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Class size: Difference between the true upper limit and true lower limit of a class interval is called the class size.
Class size remains the same for all class intervals.
For the class interval 10 - 20
Class size is 10, i.e., (20 - 10 = 10)
Class mark: Mid-value of each class interval is called its class mark.
Class mark = ½ (Upper limit + Lower limit)
For the class interval 10 - 20
Class mark = (10 + 20)/2
= 30/2
= 15
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