# Absolute Risk Reduction Calculator

Determines the difference in outcomes between the treatment and control group to show how risk decreases with treatment.

Refer to the text below the tool for more information on how to determine ARR.

ARR is used in studies to define the absolute difference in outcomes between one group, the treatment receiving one and the other group, the control one, which offers information about by how much the risk of the studied adverse outcome decreases if a certain treatment or intervention takes place.

• `ARR (%) = (ARC – ART) x 100` (if ARC and ART expressed as decimals)
• `ARR (%) = (ARC – ART)` (if ARC and ART expressed as percentage)

Where:

• ARC = Number of events in control group / Number of people in control group
• ART = Number of events in treatment group / Number of people in treatment group

## Control Group

Absolute Risk in Control Group (ARC) Known?
ARC

## Treatment Group

Absolute Risk in Treatment Group (ART) Known?
ART
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## Send Us Your Feedback

Steps on how to print your input & results:

1. Fill in the calculator/tool with your values and/or your answer choices and press Calculate.

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.

## Determining Absolute Risk Reduction

ARR, also known as risk difference (RD) is used in studies to define the absolute difference in outcomes between one group, the treatment receiving one and the other group, the control one, which offers information about by how much the risk of the studied adverse outcome decreases if a certain treatment or intervention takes place.

• `ARR (%) = (ARC – ART) x 100` (if ARC and ART expressed as 0.X)
• `ARR (%) = (ARC – ART)` (if ARC and ART expressed as percentage)

Where:

• ARC = Number of events in control group / Number of people in control group
• ART = Number of events in treatment group / Number of people in treatment group

A common alternative of expressing ARR is through the number needed to treat (NNT).

## References

Irwig L, Irwig J, Trevena L, et al. Smart Health Choices: Making Sense of Health Advice. London: Hammersmith Press; 2008. Chapter 18, Relative risk, relative and absolute risk reduction, number needed to treat and confidence intervals.

Griffith JM, Lewis CL, Hawley S, Sheridan SL, Pignone MP. Randomized trial of presenting absolute v. relative risk reduction in the elicitation of patient values for heart disease prevention with conjoint analysis. Med Decis Making. 2009; 29(2):167-74.

Carneiro AV. Relative risk, absolute risk and number needed to treat: basic concepts. Rev Port Cardiol. 2009; 28(1):83-7.

Specialty: Epidemiology

Article By: Denise Nedea

Published On: October 27, 2020 · 12:00 AM

Last Checked: October 27, 2020

Next Review: October 27, 2025