Читать книгу Medical Statistics - David Machin - Страница 85

The ‘risk’ or proportion of patients whose corn was healed or resolved by a three‐month post‐randomisation is 32/95 = 0.337 or 34% with the corn plaster and 20/94 = 0.213 or 21% in the scalpel control group. The difference in proportions or RD is 0.337–0.213 = 0.124 or 12%.

The NNT is 8.065 or 9 (rounded up to the nearest person). Thus, on average one would have to treat nine patients with corn plasters in order to expect one extra patient (compared to scalpel treatment) to have their corn resolved at a three‐month follow‐up.

Each of the above measures summarises the study outcomes, and the one chosen may depend on how the test treatment behaves relative to the control. Commonly one may chose an absolute RD for a clinical trial and a relative risk for a prospective study. In general the relative risk is independent of how common the risk factor is. Smoking increases ones risk of lung cancer by a factor of 10, and this is true in countries with a high smoking prevalence and countries with a low smoking prevalence. However, in a clinical trial, we may be interested in what reduction in the proportion of people with poor outcome a new treatment will make.