Find the mean deviation about the median for the following data; 3, 9, 5, 3, 12, 10, 18, 4, 7 , 19, 21

Here, the number of observations is11 which is odd. Arranging the data into ascending order, we get 

3,3, 4, 5,7 ,9, 10, 12, 18, 19,21

$\text{ Median }={\left(\frac{11+1}{2}\right)}^{\text{th }}\text{ or }{6}^{\text{th }}\text{ observation }=9$

The absolute values of the respective deviations from the median $\text{ i.e., }\left|x_{\mathrm{i}}-M\right|\text{ are: }$

$\begin{array}{l}6,6,5,4,2,0,1,3,9,10,12\\ \text{ Therefore, }\sum _{i=1}^{11} \left|x_i-M\right|=58\\ \text{ and M.D. }\left(M\right)=\frac{1}{11}\sum _{i=1}^{11} \left|x_i-M\right|=\frac{1}{11}\times 58=5.27\end{array}$