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Absolute risk and NNT calculator
Pubrica’s free calculator converts event counts from two trial arms into the absolute risk reduction, number needed to treat, risk ratio and odds ratio, each with a confidence interval. Enter the events and participants in the control and treatment arms to report results in clinically interpretable terms.
The tool runs in your browser. No registration, email address or file upload is required, and the formula, reference or assessment criteria used are shown with the result.
- Runs in your browser
- Nothing uploaded or stored
- No email asked for
- Method shown with the answer
- Sources dated
The Tool
Calculate Absolute Risk Reduction and Number Needed to Treat
Calculates absolute risk reduction, number needed to treat, risk ratio and odds ratio, each with a confidence interval, from two trial arms.
05
Calculates absolute risk reduction, number needed to treat, risk ratio and odds ratio, each with a confidence interval, from two trial arms.
ARR · NNT · RR · OR17
Number needed to treat
6.1pts
Absolute risk reduction
30.6%
Relative risk reduction
About 17 people would have to be treated for one additional person to avoid the event, at the baseline risk in this trial. An NNT is tied to that baseline risk and to that follow-up time: quoting it for a population at a different underlying risk, or over a different period, is the most common misuse of this number.
Method. ARR = CER − EER, NNT = 1/ARR. The interval on ARR uses the normal approximation √(CER(1−CER)/n₁ + EER(1−EER)/n₂) and is inverted for the NNT limits; where it crosses zero the limits are reported as benefit-to-infinity-to-harm, following Altman (BMJ 1998;317:1309). The risk-ratio interval uses Katz’s log method and the odds ratio Woolf’s. With zero events in an arm the ratio measures are undefined and a continuity correction is needed.
Limitations
What This Tool Does Not Cover
Use the result as a starting point. These are the situations in which it needs specialist review.
The calculators
Limitations of the calculators
The sample size calculator gives the closed-form answer for two independent groups. Cluster randomisation, interim analyses, a non-inferiority margin, repeated measures and competing risks each change the number materially, and none of them can be handled by inflating the answer afterwards. The same applies to the test chooser: it cannot know that your observations are clustered within clinics, and clustering overrides almost everything else it would tell you.
Method
Methods and Sources
The calculation or reference set behind this tool. The result panel also shows the formula or criteria used.
Risk and NNT
Altman
Katz’s log method for the risk-ratio interval, Woolf’s for the odds ratio, and Altman’s convention for reporting a number-needed-to-treat interval that crosses no difference as benefit to infinity to harm.
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