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Chapter-2: Descriptive Statistics:

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Chapter-2: Descriptive Statistics:

Tabular and Graphical PresentationsPart A

Exploratory Data Analysis

Cross-tabulations; Scatter Diagrams

Data

Qualitative Data

Quantitative Data

Tabular

Methods

Graphical

Methods

Tabular

Methods

Graphical

Methods

- Bar Graph
- Pie Chart

- Frequency
- Distribution
- Rel. Freq. Dist.
- Percent Freq.
- Distribution
- Crosstabulation

- Dot Plot
- Histogram
- Ogive
- Scatter
- Diagram

- Frequency
- Distribution
- Rel. Freq. Dist.
- Cum. Freq. Dist.
- Cum. Rel. Freq.
- Distribution
- Stem-and-Leaf
- Display
- Crosstabulation

- A stem-and-leaf display is an exploratory data
analysis method that shows both the rank order

- and shape of the distribution of the data.

- It is similar to a histogram, but it has the
- advantage of showing the actual data values.

Example: Hudson Auto Repair

- Sample of Parts Cost for 50 Tune-ups

Stem-and-Leaf Display

5

6

7

8

9

10

2 7

2 2 2 2 5 6 7 8 8 8 9 9 9

1 1 2 2 3 4 4 5 5 5 6 7 8 9 9 9

0 0 2 3 5 8 9

1 3 7 7 7 8 9

1 4 5 5 9

- Arrange the first digits of each data to the
- left of a vertical line.

- To the right of the vertical line, record the last
- digit for each item in rank order.

- Each line in the display is referred to as a stem.

- Each digit on a stem is a leaf.

Stem-and-Leaf Display

5

6

7

8

9

10

2 7

2 2 2 2 5 6 7 8 8 8 9 9 9

1 1 2 2 3 4 4 5 5 5 6 7 8 9 9 9

0 0 2 3 5 8 9

1 3 7 7 7 8 9

1 4 5 5 9

a stem

a leaf

- If we believe the display has condensed the data too
much, we can stretch thedisplay by using two stems

for each leading digit(s).

- Whenever stretched, the first value of a stem
corresponds to leaf values of 0 - 4, and the second

- value of a stem corresponds to leaf values of 5 - 9.

Stretched Stem-and-Leaf Display

5

5

6

6

7

7

8

8

9

9

10

10

2

7

2 2 2 2

5 6 7 8 8 8 9 9 9

1 1 2 2 3 4 4

5 5 5 6 7 8 9 9 9

0 0 2 3

5 8 9

1 3

7 7 7 8 9

1 4

5 5 9

Stem-and-Leaf Display—Some notes

- Leaf Units

- A single digit is used to define each leaf.

- In the preceding example, the leaf unit was 1.

- Leaf units may be 100, 10, 1, 0.1, and so on.

- Where the leaf unit is not shown, it is assumed
- to equal 1.

If we have data with values such as

8.6 11.79.49.110.211.08.8

a stem-and-leaf display of these data will be

Leaf Unit = 0.1

8

9

10

11

6 8

1 4

2

0 7

Example: Leaf Unit = 10

If we have data with values such as

1806171719741791168219101838

a stem-and-leaf display of these data will be

Leaf Unit = 10

16

17

18

19

8

The 82 in 1682

is rounded down

to 80 and is

represented as an 8.

1 9

0 3

1 7

- Thus far we have focused on methods that are used
- to summarize the data on one variable at a time.

- Cross tabulationand a scatter diagram are methods for
- summarizing the data on two (or more) variables
- simultaneously.

- Help us understand the relationship between two variables.

Crosstabulation

- A crosstabulation is a tabular summary of data for
two variables.

- Crosstabulation can be used when:
- one variable is qualitative and the other is
- quantitative,
- both variables are qualitative, or
- both variables are quantitative.

- The left and top margin labels define the classes for
- the two variables.

Cross-tabulation

- Example:
The number of Homes sold by style and price for the past two years is shown below.

qualitative

variable

quantitative

variable

Home Style

Price

Range

Colonial Log Split A-Frame

Total

18 6 19 12

55

45

< $99,000

> $99,000

12 14 16 3

30 20 35 15

Total

100

- Insights Gained from Preceding Cross-tabulation

- The greatest number of homes in the sample (19)
- are a split-level style and priced at less than or
- equal to $99,000.

- Only three homes in the sample are an A-Frame
- style and priced at more than $99,000.

- Converting the entries in the table into row percentages or column percentages can provide additional insight about the relationship between the two variables.

Cross-tabulation

Frequency distribution

for the price variable

Home Style

Price

Range

Colonial Log Split A-Frame

Total

18 6 19 12

55

45

< $99,000

> $99,000

12 14 16 3

30 20 35 15

Total

100

Frequency distribution

for the home style variable

Cross-tabulation: Row Percentages

Home Style

Price

Range

Colonial Log Split A-Frame

Total

32.73 10.91 34.55 21.82

100

100

< $99,000

> $99,000

26.67 31.11 35.56 6.67

(Colonial and > $99K)/(All >$99K) x 100 = (12/45) x 100

Simpson’s Paradox

- Often Times data in two or more cross-tabulations are
aggregated to produce a summary cross-tabulation.

- We must be careful in drawing conclusions about the
- relationship between the two variables in the
- aggregated cross-tabulation.

- Simpson’ Paradox: In some cases the conclusions
based upon an aggregated cross-tabulation can be

completely different from the un-aggregated

data.

Scatter Diagram and Trend line

Scatter Diagram and Trend line

- A scatter diagram is a graphical presentation of the
- relationship between two quantitative variables.

- One variable is shown on the horizontal axis and the
- other variable is shown on the vertical axis.

- The general pattern of the plotted points suggests the
- overall relationship between the variables.

- A trend line is an approximation of the relationship.

- A Positive Relationship

y

x

- A Negative Relationship

y

x

- No Apparent Relationship

y

x

- The Panthers football team is interested
in investigating the relationship, if any,

between interceptions made and points scored.

x = Number of

Interceptions

y = Number of

Points Scored

14

24

18

17

30

1

3

2

1

3

35

30

25

20

15

10

5

0

1

0

2

3

4

y

Number of Points Scored

x

Number of Interceptions

Example: Panthers Football Team

- Insights Gained from the Preceding Scatter Diagram

- The scatter diagram indicates a positive relationship
- between the number of interceptions and the
- number of points scored.

- Higher points scored are associated with a higher
- number of interceptions.

- The relationship is not perfect; all plotted points in
- the scatter diagram are not on a straight line.