ARR is calculated using the formula: ARR = CER - EER Where CER (Control Event Rate) is the event rate in the control group and EER (Experimental Event Rate) is the event rate in the treatment group. For example, if 20 out of 100 patients in the control group have a certain outcome and only 10 out of 100 patients in the treatment group experience the same outcome, the ARR would be: ARR = (20/100) - (10/100) = 0.2 - 0.1 = 0.1 or 10%
