Calculate the number needed to treat or harm, absolute and relative risk reduction, from event counts or rates, with Altman’s confidence interval.
About this tool
Number needed to treat or harm, with absolute and relative risk reduction.
How to use NNT / NNH Calculator
- Enter events and totals for the treatment and control groups, or the two event rates.
- Say whether the event is harmful (like death) or beneficial (like cure).
- Read the NNT or NNH, the absolute risk reduction and the confidence interval.
Worked example
If 25% of controls and 15% of treated patients have a stroke, the absolute risk reduction is 10% and the NNT is 10.
Why is NNT rounded up?
You cannot treat part of a person, and rounding up gives the more conservative estimate of benefit.
Why does the confidence interval include infinity?
When the risk difference could be zero, its inverse runs from benefit through infinity to harm. Altman (1998) recommends showing both ends.
References
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