1. What is Number Needed to Harm (NNH)?

Definition: NNH is an epidemiological measure that indicates how many patients need to be exposed to a risk factor to cause harm in one patient that would not otherwise have been harmed.

Purpose: It helps healthcare professionals understand the risk associated with treatments or exposures and make informed clinical decisions.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( NNH \) — Number Needed to Harm (dimensionless)
• \( ARR \) — Absolute Risk Reduction (dimensionless, between 0 and 1)

Explanation: The NNH is simply the reciprocal of the Absolute Risk Reduction (ARR). A lower NNH indicates a higher risk of harm.

3. Importance of NNH Calculation

Details: Understanding NNH helps balance the benefits and risks of treatments, especially when considering interventions with potential side effects.

4. Using the Calculator

Tips: Enter the Absolute Risk Reduction (ARR) as a decimal between 0 and 1 (e.g., 0.05 for 5%). The value must be > 0 and ≤ 1.

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between NNH and NNT?
A: NNH (Number Needed to Harm) measures harm, while NNT (Number Needed to Treat) measures benefit. Both use similar calculations but opposite interpretations.

Q2: What is a good NNH value?
A: Higher NNH values are better (meaning more people need to be exposed to cause one harm). Values depend on context - what might be acceptable for a life-saving drug might not be for a cosmetic treatment.

Q3: How do I calculate ARR?
A: ARR = Risk in exposed group - Risk in control group. For example, if 10% of treated patients experience side effects vs 2% in controls, ARR = 0.10 - 0.02 = 0.08.

Q4: Can NNH be less than 1?
A: No, since ARR cannot be greater than 1 (100%), NNH cannot be less than 1.

Q5: When should I use NNH?
A: Use NNH when evaluating potential harms of treatments, exposures, or interventions to help weigh risks against benefits.