Absolute risk reduction

Absolute risk reduction (ARR): The arithmetic difference between the two groups - the ones that are treated versus those not treated.

Example: Given a group of 20 people with a disease, 10 people recieve treatment and out of them 5 died. 7 out of those not treated - died. The reduction in the risk of death is 7/10 - 5/10 = 2/10 treated. This is absolute risk reduction.

Number needed to treat (NNT): It is the number of patients that need to be treated to save one event from occurring. In this case it is 10 people need to be treated to save 2 lives therefore 5 need to be treated to save one life. i.e. NNT=5. It is thus 1/ARR [1/ (2/10)].