Reports and Visualizations
Gravetter & Wallnau, Chapter 2; Young, Chapter 2
Copyright © 1997-8 by Forrest W. Young.
Grouped Frequency Distribution Tables
Oftentimes a set of data covers a wide range of values, making it unreasonable to list all of the individual potential scores that a variable might take on when construcing a frequency distribution table.
- Grouped Frequency Distribution Tables
- A grouped frequency distribution table has grouped X-value intervals in place of individual X values. The intervals correspond to a range of potential scores.
There are several rules that help guide you in constructing a grouped frequency distribution table. These rules are guidelines, not absolute requirements. The rules are:
- There should be about 10 class intervals.
- The width of each interval should be a "simple" number (2, 5, 10, 20, etc.).
- The bottom score in each class interval should be a multiple of the width.
- All intervals should be the same width.
For example, if we had the following scores:
82, 75, 88, 93, 53, 84, 87, 58, 72,
94, 69, 84, 61, 91, 64, 87, 84, 70,
76, 89, 75, 80, 73, 78, 60
Following the rules given above, we construct the grouped frequency distribution table shown at the right by using the following steps:
- We note that the maximum is 94 and the minimum is 53. Thus we have a range of 94 - 53 + 1 = 42 potential scores.
- We divide this range into about 10 class intervals (rule 1), each of which is a "simple" number (rule 2). This leads to 9 intervals with a width of 5 each.
- We make the bottom interval start at 50 since this is the first number below the minimum that is a multiple of the interval width (rule 3). The bottom inteval would be 50-54, the next one 55-59, etc. This makes all intervals the same width (rule 4).