Any data set accumulated for statistical purposes, such as the U.S. census, contains information that needs to be summarized and aggregated. Itâ€™d be impossible to list the attributes such as income and family size for each individual. Statisticians, therefore, use frequency distribution graphs to depict the data in a comprehensive manner, such as a histogram, which divides data into class intervals and counts the frequency at which all the members belonging to that class interval occur. Although there are no strict rules on how to calculate the size and number of class intervals, there are some conventional criteria that are useful.