Download Chapter 3, Part A

Slides Prepared by
JOHN S. LOUCKS
St. Edward’s University
© 2003 South-Western/Thomson LearningTM
Slide 1
Chapter 3
Descriptive Statistics:
Numerical Methods, Part A


Measures of Location
Measures of Variability
x
© 2003 South-Western/Thomson LearningTM
Slide 2
Measures of Location





Mean
Median
Mode
Percentiles
Quartiles
© 2003 South-Western/Thomson LearningTM
Slide 3
Example: Apartment Rents
Given below is a sample of monthly rent values ($)
for one-bedroom apartments. The data is a sample of 70
apartments in a particular city. The data are presented
in ascending order.
425
440
450
465
480
510
575
430
440
450
470
485
515
575
430
440
450
470
490
525
580
435
445
450
472
490
525
590
435
445
450
475
490
525
600
435
445
460
475
500
535
600
435
445
460
475
500
549
600
435
445
460
480
500
550
600
440
450
465
480
500
570
615
440
450
465
480
510
570
615
© 2003 South-Western/Thomson LearningTM
Slide 4
Mean


The mean of a data set is the average of all the data
values.
If the data are from a sample, the mean is denoted by
x.
 xi
x
n

If the data are from a population, the mean is
denoted by m (mu).
 xi

N
© 2003 South-Western/Thomson LearningTM
Slide 5
Example: Apartment Rents

Mean
 xi 34, 356
x

 490.80
n
70
425
440
450
465
480
510
575
430
440
450
470
485
515
575
430
440
450
470
490
525
580
435
445
450
472
490
525
590
435
445
450
475
490
525
600
435
445
460
475
500
535
600
435
445
460
475
500
549
600
435
445
460
480
500
550
600
440
450
465
480
500
570
615
440
450
465
480
510
570
615
© 2003 South-Western/Thomson LearningTM
Slide 6
Median


The median is the measure of location most often
reported for annual income and property value data.
A few extremely large incomes or property values
can inflate the mean.
© 2003 South-Western/Thomson LearningTM
Slide 7
Median



The median of a data set is the value in the middle
when the data items are arranged in ascending order.
For an odd number of observations, the median is the
middle value.
For an even number of observations, the median is
the average of the two middle values.
© 2003 South-Western/Thomson LearningTM
Slide 8
Example: Apartment Rents

Median
Median = 50th percentile
i = (p/100)n = (50/100)70 = 35.5
Averaging the 35th and 36th data values:
Median = (475 + 475)/2 = 475
425
440
450
465
480
510
575
430
440
450
470
485
515
575
430
440
450
470
490
525
580
435
445
450
472
490
525
590
435
445
450
475
490
525
600
435
445
460
475
500
535
600
435
445
460
475
500
549
600
435
445
460
480
500
550
600
440
450
465
480
500
570
615
440
450
465
480
510
570
615
© 2003 South-Western/Thomson LearningTM
Slide 9
Mode




The mode of a data set is the value that occurs with
greatest frequency.
The greatest frequency can occur at two or more
different values.
If the data have exactly two modes, the data are
bimodal.
If the data have more than two modes, the data are
multimodal.
© 2003 South-Western/Thomson LearningTM
Slide 10
Example: Apartment Rents

Mode
450 occurred most frequently (7 times)
Mode = 450
425
440
450
465
480
510
575
430
440
450
470
485
515
575
430
440
450
470
490
525
580
435
445
450
472
490
525
590
435
445
450
475
490
525
600
435
445
460
475
500
535
600
435
445
460
475
500
549
600
435
445
460
480
500
550
600
440
450
465
480
500
570
615
440
450
465
480
510
570
615
© 2003 South-Western/Thomson LearningTM
Slide 11
Using Excel to Compute
the Mean, Median, and Mode

Formula Worksheet
1
2
3
4
5
6
A
Apartment
1
2
3
4
5
B
C
D
E
Monthly
Rent ($)
525
Mean =AVERAGE(B2:B71)
440
Median =MEDIAN(B2:B71)
450
Mode =MODE(B2:B71)
615
480
Note: Rows 7-71 are not shown.
© 2003 South-Western/Thomson LearningTM
Slide 12
Using Excel to Compute
the Mean, Median, and Mode

