Can I please ask a quick question again on this? Is there a power test function for z-test? Obviuosly, ?power.z.test does not give me anything.
thx much ej On 10/27/06, ONKELINX, Thierry <[EMAIL PROTECTED]> wrote: > A quick answer to your questions: > > 1. Since nobody knows the "true" delta. I prefer to calculate the power for a > range of deltas. Most of the time for a range spanning - 2 * expected delta > up to 2 * expected delta. This gives an idea on how the power changes if > delta changes. > 2. ?power.t.test explains how to calculate n for a given power, delta, sd and > sig.level. A quote from ?power.t.test: "Exactly one of the parameters 'n', > 'delta', 'power', 'sd', and 'sig.level' must be passed as NULL, and that > parameter is determined from the others." > > Cheers, > > Thierry > > ---------------------------------------------------------------------------- > > ir. Thierry Onkelinx > > Instituut voor natuur- en bosonderzoek / Reseach Institute for Nature and > Forest > > Cel biometrie, methodologie en kwaliteitszorg / Section biometrics, > methodology and quality assurance > > Gaverstraat 4 > > 9500 Geraardsbergen > > Belgium > > tel. + 32 54/436 185 > > [EMAIL PROTECTED] > > www.inbo.be > > > > Do not put your faith in what statistics say until you have carefully > considered what they do not say. ~William W. Watt > > A statistical analysis, properly conducted, is a delicate dissection of > uncertainties, a surgery of suppositions. ~M.J.Moroney > > > -----Oorspronkelijk bericht----- > Van: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Namens Ethan Johnsons > Verzonden: vrijdag 27 oktober 2006 16:59 > Aan: Peter Dalgaard > CC: r-help@stat.math.ethz.ch > Onderwerp: Re: [R] Power of test > > Thank you so mcuh for the explanation, Chuck & Peter. > > Two quick questions,please. > > It states that delta = True difference in means. When the true diff > is unkown, can you use the expected diff for delta. > > If you want to know the n (number of observations) off of power.t.test > to have i.e. 80% power, how do you calculate? Is there a way to do it > in R, or use algebra? > > power.t.test(n = NULL, delta = NULL, sd = 1, sig.level = 0.05, > power = NULL, > type = c("two.sample", "one.sample", "paired"), > alternative = c("two.sided", "one.sided"), > strict = FALSE) > > Thank you, > > ej > > On 27 Oct 2006 16:37:08 +0200, Peter Dalgaard <[EMAIL PROTECTED]> wrote: > > "Ethan Johnsons" <[EMAIL PROTECTED]> writes: > > > > > What would be the R formulae for a two-sided test? > > > > > > I have a formula for a one-sided test: > > > > > > powertest <- function(a,m0,m1,n,s){ > > > t1 = -qnorm(1-a) > > > num = abs(m0-m1) * sqrt(n) > > > t2 = num/s > > > pow = pnorm(t1 + t2) > > > } > > > > > > Would you pls let me know if you know of? > > > > (Notice that power.t.test does this more accurately) > > > > For practical purposes, just halve a. Perfectionists may want you to > > add pnorm(t1 - t2), so that the total power becomes a when t2 == 0. > > > > BTW: -qnorm(1-a)==qnorm(a) > > > > -- > > 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 > > ~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907 > > > > ______________________________________________ > R-help@stat.math.ethz.ch mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.