Number needed to harm

Number Needed to Harm (NNH) is a statistical concept used in Epidemiology and Evidence-based medicine to quantify the number of patients that need to be treated with a particular intervention for one additional patient to experience a harmful event. It is the inverse of the Absolute risk reduction and is calculated from the Absolute risk increase (ARI) of a treatment or intervention.

Definition

The Number Needed to Harm is defined as the number of patients who need to be treated for one to be harmed by the treatment. It is calculated as the inverse of the Absolute Risk Increase, which is the difference in risk of a harmful event between two groups - usually a treatment group and a control group.

Calculation

The NNH is calculated using the formula:

NNH = 1 / ARI

Where ARI is the Absolute Risk Increase. The ARI is calculated as the difference in the risk of a harmful event between the treatment group and the control group.

Interpretation

The NNH is a useful measure for comparing the safety of different treatments or interventions. A lower NNH indicates a higher risk of harm, while a higher NNH indicates a lower risk of harm. It is important to note that the NNH is a population-level measure and does not predict the risk of harm for an individual patient.

Limitations

Like all statistical measures, the NNH has limitations. It assumes that the risk of harm is constant across all patients, which may not be the case in reality. It also does not take into account the severity of the harm, or the benefits of the treatment.

See also

• Number needed to treat
• Absolute risk reduction
• Relative risk reduction
• Odds ratio
• Risk ratio