# [R] Tukey multiple comparisons

john.maindonald@anu.edu.au john.maindonald at anu.edu.au
Fri Mar 24 08:24:02 CET 2000

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At 12:56 AM 24/03/00 -0500, Chong Gu wrote:
>
>   From: Douglas Bates <bates at stat.wisc.edu>
>   I am embarrassed to have to ask this but can anyone tell me of a Tukey
>   multiple comparisons procedure available for R?  I have looked through
>   the search page, through the FAQ, and in the index of V&R (1999), and
>   I still can't find such a thing.  I see there is a ptukey function and
>   a qtukey function but that is as far as I got.  Do I need to roll my
>   own?
>   --
>   Douglas Bates                            bates at stat.wisc.edu
>   Statistics Department                    608/262-2598
>   University of Wisconsin - Madison        http://www.stat.wisc.edu/~bates/
>   -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
>Doug,
>
>Try qtukey to get the quantile, then divide by sqrt(2) to get the
>multiple.
>
>Chong
>-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-

Or you may want the generalisation to nmeans>2 and df
different from 100 (= df of estimate of common variance)

> ptukey(1.96*sqrt(2),2,100,lower.tail=F)
[1] 0.0527789

The studentised range divides by the (assumed common) SE of the
means, while the equivalent t-statistic divides by the SED; hence
the sqrt(2).  If the SEDs are somewhat unequal, it may not be

ptukey(t*sqrt(2), nmeans,df,lower.tail=F)

where t is the largest of the pairwise t-statistics.
[But one must not call it t!]

John Maindonald               email : john.maindonald at anu.edu.au
Statistical Consulting Unit,  phone : (6249)3998
c/o CMA, SMS,                 fax   : (6249)5549
John Dedman Mathematical Sciences Building
Australian National University
Canberra ACT 0200
Australia

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