Data sets often include many measurements from each individual. For example, the following information might be collected from all students who completed a statistics course.

Information about students in a Statistics course
Student name   Age   Sex Test mark(%)   Maths grade   Stats grade
John 19 Male 76 B A
Michelle 20 Female 62 B C
Alex 18 Male 48 Not enrolled C
Clare 19 Female 68 A B
... ... ... ... ... ...

Some measurements are numerical (age and marks in an internal test) and they can be examined with the graphical and numerical summaries that were described in earlier chapters. Other measurements (sex, maths grade and stats grade) do not have numerical values and are called categorical data. Different methods are required to display (and get information from) batches of categorical measurements.