From: Albyn Jones <jones_at_reed.edu>

Date: Mon, 18 Apr 2011 15:14:29 -0700

Date: Mon, 18 Apr 2011 15:14:29 -0700

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

> 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.
**>
**> ______________________________________________
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**>
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-- 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 - 22:16:45 GMT

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