First, note that you are doing two separate power calculations, one with n=2 and sd = 1.19, the other with n=3 and sd = 4.35. I will assume this was on purpose. Now...
> power.t.test(n = 2, delta = 13.5, sd = 1.19, sig.level = 0.05) Two-sample t test power calculation n = 2 delta = 13.5 sd = 1.19 sig.level = 0.05 power = 0.9982097 alternative = two.sided Now, with n=2, the power is already .99. With n=1, there are zero df. So, what n corresponds to a power of .8? > power.t.test(n = 1.6305, delta = 13.5, sd = 1.19, sig.level = 0.05) Two-sample t test power calculation n = 1.6305 delta = 13.5 sd = 1.19 sig.level = 0.05 power = 0.8003734 alternative = two.sided It looks like 1.63 subjects will do the job :-) Finally, look at the power.t.test function, there is a line that explains your error message: else if (is.null(n)) n <- uniroot(function(n) eval(p.body) - power, c(2, 1e+07))$root power.t.test() is making the sensible assumption that we only care about sample sizes of at least n = 2.... albyn On Mon, Apr 18, 2011 at 02:31:19PM -0700, Schatzi wrote: > I am trying to do a power analysis to get the number of replicas per > treatment. > > If I try to get the power it works just fine: > setn=c(2,3) > sdx=c(1.19,4.35) > power.t.test(n = setn, delta = 13.5, sd = sdx, sig.level = 0.05,power = > NULL) > > If I go the other way to obtain the "n" I have problems. > sdx=c(1.19,4.35) > pow=c(.8,.8) > power.t.test(n = NULL, delta = 13.5, sd = sdx, sig.level = 0.05, power = > 0.8) > > Is there any way to do this? Thank you. > > -- > View this message in context: > http://r.789695.n4.nabble.com/Power-Analysis-tp3458786p3458786.html > Sent from the R help mailing list archive at Nabble.com. > > ______________________________________________ > 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. > -- Albyn Jones Reed College jo...@reed.edu ______________________________________________ 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.