From: David Cross <d.cross_at_tcu.edu>

Date: Mon, 18 Apr 2011 17:23:05 -0500

*> 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?
*

*>
*

*> 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:
*

*>
*

*> --
*

*> Albyn Jones
*

> Reed College

*> jones_at_reed.edu
*

*>
*

*> ______________________________________________
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*> R-help_at_r-project.org mailing list
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*> https://stat.ethz.ch/mailman/listinfo/r-help
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*> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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*> and provide commented, minimal, self-contained, reproducible code.
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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 - 22:25:47 GMT

Date: Mon, 18 Apr 2011 17:23:05 -0500

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

>> 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

>> 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. >>

> Reed College

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 - 22:25:47 GMT

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