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Odds ratio are computed from standard surveys. We must have an idea of the prevalence of exposure in the general population. (i.e. p2) and an idea of the OR, so we can compute the prevalence in the diseased (i.e. p1) These are just guesses to estimate the sample size. here p=0.5 gives the smallest sample size so estimate the p downwards, ie the smallest compatible with what is known. |