Download Section 12_2

Section 12.2
Measures of Central Tendency
Objectives
1. Determine the mean for a data set.
2. Determine the median for a data set.
3. Determine the mode for a data set.
4. Determine the midrange for a data set.
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The Mean
• Mean: The sum of the data items divided by the
number of items.
Mean =Σx ,
n
Where Σx represents the sum of all the data items
and n represents the number of items.
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Example 1
Calculating the Mean
Youngest U.S. Male Singers to Have a Number 1 Single
•
Artist/Year
Title
Age
Stevie Wonder, 1963
“Fingertips”
13
Donny Osmond, 1971
“Go Away Little Girl”
13
Michael Jackson, 1972
“Ben”
14
Laurie London, 1958
“He’s Got the Whole World in His Hands”
14
Paul Anka, 1957
“Diana”
16
Brian Hyland, 1960
“Itsy Bitsy Teenie Weenie Yellow Polkadot Bikini”
16
Shaun Cassidy, 1977
“Da Doo Ron Ron”
17
Bobby Vee, 1961
“Take Good Care of My Baby”
18
Usher, 1998
“Nice & Slow”
19
Andy Gibb, 1977
“I Just Want to be Your Everything”
19
Find the mean age of these male singers at the time of their number 1 single.
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Example 1
Calculating the Mean
Youngest U.S. Male Singers to Have a Number 1 Single
•
•
Artist/Year
Title
Age
Stevie Wonder, 1963
“Fingertips”
13
Donny Osmond, 1971
“Go Away Little Girl”
13
Michael Jackson, 1972
“Ben”
14
Laurie London, 1958
“He’s Got the Whole World in His Hands”
14
Paul Anka, 1957
“Diana”
16
Brian Hyland, 1960
“Itsy Bitsy Teenie Weenie Yellow Polkadot Bikini”
16
Shaun Cassidy, 1977
“Da Doo Ron Ron”
17
Bobby Vee, 1961
“Take Good Care of My Baby”
18
Usher, 1998
“Nice & Slow”
19
Andy Gibb, 1977
“I Just Want to be Your Everything”
19
Find the mean age of these male singers at the time of their number 1 single.
Solution: Mean =Σx = 13 +13+14+14+16+16+17+18+19+19 = 159 = 15.9 years
n
10
10
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Calculating the Mean for a Frequency
Distribution
When many data values occur more than once and a
frequency distribution is used to organize the data, we
can use the following formula to calculate the mean:
Mean =Σxf ,
n
Where
x represents each data value.
f represents the frequency of that data value.
Σxf represents the sum of all the products obtained by
multiplying each data value by its frequency.
n represents the total frequency of the distribution.
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Example 2
Calculating the Mean for a Frequency Distribution
Student’s Stress-Level Ratings
Stress Rating
x
Frequency
f
Data value x
frequency
xf
0
1
2
3
4
5
6
7
8
9
10
2
1
3
12
16
18
13
31
26
15
14
0·2 = 0
1·1 = 1
2·3 = 6
3·12 = 36
4· 16 = 64
5·18 = 90
6·13 = 78
7·31 = 217
8·26 = 208
9·15 = 135
10·14 = 140
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The table at left shows
how to derive the
information needed to
calculate the Mean from
a frequency distribution.
The final step is:
Mean =Σxf = 975 ≈ 6.46
n
151
6
The Median
Median is the data item in the middle of each set
of ranked, or ordered, data.
To find the median of a group of data items,
1. Arrange the data items in order, from smallest
to largest.
2. If the number of data items is odd, the median
is the data item in the middle of the list.
3. If the number of data items is even, the median
is the mean of the two middle data items.
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Example 3
Finding the Median
•
Find the median for each of the following
groups of data:
a. 84, 90, 98, 95, 88
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Example 3
Finding the Median
•
Find the median for each of the following
groups of data:
a. 84, 90, 98, 95, 88
• Solution:
Arrange the data items in order from smallest to
largest.
The number of data items in the list, five, is odd.
Thus, the median is the middle number.
84, 88, 90, 95, 98 The median is 90.
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Example 3 continued
b. 68, 74, 7, 13,15, 25, 28, 59, 34, 47
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Example 3 continued
b. 68, 74, 7, 13,15, 25, 28, 59, 34, 47
• Solution:
• Arrange the data items in order from smallest
to largest.
The number of data items in the list, ten, is even.
Thus, the median is the mean of the two middle
numbers.
