Relative Risk Formula Calculator

Determines the ratio of the probability of an outcome in the treatment) group to that in the control group.

Refer to the text below the tool for more information about RR, its formula and interpretation.


Relative Risk, also known as risk ratio, is defined as the ratio of the probability of an outcome in the exposed (or treatment) group to the probability of the same outcome in the unexposed (or control) group. This is an indicator of the association between exposure and outcome.


Relative Risk (RR) = ART/ARC    

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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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 Relative Risk

Relative Risk, also known as risk ratio, is defined as the ratio of the probability of an outcome in the exposed (or treatment) group to the probability of the same outcome in the unexposed (or control) group. This is an indicator of the association between exposure and outcome.

Relative Risk (RR) = ART/ARC

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
  • In research, RR is often found in the working of statistical analysis of data and can estimate the strength of association between treatments (or risk factors) and outcomes.

Relative Risk Interpretation

  • When RR = 1: Exposure does not affect the outcome;
  • When RR<1: The risk of the outcome is decreased by the exposure;
  • When RR>1: The risk of the outcome is increased by the exposure.

It is important to note however, that whilst RR provides some information about risk, it does not define the actual odds of something happening (which is in the remit of absolute risk).

 

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.

Elbarbary M. Understanding and expressing "Risk". J Saudi Heart Assoc. 2010; 22(3):159-64.


Specialty: Epidemiology

Article By: Denise Nedea

Published On: October 27, 2020

Last Checked: October 27, 2020

Next Review: October 27, 2025