Value Worksheet
1
2
3
4
5
6
A
Apartment
1
2
3
4
5
B
C
D
Monthly
Rent ($)
525
Mean
440
Median
450
Mode
615
480
E
490.80
475.00
450.00
Note: Rows 7-71 are not shown.
© 2003 South-Western/Thomson LearningTM
Slide 13
Percentiles


A percentile provides information about how the
data are spread over the interval from the smallest
value to the largest value.
Admission test scores for colleges and universities
are frequently reported in terms of percentiles.
© 2003 South-Western/Thomson LearningTM
Slide 14
Percentiles

The pth percentile of a data set is a value such that at
least p percent of the items take on this value or less
and at least (100 - p) percent of the items take on this
value or more.
• Arrange the data in ascending order.
• Compute index i, the position of the pth percentile.
i = (p/100)n
• If i is not an integer, round up.
The pth percentile is
the value in the ith position.
• If i is an integer, the pth percentile is the average of
the values in positions i and i+1.
© 2003 South-Western/Thomson LearningTM
Slide 15
Example: Apartment Rents

90th Percentile
i = (p/100)n = (90/100)70 = 63
Averaging the 63rd and 64th data values:
90th Percentile = (580 + 590)/2 = 585
425
440
450
465
480
510
575
430
440
450
470
485
515
575
430
440
450
470
490
525
580
435
445
450
472
490
525
590
435
445
450
475
490
525
600
435
445
460
475
500
535
600
435
445
460
475
500
549
600
435
445
460
480
500
550
600
440
450
465
480
500
570
615
440
450
465
480
510
570
615
© 2003 South-Western/Thomson LearningTM
Slide 16
Quartiles




Quartiles are specific percentiles
First Quartile = 25th Percentile
Second Quartile = 50th Percentile = Median
Third Quartile = 75th Percentile
© 2003 South-Western/Thomson LearningTM
Slide 17
Example: Apartment Rents

Third Quartile
Third quartile = 75th percentile
i = (p/100)n = (75/100)70 = 52.5 = 53
Third quartile = 525
425
440
450
465
480
510
575
430
440
450
470
485
515
575
430
440
450
470
490
525
580
435
445
450
472
490
525
590
435
445
450
475
490
525
600
435
445
460
475
500
535
600
435
445
460
475
500
549
600
435
445
460
480
500
550
600
440
450
465
480
500
570
615
440
450
465
480
510
570
615
© 2003 South-Western/Thomson LearningTM
Slide 18
Using Excel to Compute
Percentiles and Quartiles

1
2
3
4
5
6
Unsorted Monthly Rent ($)
A
B
C
D
Apart- Monthly
ment Rent ($)
1
525
2
440
3
450
4
615
5
480
Note: Rows 7-71 are not shown.
E
F
© 2003 South-Western/Thomson LearningTM
Slide 19
Using Excel to Compute
Percentiles and Quartiles

Sorting Data
Step 1 Select any cell containing data in column B
Step 2 Select the Data pull-down menu
Step 3 Choose the Sort option
Step 4 When the Sort dialog box appears:
In the Sort by box, make sure that
Monthly Rent ($) appears and that
Ascending is selected
In the My list has box, make sure that
Header row is selected
Click OK
© 2003 South-Western/Thomson LearningTM
Slide 20
Using Excel to Compute
Percentiles and Quartiles

1
2
3
4
5
6
Sorted Monthly Rent ($)
A
B
C
D
Apart- Monthly
ment Rent ($)
1
425
2
430
3
430
4
435
5
435
Note: Rows 7-71 are not shown.
E
F
© 2003 South-Western/Thomson LearningTM
Slide 21
Using Excel to Compute
Percentiles and Quartiles

1
2
3
4
5
6
Formula Worksheet for 90th Percentile’s Index
A
B
C
D
E
F
Apart- Monthly
Number of
ment Rent ($)
Observations Percentile Index i
1
425
70
90
=(E2/100)*D2
2
430
3
430
4
435
5
435
Note: Rows 7-71 are not shown.
© 2003 South-Western/Thomson LearningTM
Slide 22
Using Excel to Compute
Percentiles and Quartiles

