# Number Needed To Harm - Worked Example

Worked Example

The following is an example of calculating number needed to harm.

In a cohort study, individuals with exposure to a risk factor (Exposure +) are followed for a certain number of years to see if they develop a certain disease or outcome (Disease +). A control group of individuals who are not exposed to the risk factor (Exposure −) are also followed . "Follow up time" is the number of individuals in each group multiplied by the number of years that each individual is followed:

Disease + Total subjects followed Years followed^ Follow-up time Incidence
Exposure + 6054 86318 13.56^ 1,170,074 0.0703
Exposure − 32 516 21.84^ 11,270 0.0620

^ "Years followed" is a weighted average of the length of time the patients were followed.

The incidence with exposure is:

The incidence without exposure:

To determine the relative risk, divide the incidence with exposure by the incidence without exposure:

relative risk

To determine attributable risk subtract incidence without exposure from incidence with exposure:

0.0703 − 0.0620 = 0.0083 = 0.83% = attributable risk

The number needed to harm is the inverse of the attributable risk, or:

= Number needed to harm

This means that if 120 individuals are exposed to the risk factor, 1 will develop the disease that would not have otherwise.

Note that these calculations can be affected enormously by roundoff error. (If no roundoff is used in the intermediate calculations above, the final figure for the NNH is 123.)

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