3
Chapter
Describing Data Visually (Part 2)
Scatter Plots Tables Pie Charts Effective Excel Charts Maps and Pictograms Deceptive Graphs
McGraw-Hill/Irwin
Copyright Š 2009 by The McGraw-Hill Companies, Inc. All rights reserved.
Scatter Plots • A scatter plot shows n pairs of observations as dots (or some other symbol) on an XY graph. • A starting point for bivariate data analysis. • Allows observations about the relationship between two variables. • Answers the question: Is there an association between the two variables and if so, what kind of association? 3B-2
Scatter Plots Example: Birth Rates and Life Expectancy Nation
• Consider the following data:
3B-3
Birth Rate
Life Expectancy
Afghanistan
41.03
46.60
Canada
11.09
79.70
Finland
10.60
77.80
Guatemala
34.17
66.90
Japan
10.03
80.90
Mexico
22.36
72.00
Pakistan
30.40
62.70
Spain
9.29
79.10
United States
14.10
77.40
Scatter Plots Example: Birth Rates and Life Expectancy • Here is a scatter plot with life expectancy on the X-axis and birth rates on the Y-axis. • Is there an association between the two variables? • Is there a causeand-effect relationship? 3B-4
Scatter Plots Example: Aircraft Fuel Consumption • Consider five observations on flight time and fuel consumption for a twin-engine Piper Cheyenne aircraft. • A causal relationship Trip Leg Flight Time Fuel Used (hours) (pounds) is assumed since a 1 2.3 145 longer flight would consume more fuel. 2 4.2 258 3 3.6 219 4 4.7 276 5 4.9 283 3B-5
Scatter Plots Example: Aircraft Fuel Consumption • Here is the scatter plot with flight time on the X-axis and fuel use on the Y-axis. • Is there an association between variables?
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Scatter Plots Degree of Association
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Very strong association
Strong association
Moderate association
Little or no association
Scatter Plots • Scatter plots can convey patterns in data pairs that would not be apparent from a table.
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Scatter Plots
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Scatter Plots
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Scatter Plots Setting up the Excel Scatter Plot
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Scatter Plots Embellishing a Scatter Plot in Excel
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Scatter Plots • Excel makes it easy to fit a line to data on a scatter plot.
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Tables • Tables are the simplest form of data display. • A compound table is a table that contains time series data down the columns and variables across the rows.
Example: School Expenditures • Arrangement of data is in rows and columns to enhance meaning. • The data can be viewed by focusing on the time pattern (down the columns) or by comparing the variables (across the rows). 3B-14
Tables Example: School Expenditures Elementary and Secondary Year
All Schools
Colleges and Universities
Total
Public
Private
Total
Public
Private
1960
142.2
99.6
93.0
6.6
42.6
23.3
19.3
1970
317.3
200.2
188.6
11.6
117.2
75.2
41.9
1980
373.6
232.7
216.4
16.2
140.9
93.4
47.4
1990
526.1
318.5
293.4
25.1
207.6
132.9
74.7
2000
691.9
418.2
387.8
30.3
273.8
168.8
105.0
Source: U.S. Census Bureau, Statistical Abstract of the United States: 2002, p. 133. Note: All figures are in billions of constant 2000/2001 dollars.
• Units of measure are stated in the footnote. • Note merged headings to group columns. 3B-15
Tables Tips for Effective Tables 1. Keep the table simple, consistent with its purpose. - Summary tables go in the main body. - Detailed tables go in an appendix. - In a slide show, main point of table should be clear within 10 seconds, otherwise, break up table. 2. Display the data to be compared in columns. 3. Round off data to 3 or 4 significant digits. 3B-16
Tables Tips for Effective Tables 4. Table layout should guide the eye towards the desired comparison. - Use spaces or shading to separate rows or columns. - Use lines sparingly. 5. Keep row and column headings simple yet descriptive. 6. Use a consistent number of decimal digits within a column. - Right-justify or decimal align the data. 3B-17
Excel ’s Pivot Tables Excel’s
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Excel ’s Pivot Tables Excel’s
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Excel ’s Pivot Tables Excel’s
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Learning Stats Stats’’ Pivot Tables
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Pie Charts An Oft-Abused Chart • A pie chart can only convey a general idea of the data. • Pie charts should be used to portray data which sum to a total (e.g., percent market shares). • A pie chart should only have a few (i.e., 2 or 3) slices. • Each slice should be labeled with data values or percents. 3B-22
Pie Charts An Oft-Abused Chart • Consider the following charts used to illustrate an article from the Wall Street Journal. Which type is better?
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Pie Charts Pie Chart Options • Exploded and 3-D pie charts add strong visual impact but slices are hard to assess.
