Industrial Statistics with Minitab

4.2 Stratification

By default, Minitab distinguishes the groups (in this case, Sex = 1 or Sex=2) by different colors and symbols as shown above. However, when the graph is printed in black and white, the observed difference is not very clear.

If you want to change the type of symbol (and also the color and size) used for each group, follow the next steps:

First, click on any of the data points; this will select all of them. Second, click again on the same data point; this will now select solely the points of a group. Finally, double-click on the same data point to open a dialog box that will allow you to change the color, symbol and size for all points in that group.

Graph > Scatterplot… : With Groups is not the only way to get a stratified scatterplot. For instance, if you have drawn a scatterplot from Graph > Scatterplot …: Simple, you can also add a stratification variable by first double-clicking on any of the data points and then choosing the Groups tab in the dialog box that appears, as shown below:

Notice that the result is the same as the one obtained when stratification is performed directly using the commands: Graph > Scatterplot… : With Groups.