## 4.2.2.3 Class frequency

 4.2.2.3 Class Frequency

The number of observations corresponding to a particular class is known as the frequency of that class or the class frequency. In the following illustration, the frequency of the class 1000-1100 is 50 which implies that there are 50 persons having income between Rs.1000 and Rs.1100. If we add together the frequencies of all individual classes, we obtain the total frequency. Thus, in the same problem, the total frequency of the six classes is 550 which means that in all there are 550 persons whose income has been studied.

Class Mid-point or Class Mark : It is the value lying half-way between the lower and upper class limits of a class-interval. Mid-point of a class is ascertained as follows :

Mid-point of a class= For the purpose of further calculations in statistical work the mid-point of each class is taken to represent that class.

There are two methods of classifying the data according to classes, namely (i) ‘exclusive’ method and (ii) ‘inclusive’ method.