What is a QQ normality plot? How can I make one with GraphPad Prism?

Many statistical tests assume that data (or residuals) are sampled from a Gaussian distribution. Normality tests are often used to test for evidence that the distribution is not Gaussian, but these tests are less helpful than many scientists expect them to be. Many books (but not mine) and programs (including GraphPad Prism starting with version 7) suggest using a QQ normality plot (as shown above) to visually assess whether or not a distribution appears to be sampled from a Gaussian distribution. The problem is that most books give very complicated explanations of QQ plots, making it very hard to figure out what exactly they mean. In fact, the basic idea of a QQ normality plot is pretty easy to understand.

The attached six page document explains the idea behind QQ normal plots, and shows you you get Prism 6 to make one. The attached files require Prism 6, and will not work with earlier versions. The file for Figure 1 shown above is essentially a template. Replace the data with your own, and the graph will regenerate. 

Download the document (207K pdf).

Download the Prism file for Figure 1 (make a QQ normal plot from data you enter).

Download the Prism file for Figure 2 (shows examples of QQ plots from normal distributions that don't look quite linear).

Download the Prism file for Figure 3 (QQ plot from lognormal data)

(Word doc)