Number needed to treat, number needed to harm and absolute risk reduction

Number needed to treat (NNT)

NNT is the number of patients who need to receive an intervention to prevent one additional adverse outcome or to achieve one additional beneficial outcome compared to a control group.

Calculation:

NNT = 1 ÷ Absolute Risk Reduction (ARR)

The result is always rounded up to the next whole number because you can’t treat a fraction of a patient.


Event rates:

Experimental Event Rate (EER) = Number of patients with outcome in treatment group ÷ Total number in treatment group

Control Event Rate (CER) = Number of patients with outcome in control group ÷ Total number in control group

Absolute Risk Reduction (ARR):
The ARR is the absolute difference in event rates between control and treatment groups.

There are two ways to express ARR depending on the nature of the outcome:

If the outcome is undesirable (e.g., death, disease occurrence, adverse event):

ARR = CER −EER

This represents how much the risk is reduced by the treatment.

If the outcome is desirable (e.g., cure, recovery, symptom relief):

ARR = EER −CER

This can also be called the Absolute Benefit Increase (ABI) because it measures the increase in a positive outcome.

Number Needed to Harm (NNH)

NNH is the number of patients who need to be treated for one additional patient to experience a harmful or adverse event due to the intervention compared to the control.

Calculation:

NNH = 1 ÷ Absolute risk increase (ARI)

Where:

ARI = EER − CER

(Here, EER refers to the adverse event rate in the treated group, and CER is the adverse event rate in the control group.)

Interpretation:
The smaller the NNH, the greater the risk of harm associated with the treatment. A higher NNH indicates the treatment is relatively safer.