NNT & ARR Calculator
Evidence-based medicine fundamentals. Calculate the Number Needed to Treat (NNT), Absolute Risk Reduction (ARR), and Relative Risk (RR) using event rates from clinical trials. Essential for evaluating whether a statistically significant result is clinically meaningful.
Results
Control Event Rate (CER)
Experimental Event Rate (EER)
Absolute Risk Reduction (ARR)
The actual difference in risk between the two groups.
Relative Risk (RR)
The ratio of the probability of an event occurring in the experimental group vs the control group.
Number Needed to Treat (NNT)
You need to treat patients with the new treatment (instead of the control) to prevent one additional bad outcome.
Why NNT matters
Pharmaceutical marketing often highlights Relative Risk Reduction (RRR) because it produces impressive-sounding numbers. For example, if a drug reduces heart attacks from 2 in 1000 to 1 in 1000, the RRR is 50%.
However, the Absolute Risk Reduction (ARR) is only 0.1% (0.2% - 0.1%). The NNT is 1/ARR = 1000. You would need to treat 1,000 patients to prevent one heart attack.
ARR = CER - EER