There are three methods to find the mean of a grouped data. So the choice of method to be used depends on the numerical values of xi and fi. If xi and fiare sufficiently small, then the direct method is used. If xi and fi are numerically large, then we can go for the assumed mean method or step-deviation method. If the class sizes are unequal and xi are large numerically, we can still apply the step-deviation method by taking h to be a suitable divisor of all the di’s.
Modal class: A class interval in a group data is called modal class which have maximum frequency.
Mean is the arithmetic average of observations of the data but mode is the average of that observations of data which belong to the group having maximum frequency, i.e., mode is most frequent value or most popular observation.
Median class: A class interval, in which cumulative frequency is greater than and nearest to n2 is called median class.
The median of a group data can be obtained graphically as the x coordinate of the point of intersection of more and less than ogive.
