When the whole range of values is classified in some groups in the form of intervals, then each such interval is known as class interval. For example, we may want to classify a group of people according to age group. Here, the class intervals may be 15 - 19 years, 20 – 24 years, 25 – 29 years etc.
The difference between the upper and lower boundaries of a class interval is called the magnitude of the class interval.
The numerical figures used to specify the lower and upper limits of a 'class interval' are called class limits. For example, if the class interval is 15 – 19 years, the lower class limit is 15 and the upper limit is 19 in this case.