```Date: Tue, 4 Aug 1998 19:11:35 -0600 Reply-To: "Douglas J. Anderson" Sender: "SPSSX(r) Discussion" From: "Douglas J. Anderson" 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. ```

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