Additional Terminology
Lower Class Limit
– The least value that can belong to a class.
Upper Class Limit
– The greatest value that can belong to a class.
Class Width
– The difference between the upper (or lower) class limits of consecutive
classes.
All classes should have the same class width.
Class Midpoint
– The middle value of each data class.
To find the class midpoint,
average the upper and lower class limits.
upper
lower
class midpoint =
2
+
Example:
From the frequency table of statistics grades above.
The upper class limits are 99, 89, 79, 69, 59, and 49.
The lower class limits are 90, 80, 70, 60, 50, and 40.
The class midpoints are 94.5, 84.5, 74.5, 64.5, 54.5, and 44.5.
The width of each class is 10.
Creating a Frequency Table
1. Decide on the number of data classes you wish to use.
2. Divide the range of the data
(
29
range
highest value lowest value
=

by the number of classes to get an estimate of class width.
3. Decide on class bounds
4. Construct the frequency table by counting the number of data values in each class
Class Exercise:
Construct a frequency table with 6 data classes from the following data set. (p. 44 # 27)
Amount of gasoline purchased by 28 drivers:
7, 4, 18, 4, 9, 8, 8, 7, 6, 2, 9, 5, 9, 12, 4, 14, 15, 7, 10, 2, 3, 11, 4, 4, 9, 12, 5, 3
2