Sample size and power
- clinical studies such as randomised controlled
trials are designed to test whether a difference exists between two or more interventions
in terms of specific outcomes or endpoints
- in order for a study to detect
a statistically significant difference between treatments, the study must be large
enough (i.e. have enough subjects participating in the study) for a sufficient
number of endpoints of interest to occur
- the 'power' of the study is the
ability of the study to reliably detect a difference between interventions
number of subjects required to be included in the study in order to have sufficient
power must be made before a study begins
- ideally clinical study reports
should indicate that a power calculation has been made - it is common for studies
to stipulate a power of 80–90%
- the power of a study refers to its ability
to detect a difference only in that endpoint on which the power calculation is
based, i.e. the primary endpoint. The power used in the study may not be sufficient
to reliably detect differences in other (secondary) endpoints or in subgroups
power of a study can be calculated via knowledge of the type II error (a type
II error arises when the null hypothesis is accepted when it is false) (1)
(the chance that the study will detect the minimum difference as statistically
different) = 1 - type II error
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S. The principles of evidence-based medicine.Cephalalgia. 2000;20 Suppl 2:10-3.
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