Date:         Tue, 4 Aug 1998 19:11:35 -0600
Reply-To:     "Douglas J. Anderson" <Douglas.Anderson@ENMU.EDU>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@UGA.CC.UGA.EDU>
From:         "Douglas J. Anderson" <Douglas.Anderson@ENMU.EDU>
Subject:      Q-Q Normal Plots
Content-Type: text/plain; charset="us-ascii"

To Whom May Know The Answer:
 
I have been using the graduate pack edition for Power Macintosh of SPSS
6.1.1.  I have been trying to do some data analysis for my masters thesis
research with the Normal Q-Q Plots.  After doing quite a bit of reading in
the journals about these plots, I have several questions about the SPSS
method of obtaining them.
 
1)  Why do the plots place the observed value on the X-axis and the
quantiles (often labeled the Qj from the expected normal distribution in
the literature) on the Y-axis?  This is just the opposite of the standard
method of reporting the observed values on the Y-axis with the expected
normal quantiles on the X-axis.  Every textbook and literature with graphs
on Q-Q plots places the observations the opposite of the SPSS method.
 
2)  Why can I not swap axes so that my Q-Q graphs conform to the literature?
 
3)  I would like to know whether there is a way that I can gain access to
the actual quantiles that the software must compute in order to plot the
expected normal values against the observed values of my data?  I can
generate these values with a spreadsheet by long laborious calculations (I
am using the Blom method of (i - .385)/n +.25 with ties for rank broken by
the mean calculation).  In this manner, I can then plot my data as a
scatterplot where I can control the X and Y axes and plot the data in the
customary manner.  Also, I can then generate the correlation coefficient to
determine the best transform of my data, rather than rely on the
Shapiro-Wilk's statistic or K-S statistic produced by the Q-Q Plot of SPSS.
The problem is, it just takes too long to do this.  Since the software is
calculating these quantiles, is there not some way that I can write a
syntax command to output this information to a file so that I can then use
it to produce my own scatterplots of observed against expected and generate
a correlation coefficient?  (Please see Stephen W. Looney and Thomas R.
Gulledge, Jr., 1985, The American Statistician, February 1985, Vol. 39, No.
1 for the merits of this procedure.)
 
If there is anyone there reading this message who can help a graduate
student in anthropology/archaeology with this problem, then I would greatly
appreciate it.
 
Sincerely,
 
Douglas J. Anderson
Masters Candidate in Anthropology
Eastern New Mexico University
Portales, NM 88130
Douglas.Anderson@enmu.edu
 
P.S. There is only one faculty member on our entire campus who uses SPSS
and he does not use or know anything about the Q-Q plots so I have no where
else to go for help.