Choice of classes 

In examples involving grouped data, consider the following points:

• interval sizes should be chosen so that there is a sensible number of classes, generally between five and ten
• class intervals should be of equal width except occasionally for the first or last class; for example, the last group in the example above would normally go from 45 to 50
• use class intervals that match our decimal system such as 0–4, 5–9, \(\cdots\) or 0–9, 10–19, \(\cdots\) or 0–99, 100–199.