1
2
3
4
5
6
Value Worksheet for 90th Percentile’s Index
A
B
C
D
E
Apart- Monthly
Number of
ment Rent ($)
Observations Percentile
1
425
70
90
2
430
3
430
4
435
5
435
Note: Rows 7-71 are not shown.
F
Index i
63.00
© 2003 South-Western/Thomson LearningTM
Slide 23
Using Excel to Compute
Percentiles and Quartiles

1
2
3
4
5
6
Value Worksheet for 3rd Quartile’s Index
A
B
C
D
E
Apart- Monthly
Number of
ment Rent ($)
Observations Percentile
1
425
70
75
2
430
3
430
4
435
5
435
Note: Rows 7-71 are not shown.
F
Index i
52.50
© 2003 South-Western/Thomson LearningTM
Slide 24
Measures of Variability


It is often desirable to consider measures of
variability (dispersion), as well as measures of
location.
For example, in choosing supplier A or supplier B we
might consider not only the average delivery time for
each, but also the variability in delivery time for each.
© 2003 South-Western/Thomson LearningTM
Slide 25
Measures of Variability





Range
Interquartile Range
Variance
Standard Deviation
Coefficient of Variation
© 2003 South-Western/Thomson LearningTM
Slide 26
Range



The range of a data set is the difference between the
largest and smallest data values.
It is the simplest measure of variability.
It is very sensitive to the smallest and largest data
values.
© 2003 South-Western/Thomson LearningTM
Slide 27
Example: Apartment Rents

Range
Range = largest value - smallest value
Range = 615 - 425 = 190
425
440
450
465
480
510
575
430
440
450
470
485
515
575
430
440
450
470
490
525
580
435
445
450
472
490
525
590
435
445
450
475
490
525
600
435
445
460
475
500
535
600
435
445
460
475
500
549
600
435
445
460
480
500
550
600
440
450
465
480
500
570
615
440
450
465
480
510
570
615
© 2003 South-Western/Thomson LearningTM
Slide 28
Interquartile Range



The interquartile range of a data set is the difference
between the third quartile and the first quartile.
It is the range for the middle 50% of the data.
It overcomes the sensitivity to extreme data values.
© 2003 South-Western/Thomson LearningTM
Slide 29
Example: Apartment Rents

Interquartile Range
3rd Quartile (Q3) = 525
1st Quartile (Q1) = 445
Interquartile Range = Q3 - Q1 = 525 - 445 = 80
425
440
450
465
480
510
575
430
440
450
470
485
515
575
430
440
450
470
490
525
580
435
445
450
472
490
525
590
435
445
450
475
490
525
600
435
445
460
475
500
535
600
435
445
460
475
500
549
600
435
445
460
480
500
550
600
440
450
465
480
500
570
615
440
450
465
480
510
570
615
© 2003 South-Western/Thomson LearningTM
Slide 30
Variance


The variance is a measure of variability that utilizes
all the data.
It is based on the difference between the value of
each observation (xi) and the mean (x for a sample, 
for a population).
© 2003 South-Western/Thomson LearningTM
Slide 31
Variance


The variance is the average of the squared differences
between each data value and the mean.
If the data set is a sample, the variance is denoted by
s2.
s2 

2
(
x

x
)
 i
n 1
If the data set is a population, the variance is denoted
by  2.
2
(
x


)

i
2 
N
© 2003 South-Western/Thomson LearningTM
Slide 32
Standard Deviation



The standard deviation of a data set is the positive
square root of the variance.
It is measured in the same units as the data, making
it more easily comparable, than the variance, to the
mean.
If the data set is a sample, the standard deviation is
denoted s.
s  s2

If the data set is a population, the standard deviation
is denoted  (sigma).

