[R] power.t.test formula

Peter Dalgaard P.Dalgaard at biostat.ku.dk
Thu Apr 23 18:33:13 CEST 2009


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
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