The Number Needed to Harm (NNH) and Number Needed to Treat (NNT) are statistical measures used to assess the effectiveness and safety of medical interventions.

Calculating NNT:

1. Determine the outcome of interest (e.g., disease prevention, symptom relief).
2. Calculate the Absolute Risk Reduction (ARR) by subtracting the treatment group’s event rate from the control group’s event rate.
3. Calculate NNT using the formula: NNT = 1 / ARR.

Calculating NNH:

1. Determine the adverse outcome of interest (e.g., side effects, complications).
2. Calculate the Absolute Risk Increase (ARI) by subtracting the control group’s event rate from the treatment group’s event rate.
3. Calculate NNH using the formula: NNH = 1 / ARI.

Example:

Suppose a study finds:

1. Control group: 20% developed a disease (0.20)
2. Treatment group: 10% developed a disease (0.10)

Calculating ARR and NNT:

1. ARR = 0.20 – 0.10 = 0.10
2. NNT = 1 / 0.10 = 10

This means 10 patients need to receive the treatment to prevent one case of the disease.

Interpreting NNT and NNH:

1. Lower NNT values indicate more effective treatments.
2. Higher NNH values indicate safer treatments.

Keep in mind that NNT and NNH should be interpreted in context, considering factors like study design, population, and outcome severity.