Calculator Inputs
Example Data Table
| Study | Control Events | Control Total | Treatment Events | Treatment Total | CER | EER | ARR | RRR | NNT |
|---|---|---|---|---|---|---|---|---|---|
| Blood Pressure Trial | 48 | 600 | 24 | 600 | 8.00% | 4.00% | 4.00% | 50.00% | 25 |
| Cholesterol Trial | 90 | 1,000 | 70 | 1,000 | 9.00% | 7.00% | 2.00% | 22.22% | 50 |
| Prevention Program | 35 | 500 | 45 | 500 | 7.00% | 9.00% | -2.00% | -28.57% | NNH 50 |
CER = control event rate. EER = experimental or treatment event rate. Positive ARR indicates benefit. Negative ARR indicates risk increase.
Formula Used
Control Event Rate (CER) = Control Events ÷ Control Total
Treatment Event Rate (EER) = Treatment Events ÷ Treatment Total
Absolute Risk Reduction (ARR) = CER − EER
Relative Risk (RR) = EER ÷ CER
Relative Risk Reduction (RRR) = (CER − EER) ÷ CER
Number Needed to Treat (NNT) = 1 ÷ ARR, when ARR is positive
Number Needed to Harm (NNH) = 1 ÷ |ARR|, when ARR is negative
Confidence interval for ARR uses the standard error of the difference between two proportions. It helps judge how stable the observed risk difference may be.
How to Use This Calculator
- Enter a study name, outcome, and follow-up period for context.
- Select Event counts if you know events and totals.
- Select Risk percentages if you already know each group’s risk.
- Supply group totals whenever possible to estimate a confidence interval.
- Choose a confidence level and the number of displayed decimals.
- Click Calculate ARR to view summary cards, interpretation, and the Plotly graph.
- Use the Download CSV and Download PDF buttons to export the report.
FAQs
1) What does absolute risk reduction mean?
Absolute risk reduction measures the direct difference in event rates between control and treatment groups. It tells you how many fewer events happened because of treatment.
2) Why is ARR useful beside relative risk reduction?
Relative values can look impressive even when baseline risk is low. ARR shows the real-world size of the benefit in percentage points, making interpretation more practical.
3) What does a negative ARR indicate?
A negative ARR means the treatment group had more events than the control group. In that case, the treatment may be harmful, and NNH becomes more relevant than NNT.
4) How should I interpret NNT?
NNT estimates how many people need treatment to prevent one extra event. Smaller NNT values suggest a stronger absolute benefit, assuming the study design is reliable.
5) Why are group totals important?
Group totals allow the calculator to estimate the confidence interval around ARR. Without totals, you can still compute risk difference, but uncertainty is harder to quantify.
6) Can I use percentages instead of event counts?
Yes. Switch to risk percentage mode when published data already reports event rates. Add group totals too if you want a confidence interval estimate.
7) Does a confidence interval crossing zero matter?
Yes. When the ARR confidence interval crosses zero, the true effect may range from benefit to harm. That means the observed difference may not be statistically stable.
8) Is ARR enough for decision-making?
ARR is valuable, but decisions should also consider study quality, harms, cost, follow-up duration, patient preferences, and whether the outcome matters clinically.