Question

Find the mean deviation from the mean of the following data, using step deviation method.

Marks	0-10	10-20	20-30	30-40	40-50	50-60	60-70
No. of students	$6$	$5$	$8$	$15$	$7$	$6$	$3$

Answer 1

Marks	Students	Marks mean	Deviation
$0-10$	$6$	$5$	$-30$
$10-20$	$5$	$15$	$-20$
$20-30$	$8$	$25$	$-10$
$30-40$	$15$	$35=A$	$0$
$40-50$	$7$	$45$	$10$
$50-60$	$6$	$55$	$20$
$60-70$	$3$	$65$	$30$

Mean $=35+\frac{[-30\times 6+-20\times 5+-10\times 8+10\times 7+20\times 6+30\times 3]}{6}$

$=35+\frac{[280-360]}{6}=35-\frac{80}{6}$

$=21\frac{2}{3}$