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.Dalgaard@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             Øster Farimagsgade 5, Entr.B
>  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
>  (*) \(*) -- University of Copenhagen   Denmark      Ph:  (+45) 35327918
> ~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)              FAX: (+45) 35327907
>
>

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