Before the start of any controlled trial it is necessary to decide number of patients that will be needed to deliver an answer, for ethical, as well as practical reasons. This is determined by four factors:
1. The magnitude of the difference sought or expected on the primary efficacy endpoint (the target difference). For between-group studies, the focus of interest is the mean difference that constitutes a clinically significant effect.
2. The variability of the measurement of the primary endpoint as reflected by the standard deviation of this primary outcome measure. The magnitude of the expected difference (above) divided by the standard deviation of the difference, gives the standardised difference (see Fig. 4.2)
3. The defined significance level, i.e. the level of chance for accepting a Type I (a) error. Levels of 0.05 (5%) and 0.01 (1%) are common targets.
4. The power or desired probability of detecting the required mean treatment difference, i.e. the level of chance for accepting a Type II (P) error. For most controlled trials, a power of 80-90% (0.8-0.9) is frequently chosen as adequate, though higher power is chosen for some studies.
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