Group exposed to a risk factor (left) has increased risk of an adverse outcome (black) compared to the unexposed group (right). 4 individuals need to be exposed for 1 adverse outcome to occur <unk>  = 4). In medicine, the<unk>  <unk>  is an epidemiological measure that indicates how many persons on average need to be exposed to a risk factor over a specific period to cause<unk>  in an average of one person who would not otherwise have been<unk> ed. It is defined as the inverse of the absolute risk increase, and computed as , where is the incidence in the treated (exposed) group, and is the incidence in the control (unexposed) group. Intuitively, the lower the<unk> , the worse the risk factor, with 1 meaning that every exposed person is<unk> ed.<unk>  is similar to number needed to treat (NNT), where NNT usually refers to a therapeutic intervention and<unk>  to a detrimental effect or risk factor.
Number needed to harm
Number needed to treat
N of 1 trial
Minimal important difference
Relative risk reduction
Real world data
Indicative limit value
Clinical endpoint