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The ideal NNT is 1, where everyone improves with treatment and no one improves with control. A higher NNT indicates that treatment is less effective. NNT is similar to number needed to harm (NNH), where NNT usually refers to a therapeutic intervention and NNH to a detrimental effect or risk factor.
Herein, what does number needed to harm mean?
The number needed to harm (NNH) is an epidemiological measure that indicates how many persons on average need to be exposed to a risk factor over a specific period to cause harm in an average of one person who would not otherwise have been harmed.
- The NNT is the inverse of the absolute risk reduction (ARR).
- The ARR is the absolute difference in the rates of events between a given activity or treatment relative to a control activity or treatment, ie control event rate (CER) minus the experimental event rate (EER), or ARR = CER - EER.
In this manner, how do you calculate number needed to harm?
From this the value known as the number needed to harm (NNH) can be calculated by dividing 1 by the absolute risk increase, and again multiplying by 100 when the ARI is expressed as a percentage. NNH shows how many individuals would need to be treated with the drug in order for 1 to show the harmful effect.
The NNT is simply the inverse of the ARR; it can be calculated by taking 100 and dividing it by the ARR (1).