Number needed to harm

In <a href="/facts/Medicine/2fnp90Je">medicine</a>, the number needed to harm (NNH) is an <a href="/facts/Epidemiology/wgTmxoMA">epidemiological</a> measure that indicates how many persons on average need to be exposed to a <a href="/facts/Risk-factor/N0YTUyer">risk factor</a> 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 <a href="/facts/Absolute_risk_increase/BqB2f2aJ">absolute risk increase</a>, and computed as 
 
 
 
 1
 
 /
 
 (
 
 I
 
 e
 
 
 −
 
 I
 
 u
 
 
 )
 
 
 {\displaystyle 1/(I_{e}-I_{u})}
 
, where 
 
 
 
 
 I
 
 e
 
 
 
 
 {\displaystyle I_{e}}
 
 is the <a href="/facts/Incidence_(epidemiology)/2BB3FlBK"> incidence</a> in the treated (exposed) group, and 
 
 
 
 
 I
 
 u
 
 
 
 
 {\displaystyle I_{u}}
 
 is the incidence in the control (unexposed) group. Intuitively, the lower the number needed to harm, the worse the risk factor, with 1 meaning that every exposed person is harmed.
NNH is similar to <a href="/facts/Number_needed_to_treat/uGJGNpnP">number needed to treat</a> (NNT), where NNT usually refers to a positive therapeutic result and NNH to a detrimental effect or risk factor. 
Marginal metrics:

<ul><li>NNT for an additional beneficial outcome (NNTB)</li>
<li>NNT for an additional harmful outcome (NNTH)</li></ul>
are also used.

Number needed to harm open-in-new

Number needed to harm