Assessing Normality

1
Assessing Normality

• Are my data normally distributed?

2
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.

3
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.

4
There are several variations of normal
probability plots. We will only use the MINITAB
style.
Percentile
Data
5
Normal Probability Plot Interpretation
skewed left
normal
skewed right
6
How to get one in Minitab?

• Select Graph.
• Select Probability Plot.
• Select the desired variable.
• Specify Normal in the Distribution box.
• Select OK.