As you will see shortly, many of the methods for statistical inference we discuss here (and that are widely used in practise) require the data to come from a normal distribution. How do we check that?
Boxplot
We will check the assumption of normality via two graphs. The first of these we already talked about previously, namely the boxplot. Here are some boxplots for data from a normal distribution:
Here are some features of boxplots for normal data:
There are very few "outliers", and those are close to the boxplot
The left whisker, the right whisker and the box are all about equally large.
Compare this to the following boxplots of non-normal data:
Normal Probability Plot
This is a graph specifically designed to check for normality. If the data comes from a normal distribution the points should form a line. Again, let's start with some examples of normal data:
and some examples of non-normal data:
In MINITAB you can draw the normal probability plot using Graph > Probability Plot