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3-2Measures of
Variation
Instructor: Alaa saud
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Example 3-18:
A testing lab wishes to test two experimental brands of outdoor
paint to see how long each will last before fading. The testing
lab makes 6 gallons of each paint to test. Since different
chemical agents are added to each group and only six cans are
involved, these two groups constitute two small populations .
The results (in months)are shown. Find the mean of each group.
Brand A
Brand B
10
35
60
45
50
30
30
35
40
40
20
25
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The mean for brand A is
The mean for brand B is
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1- Range
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Example 3-19:
Find the ranges for the paints in Example 3-18 .
The range for brand A is
R= 60 – 10 = 50 months
The range for brand B is
R= 45 – 25 = 20 months
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Example 3-20:
The salaries for the staff of the XYZ .
Manufacturing Co are shown here. Find
the range.
Staff
Salary
Owner
$100,000
Manger
40,000
Sales
representative
30,000
workers
25,000
15,000
18,000
The range is R= $100,000- $ 15,000 = $85,000
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2-Variance
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 The variance is the average of the squares of
the distance each value is from the mean.
population
S
2
Sample
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3-The standard
deviation
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The standard deviation is the square root
of the variance.
population
S
Sample
Example 3-21:
Find the variance and standard deviation for
the data set for brand A paint in Example 3-18.
Solution :.
2 
( x   )
n
2
Brand
A
Brand
B
10
35
60
45
50
30
30
35
40
40
20
25
(10  35) 2  (60  35) 2  (50  35) 2  (30  35) 2  (40  35) 2  (20  35) 2
 
6
2


1750
 291.7
6

291.7  17.1
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Example 3-23:
• Find the sample variance and standard deviation
for the amount of European auto sales for a
sample of 6 years shown .T he data are in
million dollars.
•
11.2 , 11.9 , 12.0 , 12.8 , 13.4 , 14.3
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Note: This PowerPoint is only a summary and your main source should be the book.
Coefficient of
Variation
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The coefficient of variation , denoted by Cvar is
the standard deviation divided by the mean . T he
result is expressed as a percentage.
population
Sample
A statistic that allows you to compare standard deviations
when the units are different , as in this example is called the
coefficient of variation
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Example 3-25:
The mean of the number of sales of cars over a 3month period is 87 and the standard deviation is 5.
The mean commission is $5225 and standard
deviation is $773. Compare the variations of the two.
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Example 3-25:
The mean for the number of pages of sample of
women’s fitness magazines is 132 with a variance of
23.The mean for the number of advertisements of
sample of women’s fitness magazines is 182 with a
variance of 62. Compare the variations.
Note: This PowerPoint is only a summary and your main source should be the book.
Note: This PowerPoint is only a summary and your main source should be the book.
X  480,S  90 , approximately 68%
X  S  480  1(90)  390
X  S  480  1(90)  570
X  480,S  90
, approximately 95%
X  2S  480  2(90)  300
X  2S  480  2(90)  660
X  480,S  90
Then the data fall
between 570 and 390
Then the data fall
between 660 and 300
approximately 99.7%
X  3S  480  3(90)  210
X  3S  480  3(90)  750
Then the data fall
between 750 and 210
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• Examples:
• When a distribution is bell-shaped, approximately what
percentage of data values will fall within 1
• standard deviation of the mean?
•
•
•
•
A) 99.7%
B) 95%
C) 50%
D) 68%
• Math exam scores have a bell-shaped distribution with a
mean of 100 and standard deviation of 15 . use the
empirical rule to find the percentage of students with
scores between 70 and 130?
•
•
•
•
A) 99.7%
B) 95%
C) 50%
D) 68%
Note: This PowerPoint is only a summary and your main source should be the book.
Note: This PowerPoint is only a summary and your main source should be the book.