Number Needed to Harm Calculator
Calculates the number needed to harm from an absolute risk increase. The form displays 1/absolute risk increase beside number needed to harm, using a worked condition that can be recalculated with the labeled inputs.
Set the labeled inputs
Number Needed to Harm
Scope of the number needed to harm method
The number needed to harm page calculates the number needed to harm from an absolute risk increase.
Number Needed to Harm is limited to the statistical quantity named by the result panel. The number needed to harm calculation does not silently add a population, time horizon, causal direction, or decision threshold that is absent from the fields.
Reading the number needed to harm fields
- Risk with exposure: For number needed to harm, the worked value for risk with exposure is 0.15 risk. Treat the risk with exposure entry (0.15 risk) explicitly as a count, proportion, rate, estimate, or model parameter before comparing number needed to harm conditions. The form enforces minimum 0, maximum 1.
- Risk without exposure: For number needed to harm, the worked value for risk without exposure is 0.1 risk. Treat the risk without exposure entry (0.1 risk) explicitly as a count, proportion, rate, estimate, or model parameter before comparing number needed to harm conditions. The form enforces minimum 0, maximum 1.
The entries used for number needed to harm must refer to one coherent analysis condition. Combining incompatible populations, periods, or measurement definitions can produce valid number needed to harm arithmetic for a nonexistent study.
For a related comparison, continue with cohen kappa agreement.
Following the number needed to harm relationship
For number needed to harm, match every symbol in the relationship to a labeled field before substituting numbers. Number Needed to Harm is reported in the scale implied by the inputs and formula.
While checking number needed to harm, inspect every denominator in 1/absolute risk increase. For number needed to harm, a zero or near-zero denominator can make number needed to harm undefined or unstable.
Checking the displayed example
The default number needed to harm condition is Risk with exposure = 0.15 risk, Risk without exposure = 0.1 risk.
Increasing risk from .10 to .15 gives NNH 20.
The live calculator reports Number needed to harm 20. Repeating one intermediate step from 1/absolute risk increase provides a fixed number needed to harm reference check for later code changes.
Statistical context for number needed to harm
The harm outcome and time horizon determine how this number should be interpreted.
For number needed to harm, diagnostic and risk measures are conditional on named denominators, reference definitions, population prevalence, and follow-up time.
How to interpret the number needed to harm output
When interpreting number needed to harm, keep the two-by-two counts or source risks with the result; a ratio alone can hide very different absolute event frequencies.
As a second check for number needed to harm, reversing the numerator and denominator answers a different question, so retain the direction printed in 1/absolute risk increase.
Varying a single number needed to harm input at a time
Change risk with exposure while holding the remaining entries fixed, then state why the direction and size of the number needed to harm change are plausible from 1/absolute risk increase.
Repeat the number needed to harm exercise with risk without exposure. If a modest defensible change materially alters the interpretation, report both conditions rather than presenting that number needed to harm scenario as exact.
Documenting the number needed to harm result
Report number needed to harm using 1/absolute risk increase, followed by the entered values, units, exclusions, and analysis date. Name the number needed to harm population or dataset boundary instead of leaving it implicit.
Keep the full calculator output with the record, including Number needed to harm 20. A later number needed to harm review can then distinguish a changed input from a different convention or software implementation.
Questions about number needed to harm
What should be saved with number needed to harm?
Save the entered values and units for risk with exposure, risk without exposure, along with the analysis date, exclusions, software or formula version, and the relationship 1/absolute risk increase. That record is sufficient to rebuild this specific number needed to harm calculation.
Does number needed to harm establish a causal or population conclusion?
No. The displayed number needed to harm value is conditional on the entered data and named method. The number needed to harm design, measurement process, and assumptions determine what can be concluded beyond those values.
How should number needed to harm be rounded?
Keep the unrounded number needed to harm for subsequent arithmetic, then report only the precision supported by the source measurements and the decision context. Extra digits in number needed to harm do not correct sampling or model error.
Which input deserves the closest boundary check?
For number needed to harm, start with risk without exposure and then risk with exposure. Confirm the number needed to harm units and allowed domain because a valid-looking entry can still describe the wrong statistical setup.
Why could another program report a different number needed to harm?
A different convention for rounding, tails, ties, interpolation, parameterization, or missing values can change number needed to harm. Compare the printed number needed to harm formula and its input definitions before treating either output as wrong.