Exploded Pie Chart 3B-24
Exploded 3-D Pie Chart
Pie Charts Common Errors in Pie Chart Usage • Pie charts can only convey a general idea of the data values. • Pie charts are ineffective when they have too many slices. • Pie chart data must represent parts of a whole (e.g., percent market share).
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Effective Excel Charts • Effective visual displays - help you get your point across - are persuasive - help you see facts more clearly - help you make better decisions - make your business report stand out - increase your reputation - make you a more desirable employee • Easily learned, Excel is widely used because of its excellent graphics capabilities. 3B-26
Effective Excel Charts
Use the mouse to select (highlight) the data you want to plot.
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Effective Excel Charts Embellished Charts • Charts created in Excel can be edited to: - Improve the titles (main, X-axis, Y-axis). - Change the axis scales (minimum, maximum, demarcations). Fractional Shares, 1993-2002 7000
5827
6000
4871
5000
3834
4000 2607
3000 2000 1000
110
158
285
548
957
1551
0 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002
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Series1
- Display the data values (on the top of each bar).
Effective Excel Charts Embellished Charts • Charts created in Excel can be edited to: - Add a data table underneath the graph. Fractional Shares, 1993-2002 8000 6000 4000
Series1
2000 0
1993 1994 1995 1996 1997 1998 1999 2000 2001 2002
Series1 110 158 285 548 957 1551 2607 3834 4871 5827
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Effective Excel Charts Embellished Charts • Charts created in Excel can be edited to: - Change color or patterns in the plot or chart areas. Fractional Shares, 1993-2002 7000 6000 5000 4000
Series1
3000 2000 1000 0 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002
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Effective Excel Charts Embellished Charts • Charts created in Excel can be edited to: - Format the decimals (on the axes or data labels). 7000 6000 5000 4000 Series1
3000 2000 1000
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20 02
20 01
20 00
19 99
19 98
19 97
19 96
19 95
19 94
19 93
0
- Edit the gridlines (color, dotted or solid, patterns).
Effective Excel Charts Embellished Charts • Charts created in Excel can be edited to: - Alter the appearance of the bars (color, pattern, gap width). Fractional Shares, 1993-2002 7000 6000 5000 4000
Series1
3000 2000 1000 0 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002
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Effective Excel Charts Embellished Charts • Be careful about over-embellishing your charts.
Embellished bar chart 3B-33
Over-embellished chart
Effective Excel Charts Embellished Charts • Excel offers many other types of specialized charts.
Area (mountain) chart 3B-34
Multiple bar chart
Effective Excel Charts Embellished Charts • Other specialized Excel charts: - Radar (or Spider) chart used to compare individual performance against a benchmark.
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Effective Excel Charts Embellished Charts • Other specialized Excel charts: - Use floating bar charts to show a range of data.
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Maps and Pictograms Spatial Variation and GIS • Maps can be used for displaying many kinds of data. - Appropriate when patterns of variation across space are of interest. - Self-explanatory and revealing. - Assess patterns based on geography. • GIS (geographic information systems) combines statistics, geography and graphics. 3B-37
Maps and Pictograms
• U.S. population change by county, 1990/2000 3B-38
Maps and Pictograms Pictograms • Although entertaining, they can create visual distortion. What do you think?
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Deceptive Graphs Error 1: Nonzero Origin • A nonzero origin will exaggerate the trend.
Deceptive 3B-40
Correct
Deceptive Graphs Error 2: Elastic Graph Proportions • Keep the aspect ratio (width/height) below 2.00 so as not to exaggerate the graph. By default, Excel uses an aspect ratio of 1.8.
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Deceptive Graphs Error 3: Dramatic Title • Keep short and grab readers attention.
Error 4: Distracting Pictures • Avoid so as not to distract readers or impart an emotional slant.
Error 5: Authority Figures • Can use pictures of authority figures to impart credibility to self-serving commercial claims. 3B-42
Deceptive Graphs Error 6: 3-D and Rotated Graphs • Can make trends appear to dwindle into the distance or loom towards you.
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Deceptive Graphs Error 7: Missing Axis Demarcations • If tick marks are missing, you cannot identify individual data values.
Error 8: Missing Measurement Units or Definitions • Missing or unclear units of measurement can render a chart useless.
Error 9: Vague Source • May indicate lost citation, unknown source, or mixed data sources. Use complete source citations. 3B-44
Deceptive Graphs Error 10: Complex Graphs • Avoid if possible. Keep your main objective in mind. If necessary, break graph into smaller parts.
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Deceptive Graphs Error 11: Gratuitous Effects • Avoid too many annoying special effects when using slide shows.
Error 12: Estimated Data • Estimated points should be noted when used or avoided if possible.
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Deceptive Graphs Error 13: Area Trick • As figure height increases, so does width, distorting the area.
Deceptive 3B-47
Correct
Applied Statistics in Business & Economics
End of Chapter 3B
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