A class interval is the numerical width of one of the classes that data is classified into. We often determine a desired class interval by looking at our data, and then break it up into groups that fall inside each of the chosen ranges. The different class intervals usually have the same width, but this may not necessarily be the case.

For example, you might want to measure the spines of the echidna in the picture. After making your measurements, you might find it sensible to divide the spines up into gorups having ranges of 10 mm in length: 0-10 mm, 11-20 mm, 21-30 mm, 31-40 mm, 41-50 mm, and 51-60 mm. This would give you 6 groups, each having a class interval of 10 mm.