Study 1: In the treatment group, one out of 50 subjects died. Hence, risk of death = 1/50 = 0.02. In the control group, five out of 50 subjects died. Hence, risk of death = 5/50 = 0.1. Hence, absolute risk reduction (ARR) = 0.1−0.02 = 0.08. Number needed to treat (NNT) = 1/ARR = 1/0.08 = 12.5 The ARR and NNT for the other studies can be similarly calculated as follows:

Study Risk in experimental group Risk in control group ARR NNT
11/50 = 0.025/50 = 0.10.1−0.02 = 0.081/0.08 = 12.5
25/250 = 0.0225/250 = 0.10.1−0.02 = 0.081/0.08 = 12.5
325/1250 = 0.02125/1250 = 0.10.1−0.02 = 0.081/0.08 = 12.5
4100/2500 = 0.04250/2500 = 0.10.1−0.04 = 0.061/0.06 = 16.7