Number Needed to Treat (NNT) Calculator
This calculator computes the Number Needed to Treat (NNT), a key measure in evidence-based medicine that tells you how many patients need to be treated to prevent one additional bad outcome.
What is Number Needed to Treat?
The Number Needed to Treat (NNT) is a measure used in epidemiology and clinical trials to assess the effectiveness of a healthcare intervention. It represents the average number of patients who need to be treated to prevent one additional adverse outcome.
How to Interpret NNT
- NNT of 1: Perfect treatment - every patient benefits
- NNT of 5: Treat 5 patients to prevent 1 bad outcome
- NNT of 100: Treat 100 patients to prevent 1 bad outcome
- Lower NNT = Better treatment
Key Formulas
Control Event Rate (CER) = Control Events / Control Total
Experimental Event Rate (EER) = Experimental Events / Experimental Total
Absolute Risk Reduction (ARR) = CER - EER
NNT = 1 / ARR
Relative Risk (RR) = EER / CER
Relative Risk Reduction (RRR) = (CER - EER) / CER = 1 - RR
Example
In a clinical trial comparing a new drug to placebo for preventing heart attacks:
- Control group (placebo): 20 heart attacks out of 200 patients (10%)
- Treatment group (drug): 10 heart attacks out of 200 patients (5%)
- Absolute Risk Reduction = 10% - 5% = 5% or 0.05
- NNT = 1 / 0.05 = 20
- You need to treat 20 patients to prevent 1 heart attack
Related Concepts
- Number Needed to Harm (NNH): When the treatment causes harm rather than benefit (negative NNT)
- Absolute Risk Reduction (ARR): The absolute difference in event rates between groups
- Relative Risk Reduction (RRR): The proportional reduction in risk
- Relative Risk (RR): The ratio of event rates between groups