It seems to me, with deltas this large (relative to the SD), that a significance test is a moot point!
David Cross d.cr...@tcu.edu www.davidcross.us On Apr 18, 2011, at 5:14 PM, Albyn Jones wrote: > 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. ______________________________________________ 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.
