From: <meyerjp_at_jmu.edu>

Date: Fri 24 Sep 2004 - 05:27:09 EST

R-devel@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-devel Received on Fri Sep 24 05:31:34 2004

Date: Fri 24 Sep 2004 - 05:27:09 EST

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)

R-devel@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-devel Received on Fri Sep 24 05:31:34 2004

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