From michaell at unimelb.edu.au  Fri Aug 14 01:35:14 2009
From: michaell at unimelb.edu.au (michaell at unimelb.edu.au)
Date: Fri, 14 Aug 2009 01:35:14 +0200 (CEST)
Subject: [Rd] power.t.test (PR#13891)
Message-ID: <20090813233514.62E9E282EF50@mail.pubhealth.ku.dk>

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.

2. I've run empirical simulations and found that the power predicted by
pwr.t.test is correct and so it is clear that power.t.test is inaccurate.



[Rd] power.t.test (PR#13891)
michaell at unimelb.edu.au