Hello, Thank you, but can you understand this result? first I calculate the sd for n = 2 and then n with that sd. It should give me 2 right?
> FC = 1.5 > alfa = 0.01 > power = 0.85 > sd1 <- power.t.test( n = 2, delta = FC, sig.level = alfa, + power = power, type = "two.sample", sd = NULL)$sd > > n1 <- round( power.t.test( n = NULL, delta = FC, sig.level = alfa,power = power, type = "two.sample", sd = sd1)$n) Error in uniroot(function(n) eval(p.body) - power, c(2, 1e+07)) : f() values at end points not of opposite sign 2009/4/23 Peter Dalgaard <p.dalga...@biostat.ku.dk> > Usuario R wrote: > > Hi, > > > > Does anyone of you knows a reference for the formula used in power.t.test > > function? And also why it uses the Student's distribution instead of > Normal. > > (I know both of them can be used but don't see whether choose one or the > > other) > > It is a straightforward first-principles calculation. The t > distribution calculation is exact for normally distributed data with the > same unknown variance in both groups. Formulas based on the normal > distribution assumes the variance to be known, which overestimates small n. > > > > -- > O__ ---- Peter Dalgaard Ă˜ster Farimagsgade 5, Entr.B > c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K > (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 > ~~~~~~~~~~ - (p.dalga...@biostat.ku.dk) FAX: (+45) 35327907 > > [[alternative HTML version deleted]]
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