Cookies on this website

We use cookies to ensure that we give you the best experience on our website. If you click 'Accept all cookies' we'll assume that you are happy to receive all cookies and you won't see this message again. If you click 'Reject all non-essential cookies' only necessary cookies providing core functionality such as security, network management, and accessibility will be enabled. Click 'Find out more' for information on how to change your cookie settings.

BACKGROUND: There is debate concerning methods for calculating numbers needed to treat (NNT) from results of systematic reviews. METHODS: We investigate the susceptibility to bias for alternative methods for calculating NNTs through illustrative examples and mathematical theory. RESULTS: Two competing methods have been recommended: one method involves calculating the NNT from meta-analytical estimates, the other by treating the data as if it all arose from a single trial. The 'treat-as-one-trial' method was found to be susceptible to bias when there were imbalances between groups within one or more trials in the meta-analysis (Simpson's paradox). Calculation of NNTs from meta-analytical estimates is not prone to the same bias. The method of calculating the NNT from a meta-analysis depends on the treatment effect used. When relative measures of treatment effect are used the estimates of NNTs can be tailored to the level of baseline risk. CONCLUSIONS: The treat-as-one-trial method of calculating numbers needed to treat should not be used as it is prone to bias. Analysts should always report the method they use to compute estimates to enable readers to judge whether it is appropriate.


Journal article


Bmc med res methodol

Publication Date





Bias, Clinical Trials as Topic, Data Interpretation, Statistical, Humans, Meta-Analysis as Topic, Odds Ratio, Reproducibility of Results, Risk, Sample Size, Smoking Cessation, Statistics as Topic