Numerical data may be grouped by dividing the set of possible values into class intervals and counting the number of observations in each interval. For example, if possible test marks lie in the range 0 to 99, groups could be defined by the intervals 0–19, 20–39, 40–59, 60–79, and 80–99. It is often best (but not essential) to take the class intervals to be of equal widths.