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This JAMA Guide to Statistics and Methods explains the calculations and concepts underlying the number needed to treat (NNT) as a summary statistic of effect, and the assumptions underlying the measure that affect its interpretation.

Effectively communicating clinical trial results to patients and clinicians is a requirement for appropriate application in clinical practice. In a recent issue of JAMA, Zhao et al1 reported the results from a randomized clinical trial comparing dual antiplatelet therapy with aspirin monotherapy for preserving saphenous vein graft patency in 500 patients undergoing coronary artery bypass grafting. Dual antiplatelet therapy was found to be superior to aspirin monotherapy. The authors1 used the number needed to treat (NNT) to communicate effect size, reporting that for every 8 patients treated with dual agents rather than aspirin alone, 1 additional patient would achieve saphenous graft patency at 1 year. The NNT may be defined as the number of patients who need to be treated with one therapy vs another for 1 additional patient to have the desired outcome. Since its first description 30 years ago,2 the NNT has become an important means to express the magnitude of benefit conferred by a therapy.3


What Is the NNT?

When a clinical trial is completed, the fraction or proportion of patients experiencing the desired outcome is reported for the active and control groups. The NNT is derived from these values and indicates the magnitude of the therapy's treatment effect on the disease observed in the clinical trial. The NNT is computed by dividing 100 by the difference between the percentage response of the treatment group from that of the control group. Alternatively, the NNT is calculated by taking the reciprocal of the absolute risk reduction between the groups. The NNT indicates how many patients must be managed on average with active rather than control therapy to achieve 1 additional good outcome.

The number needed concept may be applied to many types of outcomes from both therapeutic and diagnostic studies. When a therapy increases desirable outcomes, the resulting value is the number needed to benefit (more often denoted as just NNT). When a therapy increases adverse events, the resulting value is the number needed to harm. When applied to diagnostic strategies, the resulting values are the number needed to screen for tests in asymptomatic individuals, and the number needed to diagnose for tests in symptomatic individuals.

Why Is the NNT Important?

The NNT is intuitively understandable by patients and clinicians. It is also quantitative, facilitating decision making when selecting among available therapeutic strategies. By including a 95% confidence interval (CI) around the observed NNT, the uncertainty in the benefit also can be communicated effectively.

Other well-established indices of treatment effect are not well suited for this purpose. For example, ...

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