Look at the pwr package, it has functions for 2 samples of different sizes.
Hope this helps, -- Gregory (Greg) L. Snow Ph.D. Statistical Data Center Intermountain Healthcare greg.s...@imail.org 801.408.8111 > -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r- > project.org] On Behalf Of Karl Ove Hufthammer > Sent: Friday, April 24, 2009 2:41 AM > To: r-h...@stat.math.ethz.ch > Subject: Re: [R] power.t.test formula > > Peter Dalgaard: > > >> 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. > > I didn’t know about power.t.test. A very nice function, indeed. Is > there a > similar function that handles unequal number of observations for each > group, and unequal variances? > > The help file mislead me into thinking that power.t.test handles > unequal > number of observations, and for example ‘power.t.test(c(10,3),1)’ > *does* > give some output (and not a warning or error), just not the output I > was > expecting. > > (It gives two power values, one for sample of size 10 (for each group) > and > one for samples of size 3 (for each group), while I was expecting the > power > for a sample size 10 for one group and 3 for the other group). > > -- > Karl Ove Hufthammer > > ______________________________________________ > R-help@r-project.org 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@r-project.org 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.
