Hello

I would appreciate clarity concerning which Normal distribution is

being used for the quantiles on the vertical axis of Q-Q plots when these are constructed in SPSS using the menu sequence 'Analyze --> Descriptive

Statistics --> Explore'. It seems from the scale in my plots that the

standard Normal distribution has been used. It is quite confusing that this is not fully explained in the SPSS documentation. In particular, having the

vertical scale ranging from approximately -2 to 3 while the horizontal axis is on the range of approximately 100 to 500 of my observed data seems to suggest that my above assumption is correct. Perhaps a fuller explanation is that the Normal plot which has the same standard deviation and mean as the observed data is used and then the values of the corresponding quantiles are converted to Z-scores. Is this correct?

Details would be helpful, please.

Thanks in advance!

You can see the computational details for the Q-Q plot in the Algorithms doc, which you can find from here

https://www.ibm.com/support/pages/node/3006603

Note that you can also use Analyze > Descriptive Statistics > Q-Q plot to get a different version of the plot. That is also documented in the Algorithms doc under PPLOT.

What neither of these can give you is a Q-Q plot of one variable against another or of a variable in two different groups. That functionality is available via the SPSSINC QQPLOT2 (Analyze > Descriptive Statistics > Two Variable or Group Q-Q Plot) extension command. That command requires the R Essentials.

Hey Bro,

A normal probability plot, or more specifically a quantile-quantile (Q-Q) plot, shows the distribution of the data against the expected normal distribution. For normally distributed data, observations should lie approximately on a straight line.

Thanks and Best Regards:

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Dear Jon

Thanks for your reply. I was really just looking for a simple answer regarding whether I was correct in my assumption regarding how the vertical axis is determined using the specific routine I highlighted. You appear to have directed me to a document of 1366 pages in length and it is not easy to see where the answer is to be found to my specific question.

Did you look at the specific command you are running in that document?

What is the exact name of the document to which you are referring? In my original query I specified "the menu sequence 'Analyze --> Descriptive

Statistics --> Explore'. ".

paste the syntax and look up the command name in the Algorithms doc. Or, if you prefer, use the Q-Q plot menu/procedure, which gives you a different version of the plot.