Monday, May 14, 2007

The ADHD Bell Curve

“My seven year-old son is inattentive, off-task, distractible and has a short attention span. At home, he hops from one activity to another, never picks up anything … sort of an energizer bunny. The school suggested that we have him tested for ADHD. Does he have ADHD?”

“I was told (I love this one because the person who told them is never identified) that if your child was a climber as a toddler, then they have ADHD, that you can tell true ADHD in a child if he cannot fall asleep at night … if he stays up late.”

My first thought?, to be redundant, the doctors and nurses at the hospital should have told you about these things when you brought the baby home from the hospital. Children are inattention, they do not stay on task, they hop from one activity to another. Toddlers climb and infants are not always great sleepers.

The issue is not whether a child demonstrates or does not demonstrate a specific behavior or problem, but whether the frequency, duration and/or intensity of identified behaviors or problems are within the middle of the bell curve (the norm).

An understanding of the bell curve is critical to the concept of diagnosis. Ever heard of the bell curve? Well, this is what we know; all human performances (what people do) follow the normal bell curve. An example, a few people are fast, a few people are slow, but most people fall within the middle. A few people are strong, a few people are weak, but most people fall within the middle. The bell curve can be graphed as performance (horizontal axis) by the number of people (vertical axis) and when we graph these two factors, the graph looks like a bell with the highest number of people falling in the middle (called the mean) and progressively fewer people as one moves towards the extremes or tails.

The task is not to identify whether your child is inattentive (because we know that he is) but how much does his inattention (performance) differ or deviate from the mean or average child of the same age.

It turns out that not all children who are average fall exactly on the mean so we need to establish a “tape measure” to determine how much a child deviates from the mean. This measure of deviation from the mean (or “gold standard”) is called the standard deviation. About 2/3 children fall within about one standard deviation above or below the mean. This group of children is called the “middle of the bell curve” or average range.

So, does your child have clinically significant features of inattention? It depends on how much his behavior or problems differ from the mean. Behaviors that deviate from the norm by more than two standard deviations (less than about 5% of the children) are generally labeled as having features that are clinically significant or important. While the presence of clinically significant or important features of inattention (more prevalent than about 95% of children) is important to know, it still does not nail down an ADHD diagnosis.

My point? Objective measurements of behaviors or problems with subsequent comparison to age norms (bell curves by age) is the first step in understanding whether a behavior is a clinically significant issue. In the absence of standardized observations and normative comparisons, the diagnostic process boils down to the “mean” and “standard deviation” that that the professional has in his or her head based on clinical experiences. Maybe part of our confusion regarding diagnostic processes is because each clinician develops his or her own norms (means) including normal ranges (standard deviations) without recognition of their inherent bias.

Do your child’s behavior problems fall within the norm? Does your child still drive you crazy even if it’s not clinically significant? Hmmmm…the doctors and nurses should also have told you about that part too!


Dr. Richard Dowell is a Neuropsychologist located in Pennsylvania. Dr. Dowell evaluates upwards of 400 children and adolescents each year. In addition, Dr. Dowell is recognized as one of the top Forensic Neuropsychological witnesses in the North East.

Dr. Dowell can be contacted at

For more information on Neuropsychology visit

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