Assessing Normality

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? • • • • •

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