Here the number of observations is 11 which is odd.
Arranging the data into ascending order, we have 4, 6, 7, 8, 9, 11, 13, 15, 19, 21, 25
Now, Median (M) = \(\Big(\frac{11+1}{2}\Big)^{th}\) or 6th observation = 11
The respective absolute values of the deviations from median , i.e.,|Xi - M| are 7, 5, 4, 3, 2, 0, 2, 4, 8, 10, 14
Thus, the required mean deviation about the median is
