Number Needed to Treat

The number of patients who would need to receive a treatment for one additional patient to experience a beneficial outcome.

Source: Cook & Sackett 1995

The number needed to treat is how many patients must receive a treatment for one extra person to gain the beneficial outcome. It tells a clinician the effort a benefit costs in tangible terms, so a number needed to treat of five means one in five treated benefits, while a hundred means the great majority gain nothing. This concrete framing conveys the size of a benefit more vividly than a percentage, and it aids comparison between treatments. Expressing benefit as people treated per success is its value. Sackett and colleagues (1991) describe this measure.

Source: Sackett et al. 1991

The number needed to treat is calculated as the reciprocal of the absolute risk reduction, that is one divided by the difference in the risk of the outcome between the control and treatment groups. If a treatment reduces the risk of an adverse outcome by a certain absolute amount, the number needed to treat is one divided by that amount. So the number needed to treat is calculated from the absolute risk reduction, taking its reciprocal, which converts the difference in outcome risk between groups into the number of patients who must be treated for one additional patient to benefit, linking it directly to the absolute measure of effect.

Source: Cook & Sackett 1995

The number needed to treat is interpreted as the number of patients who must be treated for one extra beneficial outcome: a small number, such as a few patients, indicates an effective treatment where benefit comes readily, while a large number indicates that many must be treated for one to benefit. So the number needed to treat is interpreted as how many treatments yield one additional good outcome, with smaller values indicating greater benefit, and it is weighed against harms and costs, since a treatment with a favourable number needed to treat may still be judged in light of its number needed to harm and its expense.

Source: Cook & Sackett 1995

The number needed to treat is useful because it translates a treatment's effect into an intuitive figure that patients and clinicians can grasp, conveying the practical benefit better than relative measures, which can seem impressive even when the absolute benefit is small. It reflects the baseline risk and links directly to the absolute risk reduction. So the number needed to treat is useful for communicating and judging the real-world benefit of a treatment, helping to weigh benefit against harm and cost, since knowing how many patients must be treated for one to benefit gives a clearer sense of value than a relative risk reduction alone.

Source: Cook & Sackett 1995

The limitations of the number needed to treat include that it depends on the baseline risk and the time period, so it varies between populations and durations and is not a fixed property of a treatment; that it requires a reliable estimate of the absolute risk reduction; and that a single figure does not capture harms or the severity of outcomes. So the number needed to treat is interpreted with attention to the baseline risk, time frame, and population to which it applies, and used alongside the number needed to harm and other information, since comparing numbers needed to treat across settings requires comparable baseline risks and the measure reflects only one benefit at a time.

Source: Cook & Sackett 1995