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Slides Prepared by JOHN S. LOUCKS St. Edward’s University. y. x. Chapter 2 Descriptive Statistics: Tabular and Graphical Presentations Part B. Exploratory Data Analysis Crosstabulations and Scatter Diagrams. Minor mods by McKim. Exploratory Data Analysis.
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Chapter 2Descriptive Statistics:Tabular and Graphical PresentationsPart B
- Exploratory Data Analysis
- Crosstabulations and
Scatter Diagrams
Minor mods by McKim
Exploratory Data Analysis
- The techniques of exploratory data analysis consist of
- simple arithmetic and easy-to-draw pictures that can
- be used to summarize data quickly.
- One such technique is the stem-and-leaf display.
Stem-and-Leaf Display
- A stem-and-leaf display shows both the rank order
- and shape of the distribution of the data.
- It is similar to a histogram on its side, but it has the
- advantage of showing the actual data values.
- The first digits of each data item are arranged to the
- left of a vertical line.
- To the right of the vertical line we 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.
The manager of Hudson Auto
would like to have a better
understanding of the cost
of parts used in the engine
tune-ups performed in the
shop. She examines 50
customer invoices for tune-ups. The costs of parts,
rounded to the nearest dollar, are listed on the next
slide.
- Sample of Parts Cost for 50 Tune-ups
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
Stretched Stem-and-Leaf Display
- If we believe the original stem-and-leaf display has
- condensed the data too much, we can stretch the
- display by using two stems for each leading digit(s).
- Whenever a stem value is stated twice, the first value
- corresponds to leaf values of 0 - 4, and the second
- value 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
- 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.
Example: Leaf Unit = 0.1
If we have data with values such as
8.6 11.7 9.4 9.1 10.2 11.0 8.8
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
If we have data with values such as
1806 1717 1974 1791 1682 1910 1838
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
Crosstabulations and Scatter Diagrams
- Thus far we have focused on methods that are used
- to summarize the data for one variable at a time.
- Often a manager is interested in tabular and
- graphical methods that will help understand the
- relationship between two variables.
- Crosstabulation and a scatter diagram are two
- methods for summarizing the data for two (or more)
- variables simultaneously.
- 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.
- In other words, any time!! (Duh.)
- The left and top margin labels define the classes for
- the two variables.
- Example: Finger Lakes Homes
The number of Finger Lakes homes sold for each 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
Crosstabulation
- Insights Gained from Preceding Crosstabulation
- 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.
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
Crosstabulation: Row or Column Percentages
- Converting the entries in the table into row percentages or column percentages can provide additional insight about the relationship between the two variables.
Crosstabulation: 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
Note: row totals are actually 100.01 due to rounding.
(Colonial and > $99K)/(All >$99K) x 100 = (12/45) x 100
Crosstabulation: Column Percentages
Home Style
Price
Range
Colonial Log Split A-Frame
60.00 30.00 54.29 80.00
< $99,000
> $99,000
40.00 70.00 45.71 20.00
100 100 100 100
Total
(Colonial and > $99K)/(All Colonial) x 100 = (12/30) x 100
Crosstabulation: Simpson’s Paradox
- Data in two or more crosstabulations are often
aggregated to produce a summary crosstabulation.
- We must be careful in drawing conclusions about the
- relationship between the two variables in the
- aggregated crosstabulation.
- Simpson’ Paradox: In some cases the conclusions
based upon an aggregated crosstabulation can be
completely reversed if we look at the unaggregated
data. suggests the overall relationship between the
variables. (Yikes, just ignore that last partial sentence.)
Judge
Verdict
Total
Appealed
Luckett Kendall
129 (86%) 110 (88%)
Upheld
Reversed
239
36
- 21 (14%) 15 (12%)
150 (100%) 125 (100%)
Total (%)
275
Kendall appears to have the better rate of upheld verdicts
Judge Luckett
Verdict
Total
Appealed
Common Pleas Municipal
29 (91%) 100 (85%)
Upheld
Reversed
129
21
- 3 (9%) 18 (15%)
32 (100%) 118 (100%)
Total (%)
150
Judge Kendall
Verdict
Total
Appealed
Common Pleas Municipal
90 (90%) 20 (80%)
Upheld
Reversed
110
15
- 10 (10%) 5 (20%)
100 (100%) 25 (100%)
Total (%)
125
Luckett has the better rate of upheld verdicts in both Common Pleas and Municipal Courts
Yet the aggregate table makes it appear the Kendall has the better rate.
What appears to have happened is that Municipal Court appeals have a higher overturn rate and Luckett just happened to try most of his cases there.
What other important factor is missing from this analysis??
- 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 trendline is an approximation of the relationship.
- Example in book is number of commercials vs sales
Example: Panthers Football Team
- Scatter Diagram
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
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.
Tabular and Graphical Procedures
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
Keep in mind…
- Frequency Distribution shows how many.
- Rel. Freq. Distribution shows what fraction.
- Percent Freq. Distribution shows what percentage.
- Can represent any of the above in tables or charts.