2
© 2003 South-Western/Thomson LearningTM
Slide 33
Coefficient of Variation


The coefficient of variation indicates how large the
standard deviation is in relation to the mean.
If the data set is a sample, the coefficient of variation
is computed as follows:
s
(100)
x

If the data set is a population, the coefficient of
variation is computed as follows:

(100)

© 2003 South-Western/Thomson LearningTM
Slide 34
Example: Apartment Rents

Variance

s 
n 1
2

( xi  x ) 2
 2 , 996.16
Standard Deviation
s  s2  2996. 47  54. 74

Coefficient of Variation
s
54. 74
 100 
 100  11.15
x
490.80
© 2003 South-Western/Thomson LearningTM
Slide 35
Using Excel to Compute the Sample Variance,
Standard Deviation, and Coefficient of Variation

Formula Worksheet
1
2
3
4
5
6
7
A
B
C
D
E
Apart- Monthly
ment Rent ($)
1
525
Mean =AVERAGE(B2:B71)
2
440
Median =MEDIAN(B2:B71)
3
450
Mode =MODE(B2:B71)
4
615
Variance =VAR(B2:B71)
5
480
Std. Dev. =STDEV(B2:B71)
6
510
C.V. =E6/E2*100
Note: Rows 8-71 are not shown.
© 2003 South-Western/Thomson LearningTM
Slide 36
Using Excel to Compute the Sample Variance,
Standard Deviation, and Coefficient of Variation

Value Worksheet
1
2
3
4
5
6
7
A
B
C
D
Apart- Monthly
ment Rent ($)
1
525
Mean
2
440
Median
3
450
Mode
4
615
Variance
5
480
Std. Dev.
6
510
C.V.
E
490.80
475.00
450.00
2996.16
54.74
11.15
Note: Rows 8-71 are not shown.
© 2003 South-Western/Thomson LearningTM
Slide 37
Using Excel’s
Descriptive Statistics Tool
Step 1 Select the Tools pull-down menu
Step 2 Choose the Data Analysis option
Step 3 Choose Descriptive Statistics from the list of
Analysis Tools
… continued
© 2003 South-Western/Thomson LearningTM
Slide 38
Using Excel’s
Descriptive Statistics Tool
Step 4 When the Descriptive Statistics dialog box
appears:
Enter B1:B71 in the Input Range box
Select Grouped By Columns
Select Labels in First Row
Select Output Range
Enter D1 in the Output Range box
Select Summary Statistics
Click OK
© 2003 South-Western/Thomson LearningTM
Slide 39
Using Excel’s
Descriptive Statistics Tool

1
2
3
4
5
6
7
8
Value Worksheet (Partial)
A
B
C
D
E
Apart- Monthly
ment Rent ($)
Monthly Rent ($)
1
525
2
440
Mean
490.8
3
450
Standard Error
6.542348114
4
615
Median
475
5
480
Mode
450
6
510
Standard Deviation 54.73721146
7
575
Sample Variance
2996.162319
Note: Rows 9-71 are not shown.
© 2003 South-Western/Thomson LearningTM
Slide 40
Using Excel’s
Descriptive Statistics Tool

Value Worksheet (Partial)
9
10
11
12
13
14
15
16
A
8
9
10
11
12
13
14
15
B
430
440
450
470
485
515
575
430
C
D
Kurtosis
Skewness
Range
Minimum
Maximum
Sum
Count
E
-0.334093298
0.924330473
190
425
615
34356
70
Note: Rows 1-8 and 17-71 are not shown.
© 2003 South-Western/Thomson LearningTM
Slide 41
Descriptive Statistics:
Numerical Methods, Part B




Measures of Relative Location and Detecting
Outliers
Exploratory Data Analysis
Measures of Association Between Two Variables
The Weighted Mean and
Working with Grouped Data
x
© 2003 South-Western/Thomson LearningTM
Slide 42
Measures of Relative Location
and Detecting Outliers




z-Scores
Chebyshev’s Theorem
Empirical Rule
Detecting Outliers
© 2003 South-Western/Thomson LearningTM
Slide 43
z-Scores


The z-score is often called the standardized value.
It denotes the number of standard deviations a data
value xi is from the mean.
xi  x
zi 
s



