Guidelines for Creating Class Intervals

Although we are not following these strict guidelines in creating class intervals for grouped frequency distributions in Ed 602, you may wish to know what they are. These guidelines are from an excellent, but currently out of print, introductory statistics textbook, Fundamentals of Statistics for the Behavioral Sciences by James V. Couch (West Publishing Company, 1987, page 17).

Guidelines for Creating Class Intervals:

1. There should be approximately 7 to 20 mutually exclusive class intervals. "Mutually exclusive" means that a score can belong to only one class interval. Two non-mutually exclusive class intervals would be 45-49 and 47-51 since the scores 47, 48, and 49 could belong to either class interval. In a grouped frequency distribution, the class intervals must be mutually exclusive.
2. Do not omit any class intervals. Just as with a regular frequency distribution, all possible scores between the largest score and the smallest score of the data set must be included in the grouped frequency distribution. Even if an interval has a frequency of zero, it is to be included in the list of class intervals.
3. The class interval size should be 3 or 5 or a multiple of 5. The class interval size is defined as the upper real limit of a class interval minus the lower real limit of the class interval. For instance, in the class interval 45-49, the class interval size is 49.5 - 44.5 = 5.
4. Pick the smallest class interval size of 3 or 5 or a multiple of 5 that also satisfies the first guideline of producing approximately 7 to 20 class intervals. In other words, if a class interval size of 25 will produce approximately 18 class intervals and a class interval size of 30 will produce approximately 12 class intervals, you should choose 25 as your class interval size. The rationale for this rule is that it is better to undersummarize your data, by using a smaller class interval size, than to oversummarize your data, which would result with a larger class interval size.
5. The class interval size should be equal for all class intervals. If the class interval size were not equal for all class intervals, then we could not perform the statistical computations which use grouped frequency distributions.
6. The lower apparent limit of each class interval should be a multiple of the class interval size. If the lowest score in the data set is 46 and a class interval size of 5 has been selected, the first class interval would be 45-49 because 45 (the lower apparent limit) is a multiple of 5 while 46 is not.