NNT is not easily understood

In a recent editorial, Misselbrook and Armstrong comment upon the concept of risk in relation to John Everyman1 and claim that the number needed to treat (NNT) for Mr Everyman is 11. It is not! If the baseline risk of cardiovascular disease (CVD) events is 27.5% (‘25–30%’) and 24.5% (‘22–27%’) after treatment, the absolute risk reduction is 27.5 – 24.5% = 3% and the NNT = 1/0.03 = 33. The relative risk reduction is (27.5 – 24.5) × 100/27.5 which is ∼11% and not 9%. The authors state that “the ‘personal probability of benefit’ (PPB) may make the probability as meaningful as possible to the individual patient”.1 In their example, the NNT is 11, and %PPB is claimed to be 1/NNT × 100 = 9%. This implies, according to Misselbrook and Armstrong, that the patient “would have a 9% chance of benefiting from treatment”. Unfortunately, this interpretation is not correct.