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number needed to treat
The number needed to treat (NNT) is an epidemiological measure used in communicating the effectiveness of a health-care intervention, typically a treatment with medication. The NNT is the average number of patients who need to be treated to prevent one additional bad outcome (e.g. the number of patients that need to be treated for one to benefit compared with a control in a clinical trial). It is defined as the inverse of the absolute risk reduction. It was described in 1988. The ideal NNT is 1, where everyone improves with treatment and no one improves with control. The higher the NNT, the less effective is the treatment.〔(【引用サイトリンク】title=Number Needed to Treat )〕
Variants are sometimes used for more specialized purposes. One example is number needed to vaccinate.
NNT values are time-specific. For example, if a study ran for 5 years and another ran for 1 year the NNT values would not be directly comparable.
==Derivation==
NNT is the statistical inverse of the absolute risk reduction i.e. 1/absolute risk reduction. In general, NNT is computed with respect to two treatments ''A'' and ''B'', with ''A'' typically the intervention and ''B'' the control (e.g., ''A'' might be a 5-year treatment with a drug, while ''B'' is no treatment). A defined endpoint has to be specified (e.g., the appearance of colon cancer in a five-year period). If the probabilities ''pA'' and ''pB'' of this endpoint under treatments ''A'' and ''B'', respectively, are known, then the NNT is computed as 1/(''pB'' – ''pA''). NNT is a number between 1 and ∞; effective interventions have a low NNT. A negative number would not be presented as a NNT, rather, as the intervention is harmful, it is expressed as a number needed to harm (NNH). The units of the aforementioned probabilities are expressed as number of events per subject (see worked out example below); therefore, the inverse NNH will be number of subjects per event.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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