The cumulative frequency is the sum of all simple frequencies.
So, to determine the cumulative frequency of each class, you have to add the simple frequency of the class to the cumulative frequency of the previous class.
The cumulative frequency of the first class is equal to the simple frequency since before the first category of the variable there is no observed frequency.
xi → value of the variable
fi → simple frequency of the i-class
Fi → cumulative frequency of the i-class
For the first class, the cumulative and simple frequencies are equal
For the second class, the cumulative frequency is equal to the sum of the cumulative frequency of the previous class and the simple frequency of the second class:
The cumulative frequency for the third class is equal to the sum of the cumulative frequency of the second class and the simple frequency of the third class:
The cumulative frequency of the last class of the frequency table corresponds to the sample size of the data set used to construct the table.