Date:         Wed, 19 Feb 1997 10:42:48 -0600
Reply-To:     "Nichols, David" <nichols@SPSS.COM>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@UGA.CC.UGA.EDU>
From:         "Nichols, David" <nichols@SPSS.COM>
Subject:      Re: Post-hoc query

>----------
>From:  William B. Ware[SMTP:wbware@EMAIL.UNC.EDU]
>Sent:  Wednesday, February 19, 1997 6:15 AM
>To:    Multiple recipients of list SPSSX-L
>Subject:       Re: Post-hoc query
>
>On Thu, 20 Feb 1997, John Reece wrote:
>
>> A simple query: Is the Tukey B post-hoc test the same as the post-hoc test
>> often called the Tukey-Kramer? If _not_, then what is the preferred method
>> for analysing post-hoc when sample sizes are unequal (sometimes wildly)?
>> Does it involve taking a harmonc mean of all groups?
>
>The Tukey B procedure is a compromise between the Tukey A procedure and
>the studentized Newman-Keuls procedure... The Tukey A uses the same
>critical value for each pairwise contrast, no matter how many means are in
>the "range."  The SNK is based on a "layered" approach, where the critical
>value varies with the number of means in the range.  The Tukey B procedure
>uses the average of Tukey B and SNK.  For example, consider the
>"manufactured" example below...
>
>#means in range         2       3       4
>
>Tukey A                 4       4       4
>
>SNK                     4       5       6
>
>Tukey B                 4       4.5     5.0
>
>**********
>
>In my opinion, the most appropriate is Tukey A.  As for the Tukey-Kramer,
>it is a modification of the Tukey A.  It uses the harmonic mean of the
>samples sizes of the two groups being contrasted, rather than the harmonic
>mean of all sample sizes.  It is the default in SPSS when you run Tukey's
>A.
>
>
>_____________________________________________________________________________
>_
>
>William B. Ware, Professor and Chair                   Educational Psychology
>CB# 3500                                               EMAIL:
>wbware@unc.edu
>University of North Carolina                           PHONE:
>(919)-966-5266
>Chapel Hill, NC      27599-3500                        FAX:
>(919)-962-1533
>
>URL:http://www.unc.edu/~wbware/
>_____________________________________________________________________________
>_
 
 
Bill's explanation is accurate. To elaborate, the default method in
pre-7.x
releases of SPSS, using the individual cell sizes or their harmonic mean
rather
than the harmonic mean of all cell sizes, is also known as the
Tukey-Kramer
method, and it is to be generally recommended in opposition to other
variants
on Tukey methods. What Bill refers to here as Tukey A is sometimes
called Tukey's
HSD (Honestly Significant Difference), and I think we refer to it that
way in
some of our documentation. In Release 7.0 and above, we actually give
two forms
of results for the Tukey approach. One does all pairwise comparisons
using the
Tukey A/HSD or Tukey-Kramer approach for unequal N, while the other
gives
homogeneous subsets based on the harmonic mean approach. For the Tukey-B
option,
we give only the homogeneous subsets results, since the test is by
definition a
sequential test.
 
David Nichols
Senior Support Statistician
SPSS Inc.
nichols@spss.com