 studies. These studies included records of more than a million children in North America and Europe over a 36-year period. But critics say the 7 percent finding may be flawed. That’s because the studies used many different standards to decide if a child had ADHD. Some doctors determine a child has ADHD based on how his parents and teachers describe his behavior. Others do so by observing him over time or even in a brief office visit. Still others may decide a child has ADHD based on how he responds to medication. But there’s no set way to decide whether a child has ADHD. “There’s no blood test, brain scan, no X-ray, and no definitive way to say whether you’re in or out,” Horowitz says. “There is no screening other than the doctor’s eyes on the kid and what behaviors they choose to rule in or rule out.” Horowitz says doctors often use one of half a dozen or so questionnaires to help screen for ADHD. Commonly used questionnaires include the Vanderbilt, the Conners and the Achenbach. But the criteria differ in each. The result is that we have many different estimates of how many kids have ADHD. And a big problem with that is that it leaves parents and teachers uncertain. “If I’m a parent, I’m going to say, ‘How can I know for sure if my child has ADHD?’” says Horowitz. “I’m going to want to know a lot more before I even make a request for my kid to be evaluated.” But this problem may be solved someday soon. Neuroscientists and medical researchers are starting to take a closer look at ADHD. They want to find out how best to identify and treat it. “All of those areas of research and inquiry are getting to a better place,” Horowitz says. And once everyone agrees about how to identify ADHD, figuring out how many kids have it should be easier. Any opinions, views, information and other content contained in blogs on Understood.org are the sole responsibility of the writer of the blog, and do not necessarily reflect the views, values, opinions or beliefs of, and are not endorsed by, Understood.