Procedure for constructing a frequency table

Follow the procedure for constructing a frequency table given below for a set of quantitative data.

  • Pick a number between 5 and 20 for the number of classes you want to use. You are free to choose any number you like. However, this number will likely increase as the size of a data set increases.
  • Calculate the class width using the formula below. Round the answer either up or down to get a convenient number. The class width determines the size of the classes. The goal is to have classes that have the same size. For example, the two classes 6-9 and 14-17 have the same width of 4.
Class width   =
Highest value - Lowest value / Number of classes
 
  • Pick a number that you will use as the lower limit of the first class. Choose either the lowest number or a number a little smaller than the lowest number.

  • Add the class width to the lower limit of the first class to get the lower limit of the second class. Add the class width to the lower limit of the second class in order to get the lower limit of the third class. Keep doing this until you have found the lower limit of the last class. Subtract 1 from the class width. Add the result to the lower limit of the first class in order to find the upper limit of the first class. To find all other upper limits, use the same technique used above to find all the lower limits.
  • Find the frequency of each class and then put the classes along with the corresponding frequencies in a table.

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