Deciding how many classes to use for grouping the data is a compromise between the extremes of too much detail (each observation in its own category) and insufficient detail (only one category). Most frequency tables are constructed according to the following guidelines:
- For most data, 6 to 15 classes are enough.
- Class intervals (lengths) should be equal. Intervals such as 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50, etc are desirable.
- The starting point for each class should be divisible by the interval; for example, in the class 15–20, the starting point, 15, is divisible by the interval, 5.
- Each observation must fit into only one class.
- When many points fall around a certain value, make this value the approximate center of the frequency distribution.
For the data in our example, the minimum is 65, and the maximum is 114, a range of about 50. We can, therefore, choose intervals of 5 and have ten of them. Our classes are 65–70, 70–75, etc.
