From meyerjp at jmu.edu  Thu Sep 23 21:27:09 2004
From: meyerjp at jmu.edu (meyerjp@jmu.edu)
Date: Thu Sep 23 21:27:13 2004
Subject: [Rd] noncentrality paramter in power.anova.test (PR#7244)
Message-ID: <20040923192709.3D66110FBC@slim.kubism.ku.dk>

Full_Name: 
Version: 1.9.0
OS: Windows XP
Submission from: (NULL) (134.126.93.191)


I believe the noncentrality parameter, lambda, in power.anova.test is incorrect.
The noncentrality paramter is defined as lambda <- (groups - 1) * n *
(between.var/within.var). The n should not be there. The function pf defines the
noncentrality parameter as the sum of squares of the means. Therefore, the
noncentrality paramter, lambda, in power.anova.test should be lambda <- (groups
- 1) * (between.var/within.var). The following code shows the difference in
results. I've compared a few power computations using the new lambda to values
from a text on power. The new lambda produces more correct values for power.


groups <- 3
n <- 5
between.var <- 10.295
within.var <- 1
sig.level <- 0.05


#The first, incorrect lambda is used in power.anova.test
#incorrect labmda
lambda <- (groups - 1) * n * (between.var/within.var)
pf(qf(sig.level, groups - 1, (n - 1) * groups, lower = FALSE), 
            groups - 1, (n - 1) * groups, lambda, lower = FALSE)

#This correct lambda should be used instead
lambda <- (groups - 1) * (between.var/within.var)
pf(qf(sig.level, groups - 1, (n - 1) * groups, lower = FALSE), 
      		groups - 1, (n - 1) * groups, lambda, lower = FALSE)


[Rd] noncentrality paramter in power.anova.test (PR#7244)
meyerjp@jmu.edu