Class Intervals
(A worked Problem:  Procedure explained in more detail in Chapter 2)

Create class intervals for the following distribution of 15 scores:

46, 48, 54, 58, 59, 64, 65, 77, 77, 79, 84, 89, 89, 91, 97

I. Find the high and low score and then compute the Range:

Range = High Score - Low Score

Range = 97 - 46  = 51

II. Because the Range is greater than 20 we will create a group frequency distribution

III. Next we must choose the interval size ( i )

A. i must be an easy to count by number (2, 3, 5, 10, 20, 25, 50, 100,...)

B. When i is divided into the range will result in between 10 and 20 intervals

C. Choose i = 5

D. Number of intervals = Range / i

Number of intervals = 51 / 5 = 10.2

(This will give us about 11 intervals)

IV. Choose the Apparent Limits for the highest interval

A. The lower apparent limit (LAL) of the top interval

1. All lower apparent limits must be a multiple of i

2. Choose the closest multiple of i to the highest score.....95

(remember not to choose a number greater than the high score)

B. The upper apparent limit (UAL) of the top interval = LAL + i -1

UAL = 95 + 5 - 1 = 99

V. Create the apparent limits of the class intervals

Apparent Limits
95 - 99
90 - 94
85 - 89
80 - 84
75 - 79
70 - 74
65- 69
60 - 64
55 - 59
50 - 54
45 - 49

Hint:  Always remember to choose an interval size (i) that is easy to count by.

Hint:  Remember that the lowest interval must contain the lowest score and
            the highest interval must contain the highest score.

Hint:  Remember that the Apparent Limits do not have to be equal to the highest or lowest scores.

Copyright © 2004 by Mark W. Vernoy