What is the rationale behind having both the delta and sd parameters for the power.t.test function? For the relevant noncentrality parameter, we only need the ratio delta/sd. If my effect size is given as Cohen's d, then I only got that ratio and not sd.
As far as I see, in such a case, I can specify delta=d and leave sd at its default value 1. Is this correct or am I missing something? Thanks.
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