Evidence-based medicine
Number Needed to Treat (NNT)
NNT communicates the clinical impact of an intervention by describing how many patients must receive it to prevent one adverse event over a specified time. Lower NNTs indicate more effective interventions (assuming acceptable risk and cost).
Proportion experiencing outcome without intervention.
Proportion experiencing outcome with intervention.
Optional descriptor (e.g., “myocardial infarction”, “stroke”).
Example: “1 year”, “5 years”, “hospital stay”.
Absolute metrics
- ARR: 6.0%
- RRR: 40.0%
Number needed to treat
16.7
NNT = 1 ÷ ARR
Evaluate resource availability, patient preferences, and side effects to decide on implementation.
Interpret alongside risks (Number Needed to Harm) and patient preferences.
How to Use This Calculator
Extract event rates from clinical studies
Use absolute event rates, not relative risk reductions, from randomized trials or meta-analyses.
Calculate ARR and NNT
ARR = Control event rate − Treatment event rate. NNT = 1 ÷ ARR (round up to whole persons).
Contextualize with harms and costs
NNT alone is insufficient. Compare with Number Needed to Harm, treatment cost, access, and patient values.
Formula
ARR = CER − TER
RRR = (CER − TER) ÷ CER
NNT = 1 ÷ ARR (rounded up to the nearest whole number)
CER = control event rate, TER = treatment event rate (both expressed as proportions).
Full Description
The NNT metric translates clinical trial results into patient-centered terms. Lower NNT values indicate greater absolute benefit, but interpretation requires considering harms, cost, feasibility, and patient preferences. Negative values (i.e., TER > CER) produce the Number Needed to Harm (NNH), indicating increased risk with treatment.
When event rates vary across populations, NNT will change accordingly. Always cite the population, baseline risk, and time horizon when communicating NNT. Clinical practice guidelines often summarize NNTs for common preventive therapies (e.g., statins, antihypertensives).
Frequently Asked Questions
Should NNT be rounded?
Yes. Report whole numbers (always round up) to reflect individuals. NNT of 7.2 is communicated as “8 people”.
What if TER is higher than CER?
The intervention increases risk; compute NNH = 1 ÷ (TER − CER) and describe the harm.
Can NNT vary across risk strata?
Yes. Higher baseline risk (CER) yields larger ARR and smaller NNT. Tailor benefit discussions to patient risk level.
How does NNT relate to shared decision-making?
NNT helps patients weigh absolute benefits against side effects and costs. Present NNT alongside NNH and patient goals.