How Should We Determine the Class Limits?

Class limits must be definite and clearly stated. Open-ended classes such as "70 and over" or "less than 10" are generally avoided. Limits should also be set so that the observations in each class tend to gather around the middle of the interval.

There are two methods of forming class intervals:

• Inclusive class intervals — values equal to both the lower and the upper limit are counted in that same class (e.g. classes written 0–10, 11–20, 21–30, …, 91–100).
• Exclusive class intervals — a value equal to a class limit is excluded from that class (e.g. classes written 0–10, 10–20, 20–30, …, 90–100).

For discrete variables either method can be used. For continuous variables the inclusive style is very often chosen (with the continuity later restored by an adjustment). …