# 6.1 Class Intervals

6.1 Class Interval

1. Data that consist of the measurement of a quantity can be grouped into few classes and the range of each class in known as the class interval.

(A) Class Limits and Boundaries

Lower Limit and Upper Limit
2. For class interval, for example 30 – 39, the smaller value (30) is known as the lower limit while the larger value (39) is known as the upper limit.

Lower Boundary and Upper Boundary
3. The lower boundary of a class interval is the middle value between the lower limit of the class interval and the upper limitof the class before it.

4. The upper boundary of a class interval is the middle value between the upper limit of the class interval and the lower limitof the class after it.

Example:
 20 – 29 30 – 39 40 – 49
$\begin{array}{l}\text{Lower boundary of the class}30-39\\ =\frac{29+30}{2}=29.5\\ \\ \text{Upper boundary of the class}30-39\\ =\frac{39+40}{2}=39.5\end{array}$

(B) Class size

5. The class size is the difference between the upper boundary and the lower boundary of the class.

Example:
Size of class interval 30 – 39
= Upper boundary – Lower boundary
= 39.5 – 29.5
= 10