### 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; between five and ten classes are commonly used
• 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.