A data value less than the sample mean will have a
z-score less than zero.
A data value greater than the sample mean will have
a z-score greater than zero.
A data value equal to the sample mean will have a
z-score of zero.
© 2003 South-Western/Thomson LearningTM
Slide 44
Example: Apartment Rents

z-Score of Smallest Value (425)
xi  x 425  490.80
z

 1. 20
s
54. 74
Standardized Values for Apartment Rents
-1.20
-0.93
-0.75
-0.47
-0.20
0.35
1.54
-1.11
-0.93
-0.75
-0.38
-0.11
0.44
1.54
-1.11
-0.93
-0.75
-0.38
-0.01
0.62
1.63
-1.02
-0.84
-0.75
-0.34
-0.01
0.62
1.81
-1.02
-0.84
-0.75
-0.29
-0.01
0.62
1.99
-1.02
-0.84
-0.56
-0.29
0.17
0.81
1.99
-1.02
-0.84
-0.56
-0.29
0.17
1.06
1.99
-1.02
-0.84
-0.56
-0.20
0.17
1.08
1.99
-0.93
-0.75
-0.47
-0.20
0.17
1.45
2.27
-0.93
-0.75
-0.47
-0.20
0.35
1.45
2.27
© 2003 South-Western/Thomson LearningTM
Slide 45
Chebyshev’s Theorem
At least (1 - 1/k2) of the items in any data set will be
within k standard deviations of the mean, where k is
any value greater than 1.
• At least 75% of the items must be within
k = 2 standard deviations of the mean.
• At least 89% of the items must be within
k = 3 standard deviations of the mean.
• At least 94% of the items must be within
k = 4 standard deviations of the mean.
© 2003 South-Western/Thomson LearningTM
Slide 46
Example: Apartment Rents

Chebyshev’s Theorem
Let k = 1.5 with
x = 490.80 and s = 54.74
At least (1 - 1/(1.5)2) = 1 - 0.44 = 0.56 or 56%
of the rent values must be between
x - k(s) = 490.80 - 1.5(54.74) = 409
and
x + k(s) = 490.80 + 1.5(54.74) = 573
© 2003 South-Western/Thomson LearningTM
Slide 47
Example: Apartment Rents

Chebyshev’s Theorem (continued)
Actually, 86% of the rent values
are between 409 and 573.
425
440
450
465
480
510
575
430
440
450
470
485
515
575
430
440
450
470
490
525
580
435
445
450
472
490
525
590
435
445
450
475
490
525
600
435
445
460
475
500
535
600
435
445
460
475
500
549
600
435
445
460
480
500
550
600
440
450
465
480
500
570
615
440
450
465
480
510
570
615
© 2003 South-Western/Thomson LearningTM
Slide 48
Empirical Rule
For data having a bell-shaped distribution:
• Approximately 68% of the data values will be
within one standard deviation of the mean.
© 2003 South-Western/Thomson LearningTM
Slide 49
Empirical Rule
For data having a bell-shaped distribution:
• Approximately 95% of the data values will be
within two standard deviations of the mean.
© 2003 South-Western/Thomson LearningTM
Slide 50
Empirical Rule
For data having a bell-shaped distribution:
• Almost all (99.7%) of the items will be
within three standard deviations of the mean.
© 2003 South-Western/Thomson LearningTM
Slide 51
Example: Apartment Rents

Empirical Rule
Within +/- 1s
Within +/- 2s
Within +/- 3s
425
440
450
465
480
510
575
430
440
450
470
485
515
575
430
440
450
470
490
525
580
Interval
436.06 to 545.54
381.32 to 600.28
326.58 to 655.02
435
445
450
472
490
525
590
435
445
450
475
490
525
600
435
445
460
475
500
535
600
435
445
460
475
500
549
600
% in Interval
48/70 = 69%
68/70 = 97%
70/70 = 100%
435
445
460
480
500
550
600
440
450
465
480
500
570
615
440
450
465
480
510
570
615
© 2003 South-Western/Thomson LearningTM
Slide 52
Detecting Outliers





An outlier is an unusually small or unusually large
value in a data set.
A data value with a z-score less than -3 or greater
than +3 might be considered an outlier.
It might be an incorrectly recorded data value.
It might be a data value that was incorrectly included
in the data set.
It might be a correctly recorded data value that
belongs in the data set !
© 2003 South-Western/Thomson LearningTM
Slide 53
Example: Apartment Rents

