Clinical research & EBM · Reference
What is the number needed to treat?
The number needed to treat is a measure of an intervention’s effect, defined as the number of people who would need to receive it for one additional person to benefit. It is the reciprocal of the absolute risk reduction and helps express effect sizes intuitively.
How NNT is defined
The number needed to treat is the reciprocal of the absolute risk reduction (ARR), the difference in the rate of an outcome between the control group and the intervention group. If an intervention lowers the outcome rate by an absolute 5 percentage points (an ARR of 0.05), the NNT is 1 ÷ 0.05 = 20: on average 20 people would need to receive it for one additional good outcome. A smaller NNT indicates a larger effect. NNT is presented as a methodological measure of effect size, not as advice about whether any individual should receive an intervention.
Why NNT is useful
NNT is valued because it is more intuitive than relative measures. A relative risk reduction of 50% sounds impressive but says nothing about how common the outcome is; the same relative reduction can correspond to a very large or a very small absolute benefit. By anchoring the effect in absolute terms, the NNT makes the scale of benefit clearer and guards against overstating effects from relative figures alone. It is therefore widely used to communicate the practical size of an effect found in a trial.
Cautions when interpreting NNT
NNT must be read with care. It depends on the baseline risk in the population studied, so an NNT from one trial does not transfer automatically to populations with different underlying risk. It also depends on the time horizon over which the outcome is measured, so an NNT is only meaningful alongside its follow-up period. Like any estimate it carries uncertainty and should be reported with a confidence interval. The mirror-image measure for harms is the number needed to harm (NNH), the reciprocal of the absolute risk increase of an adverse outcome.
Key facts
At a glance
- Definition: People treated for one additional good outcome
- Formula: NNT = 1 ÷ absolute risk reduction
- Smaller NNT: Indicates a larger effect
- Contrast: More intuitive than relative risk reduction
- Depends on: Baseline risk and follow-up time
- Harm version: Number needed to harm (NNH)
Common questions
FAQ
How is the number needed to treat calculated?+
The NNT is the reciprocal of the absolute risk reduction — that is, 1 divided by the difference in outcome rates between the control and intervention groups. For example, an absolute risk reduction of 0.05 gives an NNT of 20.
Why is NNT preferred over relative risk reduction?+
A relative risk reduction does not reveal how common the outcome is, so the same percentage can mean a large or tiny absolute benefit. NNT anchors the effect in absolute terms, which makes the practical size of the benefit clearer and harder to overstate.
Does an NNT from one study apply everywhere?+
No. The NNT depends on the baseline risk of the population and on the follow-up period over which the outcome was measured. An NNT should always be interpreted with its time horizon and its underlying population in mind, and reported with a confidence interval.
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