7, 13,15 ,25, 28, 34, 47, 59, 68, 74
The median is 28 + 34 = 62 = 31
2
2
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Example 4
Finding the Median using the Position Formula
• Position of the Median: If n items are arranged in order,
from smallest to largest, the median is the value in the
n+1 position.
2
• Listed below are the points scored per season by the 13
top point scorers in the National Football League. Find
the median points scored per season for the top 13
scorers. The data items are arranged from smallest to
largest:
144, 144, 145, 145, 145, 146, 147, 149, 150, 155, 161, 164, 176
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Example 4
Finding the Median using the Position Formula
• Position of the Median: If n items are arranged in order,
from smallest to largest, the median is the value in the
n+1 position.
2
• Listed below are the points scored per season by the 13
top point scorers in the National Football League. Find
the median points scored per season for the top 13
scorers. The data items are arranged from smallest to
largest:
144, 144, 145, 145, 145, 146, 147, 149, 150, 155, 161, 164, 176
• Solution:
The median is in the n+1 position = 13+1 = 7th position.
2
2
The median is 147.
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Example 5
Finding the Median for a Frequency
Distribution
Stress Rating Frequency
x
f
0
2
1
1
2
3
3
12
4
16
5
18
6
13
7
31
8
26
9
15
10
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There are 151 data items in this table so n = 151.
The median is the value in the
151 + 1 = 152 = 76th position
2
2
Rather than write all the data items out and count
them, we can count the frequencies until we
reach the 76th data item.
x sum of frequency
0&1: 2 + 1 = 3
2:
3+3=6
3:
6 + 12 = 18
4: 18 + 16 = 34
5: 34 + 18 = 52
6: 52 + 13 = 65
7: 65 + 31 = 91 Stop counting!
The 76th data item within the stress rating
rating of 7. The median stress rating is 7.
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Example 6
Comparing the Median and the Mean
Five employees in a manufacturing plant earn salaries of
$19,700, $20,400, $21,500, $22,600 and $23,000 annually.
The section manager has an annual salary of $95,000.
a. Find the median annual salary for the six people
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Example 6
Comparing the Median and the Mean
Five employees in a manufacturing plant earn salaries of
$19,700, $20,400, $21,500, $22,600 and $23,000 annually.
The section manager has an annual salary of $95,000.
a. Find the median annual salary for the six people
Solution: First arrange the salaries in order.
$19,700, $20,400, $21,500, $22,600 $23,000 $95,000
Since there is an even number of data items, six, the
median is the average of the two middle items.
Median = $21,500 + $22,600 = $44,100 = $22,050
2
2
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Example 6 continued
b. Find the mean annual salary for the six people.
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Example 6 continued
b. Find the mean annual salary for the six people.
Solution: We find the mean annual salary by adding the six
annual salaries and dividing by 6.
Mean =
19,700+ $20,400+ $21,500+ $22,600+ $23,000+ $95,000
6
=$202,200
6
= $33,700
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The Mode
• Mode is the data value that occurs most often in
a data set.
– If no data items are repeated, then the data
set has no mode.
– If more than one data value has the highest
frequency, then each of these data values is a
mode.
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Example 7
Finding the mode
• Find the mode for the following groups of data:
a. 7,2,4,7,8,10
b. 2,1,4,5,3
c. 3,3,4,6,6
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Example 7
Finding the mode
• Find the mode for the following groups of data:
a. 7,2,4,7,8,10
The mode is 7
b. 2,1,4,5,3
There is no mode
c. 3,3,4,6,6
The modes are 3 and 6 (bimodal)
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The Midrange
• Midrange is found by adding the lowest and highest
data values and dividing the sum by 2.
Midrange =lowest data value + highest data value
2
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Example 8
Finding the Midrange
In 2006, the New York Yankees had the greatest payroll, a
record $194,663,100. The Florida Marlins were the worst
paid team, with a payroll of $14,998,500.
Find the midrange for the annual payroll of major league
baseball teams in 2006.
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Example 8
Finding the Midrange
In 2006, the New York Yankees had the greatest payroll, a
record $194,663,100. The Florida Marlins were the worst
paid team, with a payroll of $14,998,500.
Find the midrange for the annual payroll of major league
baseball teams in 2006.
Solution:
Midrange = lowest annual payroll + highest annual payroll
2
= 14,998,500 + 194,663,100
2
= $209,661,600 = $104,830,800
2
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HW #4-64 every 4
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