Ask the Professor Forum

Hi Martina,

If there is a small number of classes involved (for example, just five classes), it is usually best to go to the next highest whole number. That means you can go from 7 to 8 safely, but you should always check. For example, say we have a maximum value of 40 and a minimum value of 5, and we would like to have a table with five classes. The smallest class width that could work is (40 - 5)/5 = 35/5 = 7. When we check 7, we can see it doesn't work:

  5 - 11  

12 - 18

19 - 25

26 - 32

33 - 39

Notice that 40 (our largest value) does not fit into the last class. This means we can try 8 instead.

5 - 12

13 - 20

21 - 28

29 - 36

37 - 44

Notice that now 40 fits nicely into our last class. This will usually work well, unless you have many classes. If you have 18 classes, going from 7 to 8 would probably not work out too well. Usually, with that many classes, the last two or three classes are empty if you use a width of 8 when the calculation yields a value of 7. It is in those situations that it becomes necessary to use a decimal like 7.1 or 7.2. 

Here is another post on the topic if you need more information:

http://www.statsprofessor.com/atpquestion.php?qid=102

Professor McGuckian