Detecting Outliers
The most extreme z-scores are -1.20 and 2.27.
Using |z| > 3 as the criterion for an outlier,
there are no outliers in this data set.
Standardized Values for Apartment Rents
-1.20
-0.93
-0.75
-0.47
-0.20
0.35
1.54
-1.11
-0.93
-0.75
-0.38
-0.11
0.44
1.54
-1.11
-0.93
-0.75
-0.38
-0.01
0.62
1.63
-1.02
-0.84
-0.75
-0.34
-0.01
0.62
1.81
-1.02
-0.84
-0.75
-0.29
-0.01
0.62
1.99
-1.02
-0.84
-0.56
-0.29
0.17
0.81
1.99
-1.02
-0.84
-0.56
-0.29
0.17
1.06
1.99
-1.02
-0.84
-0.56
-0.20
0.17
1.08
1.99
-0.93
-0.75
-0.47
-0.20
0.17
1.45
2.27
-0.93
-0.75
-0.47
-0.20
0.35
1.45
2.27
© 2003 South-Western/Thomson LearningTM
Slide 54
Exploratory Data Analysis


Five-Number Summary
Box Plot
© 2003 South-Western/Thomson LearningTM
Slide 55
Five-Number Summary





Smallest Value
First Quartile
Median
Third Quartile
Largest Value
© 2003 South-Western/Thomson LearningTM
Slide 56
Example: Apartment Rents

Five-Number Summary
Lowest Value = 425
First Quartile = 450
Median = 475
Third Quartile = 525
Largest Value = 615
425
440
450
465
480
510
575
430
440
450
470
485
515
575
430
440
450
470
490
525
580
435
445
450
472
490
525
590
435
445
450
475
490
525
600
435
445
460
475
500
535
600
435
445
460
475
500
549
600
435
445
460
480
500
550
600
440
450
465
480
500
570
615
440
450
465
480
510
570
615
© 2003 South-Western/Thomson LearningTM
Slide 57
Box Plot



A box is drawn with its ends located at the first and
third quartiles.
A vertical line is drawn in the box at the location of
the median.
Limits are located (not drawn) using the interquartile
range (IQR).
• The lower limit is located 1.5(IQR) below Q1.
• The upper limit is located 1.5(IQR) above Q3.
• Data outside these limits are considered outliers.
… continued
© 2003 South-Western/Thomson LearningTM
Slide 58
Box Plot (Continued)


Whiskers (dashed lines) are drawn from the ends of
the box to the smallest and largest data values inside
the limits.
The locations of each outlier is shown with the
symbol * .
© 2003 South-Western/Thomson LearningTM
Slide 59
Example: Apartment Rents

Box Plot
Lower Limit: Q1 - 1.5(IQR) = 450 - 1.5(75) = 337.5
Upper Limit: Q3 + 1.5(IQR) = 525 + 1.5(75) = 637.5
There are no outliers.
37
5
40
0
42
5
45
0
47
5
50
0
52
5
550
575 600
625
© 2003 South-Western/Thomson LearningTM
Slide 60
Measures of Association
between Two Variables


Covariance
Correlation Coefficient
© 2003 South-Western/Thomson LearningTM
Slide 61
Covariance



The covariance is a measure of the linear association
between two variables.
Positive values indicate a positive relationship.
Negative values indicate a negative relationship.
© 2003 South-Western/Thomson LearningTM
Slide 62
Covariance

If the data sets are samples, the covariance is denoted
by sxy.
 ( xi  x )( yi  y )
sxy 
n 1

If the data sets are populations, the covariance is
denoted by  xy .
 xy
 ( xi   x )( yi   y )

N
© 2003 South-Western/Thomson LearningTM
Slide 63
Correlation Coefficient




The coefficient can take on values between -1 and +1.
Values near -1 indicate a strong negative linear
relationship.
Values near +1 indicate a strong positive linear
relationship.
If the data sets are samples, the coefficient is rxy.
rxy 

sxy
sx s y
If the data sets are populations, the coefficient is
 xy
 xy

 x y
 xy
.
© 2003 South-Western/Thomson LearningTM
Slide 64
Using Excel to Compute the
Covariance and Correlation Coefficient

