Michael Dewey wrote:
At 00:35 14/08/2009, micha...@unimelb.edu.au wrote:
Full_Name: Michael J. Lew
Version: R version 2.9.1 (2009-06-26)
OS: OS X
Submission from: (NULL) (210.49.195.149)
The default function power.t.test gives inaccurate values,
particularly with
large sig.level.
There are two reasons for me to propose that power.t.test is inaccurate:
1. When sig.level approaches 1, so should the power. However,
power.t.test('n'=5,'d'=1,'sig.level'=1) yields a power of 0.9430769. I've
compared it to the equivalent function in the 'pwr' package,
pwr.t.test, and
while power.t.test gives a smooth function with respect to sig.level, it
deivates systematically more and more as sig.level is increased.
pwr.t.test('n'=5,'d'=1,'sig.level'=1) gives the correct power of 1.
I think you need to set strict=TRUE when your example gives a power of 1
here.
Thanks. Yes. strict=TRUE was designed for people bothered by the
behaviour as d approaches zero, but this is essentially the same issue.
I do find it somewhat odd that people take Neyman-Pearson theory so
literally that they insist on counting rejections with the opposite sign
as the truth as "successes"...
--
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