Calculating the number needed to treat for trials where the outcome is time to an event

BMJ 1999;319:1492-1495 ( 4 December )

Education and debate

Calculating the number needed to treat for trials where the outcome is time to an event

Douglas G Altman, professor of statistics in medicine ^a, Per Kragh Andersen, professor of biostatistics ^b.

^a Imperial Cancer Research Group Medical Statistics Group, Centre for Statistics in Medicine, Institute of Health Sciences, Headington, Oxford OX3 7LF, ^b Department of Biostatistics, University of Copenhagen, DK-2200 Copenhagen N, Denmark

Correspondence to: D G Altman d.altman@icrf.icnet.uk

The first 150 words of the full text of this article appear below.

The number of patients who need to be treated to prevent one additional event (number needed to treat; NNT) has become a widelyused measure of treatment benefit derived from the results ofrandomised controlled trials with a binary outcome. ¹ ² Weshow how to obtain a number needed to treat for studies wherethe primary outcome is the time to an event. We consider primarilythe situation where there is no access to raw data, for example,when reviewing a published study, and also how to proceed whengiven the raw data.

Table Removed (Available Only in the Full Text)

Time to event data

As noted previously, for studies with binary outcome the number needed to treat will vary according to the length of followup.³ For studies of survival this relation with time is moreexplicit. There is no single number needed to treat; rather itcan be calculated at any time point after the start of treatment.Often there are . . . [Full text of this article]

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