1
2
3
4
5
6
7
8
Formula Worksheet
A
Average
Drive
277.6
259.5
269.1
267.0
255.6
272.9
B
18-Hole
Score
69
71
70
70
71
69
C
D
E
Pop. Covariance =COVAR(A2:A7,B2:B7)
Samp. Correlation =CORREL(A2:A7,B2:B7)
© 2003 South-Western/Thomson LearningTM
Slide 65
Using Excel to Compute the
Covariance and Correlation Coefficient

1
2
3
4
5
6
7
8
Value Worksheet
A
Average
Drive
277.6
259.5
269.1
267.0
255.6
272.9
B
18-Hole
Score
69
71
70
70
71
69
C
D
E
Pop. Covariance
Samp. Correlation
-5.9
-0.9631
© 2003 South-Western/Thomson LearningTM
Slide 66
The Weighted Mean and
Working with Grouped Data




Weighted Mean
Mean for Grouped Data
Variance for Grouped Data
Standard Deviation for Grouped Data
© 2003 South-Western/Thomson LearningTM
Slide 67
Weighted Mean



When the mean is computed by giving each data
value a weight that reflects its importance, it is
referred to as a weighted mean.
In the computation of a grade point average (GPA),
the weights are the number of credit hours earned for
each grade.
When data values vary in importance, the analyst
must choose the weight that best reflects the
importance of each value.
© 2003 South-Western/Thomson LearningTM
Slide 68
Weighted Mean
x =  wi xi
 wi
where:
xi = value of observation i
wi = weight for observation i
© 2003 South-Western/Thomson LearningTM
Slide 69
Grouped Data




The weighted mean computation can be used to
obtain approximations of the mean, variance, and
standard deviation for the grouped data.
To compute the weighted mean, we treat the
midpoint of each class as though it were the mean of
all items in the class.
We compute a weighted mean of the class midpoints
using the class frequencies as weights.
Similarly, in computing the variance and standard
deviation, the class frequencies are used as weights.
© 2003 South-Western/Thomson LearningTM
Slide 70
Mean for Grouped Data

Sample Data
fM

x
f
i
fM


i
i
i

Population Data
i
N
where:
fi = frequency of class i
Mi = midpoint of class i
© 2003 South-Western/Thomson LearningTM
Slide 71
Example: Apartment Rents
Given below is the previous sample of monthly rents
for one-bedroom apartments presented here as grouped
data in the form of a frequency distribution.
Rent ($) Frequency
420-439
8
440-459
17
460-479
12
480-499
8
500-519
7
520-539
4
540-559
2
560-579
4
580-599
2
600-619
6
© 2003 South-Western/Thomson LearningTM
Slide 72
Example: Apartment Rents

Mean for Grouped Data
Rent ($)
420-439
440-459
460-479
480-499
500-519
520-539
540-559
560-579
580-599
600-619
Total
fi
8
17
12
8
7
4
2
4
2
6
70
Mi
429.5
449.5
469.5
489.5
509.5
529.5
549.5
569.5
589.5
609.5
f iMi
3436.0
7641.5
5634.0
3916.0
3566.5
2118.0
1099.0
2278.0
1179.0
3657.0
34525.0
34, 525
 493. 21
70
This approximation
differs by $2.41 from
the actual sample
mean of $490.80.
x
© 2003 South-Western/Thomson LearningTM
Slide 73
Variance for Grouped Data

Sample Data
2
f
(
M

x
)

i
i
s2 
n 1

Population Data
2
f
(
M


)

i
i
2 
N
© 2003 South-Western/Thomson LearningTM
Slide 74
Example: Apartment Rents

Variance for Grouped Data
s2  3, 017.89

Standard Deviation for Grouped Data
s  3, 017.89  54. 94
This approximation differs by only $.20
from the actual standard deviation of $54.74.
© 2003 South-Western/Thomson LearningTM
Slide 75
End of Chapter 3
© 2003 South-Western/Thomson LearningTM
Slide 76