Question
Find out the arithmetic mean using the step deviation method.
Item
Frequency
More than 0
28
More than 10
24
More than 20
14
More than 30
4
Hint: In the above problem, cumulative frequencies are given. Firstly, it will be changed into exclusive class interval and thereafter we will calculate arithmetic mean using the step deviation method.

Answer

Calculation of Arithmetic Mean by Step Deviation Method
Items
Mid-Value
(m)
Frequency (f)
dm(m - A)
(A = 15)
$\text{d}'\text{m}\Big(\frac{\text{dm}}{\text{c}}\Big)$
(c = 10)
fd'm
0-10
5
28 - 24 = 4
-10
-1
-4
10-20
15
24 - 14 = 10
0
0
0
20-30
25
14 - 4 = 10
+10
+1
+10
30-40
35
4
+20
+2
+8
 
 
$\Sigma\text{f}=28$
 
 
$\Sigma\text{f}\text{d}'\text{m}=+14$
$\therefore\ \overline{\text{X}}=\text{A}+\frac{\Sigma\text{fd}'\text{m}}{\Sigma\text{f}}\times\text{c}$
$=15+\frac{14}{28}\times10=15+5$
$\Rightarrow\ \overline{\text{X}}=20$

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