Calculate absolute risk reduction, risk increase, control event rate or experimental event rate from the other two values in percent or proportion.
How to interpret the result
ARR is calculated as CER − EER. If the result is positive, the experimental group had a lower event rate than the control group (risk reduction). If the result is negative, the experimental group had a higher event rate (risk increase, often reported as ARI).
When interpreting results, consider the study design, the baseline risk, and statistical uncertainty (such as confidence intervals) rather than relying on a single point estimate.
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Absolute Risk Reduction Formula
The following equation is used to calculate an absolute risk reduction (or increase) as a signed difference in event rates (in percentage points or proportion).
ARR = CER - EER
- Where ARR is the absolute difference in event rates (in % points or proportion); if negative, it indicates an absolute risk increase (ARI)
- CER is the control event rate (in % or proportion)
- EER is the experimental event rate (in % or proportion)
Absolute Risk Reduction Definition
An absolute risk reduction is defined as the difference between the control event rate and the experimental event rate (reported as a percentage point difference or as a proportion).
Absolute Risk Reduction Example
How to calculate absolute risk reduction?
- First, determine the control rate.
Calculate the percentage rate of the control event rate.
- Next, determine the experimental event rate.
Calculate the experimental event rate.
- Finally, calculate the absolute risk reduction.
Calculate the absolute risk reduction using the formula above.
An absolute risk reduction, or ARR for short, is a measure of the absolute difference between a control group and a group receiving a treatment to prevent the event from happening.
Absolute risk is a ratio of the number of people that have a particular event, typically a medical event, compared to the total number of people that it could happen to.
