Number needed to harm
In medicine, the number needed to harm (NNH) 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 harm in an average of one person who would not otherwise have been harmed. It is defined as the inverse of the absolute risk increase, and computed as 1 / ( I e − I u ) {\displaystyle 1/(I_{e}-I_{u})} , where I e {\displaystyle I_{e}} is the incidence in the treated (exposed) group, and I u {\displaystyle I_{u}} is the incidence in the control (unexposed) group.