Understanding the Effect Size of ADHD Medications: Implications for Clinical Care

Stephen V. Faraone, PhD


September 18, 2003

In This Article

Number Needed to Treat

Another statistic having a straightforward interpretation is the number needed to treat, which is the number of patients who need to be treated to prevent one bad outcome. It is computed as follows:

Number Needed to Treat = 1
Percent Improved on Drug -
Percent Improved on Placebo

If the formula gives a noninteger number, we would round it to the next higher number. In the context of ADHD, the number computed from the formula is the number of ADHD patients a physician would need to treat to be assured that he or she has prevented one deleterious outcome with the treatment. In this example, the deleterious outcome would be not showing improvement in ADHD symptoms. Ideally, the number needed to treat would be 1, which would mean that every patient improved to a point where they were better than 90% of untreated patients. However, due to incomplete responses in some treated patients and the presence of placebo responses, the number needed to treat usually exceeds 1. For the classes of medicine used to treat ADHD, number needed to treat values are 4 for the 2 stimulant classes and 7 for nonstimulants.


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