Absolute risk reduction (ARR)

The absolute risk of an outcome associated with exposure to an agent (e.g. a therapy) is of less value than the difference in absolute risk between exposed and unexposed groups. This difference – known as the absolute risk reduction (ARR) where the agent confers protection to the outcome of interest – is of fundamental importance to economic evaluations. ARR drives the incremental effect in an incremental cost-effectiveness ratio and is the reciprocal of the number needed to treat, a metric used to communicate effectiveness in evidence-based medicine.