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Mean and Standard Deviation of
Grouped Data
• Make a frequency table
• Compute the midpoint (x) for each class.
• Count the number of entries in each class
(f).
• Sum the f values to find n, the total
number of entries in the distribution.
• Treat each entry of a class as if it falls at
the class midpoint.
Sample Mean for a Frequency
Distribution
xf

x 
n
x = class midpoint
Sample Standard Deviation for
a Frequency Distribution
s
 ( x  x) f
2
n 1
Computation Formula for
Standard Deviation for a
Frequency Distribution
SSx
s
n 1
where SSx   x

xf 

f
2
2
n
Calculation of the mean of
grouped data
Ages:
f
30 - 34 4
35 - 39 5
x
xf
32
128
37
185
42
xf 84
= 820
40 - 44 2
45 - 49 9
f = 20
Mean of Grouped Data
xf  xf

x

n
f
820

 41 . 0
20
Calculation of the standard
deviation of grouped data
Ages:
f
x
x – mean
(x – mean)2
(x – mean)2 f
30 – 34 4
32
–9
81
324
35 – 39 5
37
–4
16
80
40 – 44 2
42
1
1
2
45 - 49 9
47
 f = 20
6
36
324
Mean
 (x – mean)2 f = 730
Calculation of the standard
deviation of grouped data
  x  x   730
f = n = 20
2
( x  x) f

s

2
n 1
730
20  1
 38 . 42  6 . 20
Computation Formula for
Standard Deviation for a
Frequency Distribution
SSx
s
n 1
where SSx   x

xf 

f
2
2
n
Computation Formula for
Standard Deviation
x
f
xf
x2f
32
4
128
4096
5
37
2
6845
185
9
42
f = 20
3528
19881
xf84= 820
x2f =
34350
Computation Formula for
Standard Deviation for a
Frequency Distribution
where
SS
  x f 
2
x

xf
n

2
820 2
34350 
 730
20
SS x
730
s 

 6 . 20
n1
20  1

Weighted Average
Average calculated where some
of the numbers are assigned more
importance or weight
Weighted Average
xw

Weighted Average 
w
where w  the weight of the data value x.
Compute the Weighted
Average:
•
•
•
•
•
•
Midterm weight = 25%
Term paper weight = 25%
Final exam weight = 50%
Compute the Weighted Average:
• Midterm
• Term Paper
• Final exam
x
92
80
88
w
.25
.25
.50
1.00
 xw  87  87  Weighted Average
 w 1.00
xw
23
20
44
87
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