Sample size og number needed to treat--en statistisk kasuistik

The sample size necessary in each group to detect a significant effect of a vaccine on a given disease in a randomised clinical trial (RCT) can be calculated from the observed disease incidence without vaccine, the expected incidence with vaccine, and the desired level of significance and statistical power of the study. In this example, sample size is calculated for an RCT of the effect of pneumococcal vaccine on the incidence of pneumococcal sepsis. The number of persons needed to be vaccinated to prevent one episode of pneumococcal sepsis can be similarly calculated from the observed incidences with and without vaccine.