# [R] Power Analysis

Albyn Jones jones at reed.edu
Tue Apr 19 00:37:05 CEST 2011

```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
> >
> > 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.
> >>
> >> ______________________________________________
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> >> and provide commented, minimal, self-contained, reproducible code.
> >>
> >
> > --
> > Albyn Jones
> > Reed College
> > jones at reed.edu
> >
> > ______________________________________________
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