Calculate the mean deviation about the mean of following frequency distribution:

Class Interval	0-4	4-8	8-12	12-16	16-20
Freq.	4	6	8	5	2

Class Interval	$f_i$	$x_i$	$f_i{x}_i$
0-4	4	2	8
4-8	6	6	36
8-12	8	10	80
12-16	5	14	70
16-20	2	18	36
Total	$\sum f_i=25$		$\sum f_i{x}_i=230$

Mean $\left(\overline{x}\right)=\frac{\sum f_i{x}_i}{\sum f_i}=\frac{230}{25}=9.2$


$\overline{x}=9.2$

Class Interval	$f_i$	$x_i$	$f_i{x}_i$	$d_i=|x_i-\overline{x}|$	$f_i{d}_i$
0-4	4	2	8	7.2	28.8
4-8	6	6	36	3.2	19.2
8-12	8	10	80	0.8	6.4
12-16	5	14	70	4.8	24.0
16-20	2	18	36	8.8	17.6
Total	$\sum f_i=25$		$\sum f_i{x}_i=230$		$\sum f_i{d}_i=96$

Mean deviation $=\frac{\sum f_i{d}_i}{\sum f_i}=\frac{96}{25}=3.84$

Hence, required mean deviation about mean = 3.84