Finding the median and quartiles for grouped data

A clear description of how to find the median and quartiles of grouped data.

Median for Grouped Data

The median for grouped data can be found either by interpolation or by using a formula.
The formula is
Median =LB+w((1/2 n-∑ f_b)/f_m )
where
        LB is the lower boundary or limit of the class containing the median
        w is the width of the median class interval
        n is the total frequency, or the number of values
        ∑ f_b is the number of values below LB
        f_m is the number of values containing the median