Absolute Risk Reduction (ARR) Calculator

Absolute Risk Reduction and Excess Risk are commonly referred as Risk Difference. It is generally the difference in the risk between two different activities or treatment.

Risk Difference Calculation

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Absolute Risk Reduction = Control Event Rate - Experimental Event Rate
Control Event Rate = c / (c + d) Experimental Event Rate = a / (a + b)
Where, a = Experimental Group Size b = Control Group Size c = Events in Experimental Group d = Events in Control Group

The risk difference can be between a control group's event rate (CER) and the experimental group's event rate (EER). Commonly calculated between treatments A and B, where A is a drug and B without any drug.


If the experimental group size is 12 and control group size is 13 and the events in experimental group is 2 and control group is 3, then

Control Event Rate

CVR = c / c + d
CVR = 2 / (2+3)
CVR = 0.4

Experimental Event Rate

EER = a / a+c
EER = 12 / 12 + 13
EER = 0.48

Excess Risk (ARR)

ER = Control Event Rate - Experimental Event Rate
= 0.4 - 0.48 = -0.08
= -0.08

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