Yes, Richard Savage used to call this "inter ocular data"; the answer should leap up and strike you right between the eyes...
albyn On Mon, Apr 18, 2011 at 05:23:05PM -0500, David Cross wrote: > 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. > > -- 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.
