Assessing Normality
Are my data normally distributed?
Evidence for Normality
• Mean and median should not be too
different.
• Histogram or stem-and-leaf plot should look
symmetric.
• Box plot should look symmetric.
• Normal probability plot should roughly be
a straight line.
Evidence against Normality
• Mean and median are wildly different.
• Histogram or stem-and-leaf plot look
skewed.
• Box plot should look skewed.
• Normal probability plot does not follow a
straight line.
There are several variations of normal probability plots.
We will only use the MINITAB style.
Data
Normal Probability Plot Interpretation
skewed left
normal
skewed right
How to get one in Minitab?
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Select Graph.
Select Probability Plot.
Select the desired variable.
Specify Normal in the Distribution box.
Select OK.