Frequency is the number of times a particular observation occurs in data.

Class Interval

Data can be grouped into class intervals such that all observations in that range belong to that class.

Central tendencies


It is the average of “n” numbers, which is calculated by dividing the sum of all the numbers by n.

The mean  \bar x of n values  x_1,x_2,x_3,.....x_n is given by

\bar x=\dfrac{x_1,x_2,x_3....x_n}{n}


If we arrange the numbers in an ascending or descending order then the middle number of the series will be median. If the number of series is even then the median will be the average of two middle numbers.

If n is odd then the median is \left(\dfrac{n+1}{2}\right)^{th} observation.

If the n is even then the median is the average of \left(\dfrac{n}{2}\right)^{th} and \left(\dfrac{n+1}{2}\right)^{th} observation.


The number which appears most frequently in the series then it is said to be the mode of n numbers.

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