From: Albyn Jones <jones_at_reed.edu>

Date: Mon, 18 Apr 2011 15:37:05 -0700

Date: Mon, 18 Apr 2011 15:37:05 -0700

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.cross_at_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_at_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
**> > jones_at_reed.edu
**> >
**> > ______________________________________________
**> > R-help_at_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 jones_at_reed.edu ______________________________________________ R-help_at_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.Received on Mon 18 Apr 2011 - 22:40:04 GMT

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