Date:         Fri, 15 Nov 1996 14:03:47 +1100
Reply-To:     Patrick Wig <patwig@DEAKIN.EDU.AU>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@UGA.CC.UGA.EDU>
From:         Patrick Wig <patwig@DEAKIN.EDU.AU>
Subject:      Wilcoxon handling of "ties" or equal ranks

Hello all,
 
A lurker awakes...
 
Because of site license restrictions, I don't have ready access to the full
manual set for SPSS 6.0 for Windows 3.1.  I've checked all the information
provided in the online help, and scanned through 13.5 MB of back messages in
this group dating back to June 1994, but not found an answer to the
following question.
 
The Wilcoxon signed-ranks test specifies one assumption: That the dependent
variable is continuous.  One symptom of a discontinuous dependent variable
is a large number of ties in the signed ranks.
 
Two solutions are recommended:  1)  Discard ties and reduce N; or 2) Retain
ties, divide them equally among positive and negative signs, and discard one
if there is an odd number.  This second option is considered more
conservative and therefore superior (at least by Gravetter & Wallnau, 1992),
since ties can be interpreted as evidence in support of the null hypothesis
and preserving them in this fashion increases the likelihood of retaining
the null hypothesis.  A third option is to find a better means of analysing
the data *grin*, but I'm interested in the first two options particularly.
 
Here comes the punch-line:  Which of the first two solutions is used by SPSS
when calculating the Wilcoxon Signed-Ranks test for related samples?  The
output does not make the decision clear.
 
Thanks in advance for your help,
 
Pat
 
 
-------------------
Gravetter, F. J., & Wallnau, L. B. (1992).  _Statistics for the
        Behavioral Sciences_ (3rd ed.).  St. Paul: West.
 
Patrick Wig
snail-mail:  School of Psychology,
             Deakin University, Geelong, Vic.,
             Australia 3217
e-mail:      patwig@deakin.edu.au
Phone:       x71189 (613 5227 1189)
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