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(i) Variable: Any character that can vary from one individual to other is called variable.

(ii) Class-interval: In the data of each group into which raw data is considered is called a Class-interval.

(iii) Class-size: The difference between the upper class limit and the lower class limit is called the class size of the class.

Class-mark: The middle value of the class is called the class-mark.
Class mark =

(iv) Frequency: The number of observations corresponding to class is called its frequency.

(v) Class-limit: Each class is bounded by two figures, called the class limits. The figures on the left side of the classes are called lower limits while the figures on the right sides are called the upper limits.

(vi) True class limits: If classes are inclusive e.g., 15-19, 20-24, 25-29, 30-34

Then, true lower limit of class = Lower limit of class – 0.5

And, true upper limit of class = Upper limit of class + 0.5

e.g., True limits of class 15-19 are 14.5-19.5