[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Epidemiology](https://durgical.com/en/epidemiology/) › [Measures of frequency and association](https://durgical.com/en/epidemiology/areas/measures/) › Number needed to treat

# Number needed to treat

- Measure

How many people must be treated, over a given time, to prevent one more event than without treatment.

Updated Nov 5, 2026

## Key points

- NNT = 1 / ARR (with the ARR as a proportion, not a percentage).
- Round up to the next whole number: 16.7 → 17.
- Always state the outcome and the period: an NNT of 17 over one year to prevent one fall.
- The lower, the more effective; the ideal NNT is 1.
- Higher baseline risk raises the ARR and lowers the NNT.

## What it is

The number needed to treat (NNT) is the inverse of the absolute risk reduction. It turns a trial result into an intuitive figure: how many patients must receive the intervention, over the study period, for one of them to benefit. A low NNT means a large effect. NNTs can only be compared between studies with the same outcome, the same period and similar populations.

## Formula and reading

- NNT = 1 / (CER − EER). With the ARR as a percentage: NNT = 100 / ARR.
- If the CI of the ARR includes 0, the CI of the NNT includes infinity and crosses over to harm.
- From the RRR: NNT = 1 / (RRR × CER).

## Worked example

- Data: in a care home, an exercise programme is tested for one year. Falls: 18% in controls and 12% with the programme.
- ARR = 0.18 − 0.12 = 0.06.
- NNT = 1 / 0.06 ≈ 16.7.
- Round up: NNT = 17 over one year.
- Another programme achieves an ARR of 0.025 over one year: NNT = 1 / 0.025 = 40.
- Reading: 17 residents must take part for a year to prevent one fall; the first programme does more than the second.

## Markers

- NNT
- 1/ARR

## How to read it

- The NNT helps explain benefits to patients in concrete terms.
- It is set beside the number needed to harm (NNH) to weigh benefit against harm.

## Common pitfalls

- Calculating the NNT from the relative reduction instead of the absolute one.
- Rounding down, which exaggerates the benefit.
- Comparing NNTs from studies with different periods or outcomes.

## Memory aid

One over the difference: subtract first for the ARR, then flip it and round up.

## Test yourself

Think of the answer, then tap the question to see it.

## Control risk 10% and treated risk 6%. What is the NNT?

ARR = 0.04; NNT = 1 / 0.04 = 25.

## Why is the NNT rounded up?

To avoid overstating benefit: nobody can treat a fraction of a person, and rounding down overestimates the effect.

## What is missing from the statement that the NNT is 50?

The period and the outcome: for example, 50 treated for 5 years to prevent one myocardial infarction.

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## Connections

## Related 4

- [Absolute risk reduction](https://durgical.com/en/epidemiology/absolute-risk-reduction/)
- [Number needed to harm](https://durgical.com/en/epidemiology/number-needed-to-harm/)
- [Relative risk reduction](https://durgical.com/en/epidemiology/relative-risk-reduction/)
- [Randomised controlled trial](https://durgical.com/en/epidemiology/randomised-controlled-trial/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.

Sources: [Chapter 15: Interpreting results and drawing conclusions](https://www.cochrane.org/authors/handbooks-and-manuals/handbook/current/chapter-15) (Cochrane, 2024); [Chapter 6: Choosing effect measures and computing estimates of effect](https://www.cochrane.org/authors/handbooks-and-manuals/handbook/current/chapter-06) (Cochrane, 2024).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

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Source: https://durgical.com/en/epidemiology/number-needed-to-treat/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/number-needed-to-treat/