in case-control intervention study (2 groups). What should be selected to calculate power sample size:
1- Data of primary outcome from previous published article
2- Minimal clinically importance difference for the primary outcome
3- Prevalence of the health condition
You need:
1. the magnitude of the effect you wish to detect.
2.the alpha probability for the significance of the effect you wish to detect
3.an estimate of the variance of the variable you are studying
1.is from prior content knowledge of the field
2.is determined by the investigator,possibly also with prior content knowledge
3.is estimated from previous studies.
then you can determine (a) power of study given a known sample size OR
(b)sample size required to determine specified effect at given alpha level
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