3B-1. Describing Data Visually (Part 2) Scatter Plots Scatter Plots Tables Pie Charts Pie Charts...

Describing Data Visually (Part Describing Data Visually (Part 2)2)

Describing Data Visually (Part Describing Data Visually (Part 2)2)

Scatter Plots

Tables

Pie ChartsPie Charts

Effective Excel ChartsEffective Excel Charts

Maps and PictogramsMaps and Pictograms

Deceptive GraphsDeceptive Graphs

Chapter3B3B3B3B

McGraw-Hill/Irwin © 2008 The McGraw-Hill Companies, Inc. All rights reserved.

3B-3

• A A scatter plotscatter plot shows shows nn pairs of observations as dots (or some other symbol) on an pairs of observations as dots (or some other symbol) on an XYXY graph. graph.

• A starting point for bivariate data analysis.A starting point for bivariate data analysis.

• Allows observations about the relationship between two variables.Allows observations about the relationship between two variables.

• Answers the question: Is there an association between the two variables and if so, Answers the question: Is there an association between the two variables and if so, what kind of association?what kind of association?

Scatter PlotsScatter PlotsScatter PlotsScatter Plots

3B-4

• Consider the Consider the following data:following data:

NationNation Birth RateBirth Rate Life ExpectancyLife Expectancy

AfghanistanAfghanistan 41.0341.03 46.6046.60

CanadaCanada 11.0911.09 79.7079.70

FinlandFinland 10.6010.60 77.8077.80

GuatemalaGuatemala 34.1734.17 66.9066.90

JapanJapan 10.0310.03 80.9080.90

MexicoMexico 22.3622.36 72.0072.00

PakistanPakistan 30.4030.40 62.7062.70

SpainSpain 9.299.29 79.1079.10

United United StatesStates 14.1014.10 77.4077.40

Example: Birth Rates and Life ExpectancyExample: Birth Rates and Life Expectancy


3B-5

• 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 cause-and-effect relationship?

Example: Birth Rates and Life ExpectancyExample: Birth Rates and Life Expectancy


3B-6

• Consider five observations on flight time and fuel consumption for a twin-engine Consider five observations on flight time and fuel consumption for a twin-engine Piper Cheyenne aircraft. Piper Cheyenne aircraft.

Trip LegTrip Leg Flight Time Flight Time (hours)(hours)

Fuel Used Fuel Used (pounds)(pounds)

11 2.32.3 145145

22 4.24.2 258258

33 3.63.6 219219

44 4.74.7 276276

55 4.94.9 283283

• A causal relationship A causal relationship is assumed since a is assumed since a longer flight would longer flight would consume more fuel.consume more fuel.

Example: Aircraft Fuel ConsumptionExample: Aircraft Fuel Consumption


3B-7

• Here is the scatter plot with flight time on the Here is the scatter plot with flight time on the XX-axis and fuel use on the -axis and fuel use on the YY-axis. -axis.

• Is there an Is there an association association between between variables?variables?

Example: Aircraft Fuel ConsumptionExample: Aircraft Fuel Consumption


3B-8

Degree of AssociationDegree of Association


Very strong associationVery strong association Strong associationStrong association

Moderate associationModerate association Little or no associationLittle or no association

3B-9

• Scatter plots can be helpful when policy decisions need to be made.

• For example, compare traffic fatalities resulting from crashes per million vehicles sold between 1995 and 1999.

• Do SUV’s create a greater risk to the drivers of both cars?

Policy MakingPolicy Making


3B-11

• TablesTables are the simplest form of data display. are the simplest form of data display.

• Arrangement of data is in rows and columns to enhance meaning.Arrangement of data is in rows and columns to enhance meaning.

• A A compound tablecompound table is a table that contains time series data down the columns and is a table that contains time series data down the columns and variables across the rows.variables across the rows.

• The data can be viewed by focusing on the time pattern (down the columns) or by The data can be viewed by focusing on the time pattern (down the columns) or by comparing the variables (across the rows).comparing the variables (across the rows).

Example: School ExpendituresExample: School Expenditures

TablesTablesTablesTables

3B-12

Elementary and SecondaryElementary and Secondary Colleges and UniversitiesColleges and Universities

YearYearAll All

SchoolsSchools TotalTotal PublicPublic PrivatePrivate TotalTotal PublicPublic PrivatePrivate

19601960 142.2142.2 99.699.6 93.093.0 6.66.6 42.642.6 23.323.3 19.319.3

19701970 317.3317.3 200.2200.2 188.6188.6 11.611.6 117.2117.2 75.275.2 41.941.9

19801980 373.6373.6 232.7232.7 216.4216.4 16.216.2 140.9140.9 93.493.4 47.447.4

19901990 526.1526.1 318.5318.5 293.4293.4 25.125.1 207.6207.6 132.9132.9 74.774.7

20002000 691.9691.9 418.2418.2 387.8387.8 30.330.3 273.8273.8 168.8168.8 105.0105.0

Source: U.S. Census Bureau, Source: U.S. Census Bureau, Statistical Abstract of the United States: 2002Statistical Abstract of the United States: 2002 , p. 133. , p. 133. Note: All figures are in billions of constant 2000/2001 dollars. Note: All figures are in billions of constant 2000/2001 dollars.

• Units of measure are stated in the footnote.Units of measure are stated in the footnote.• Note merged headings to group columns.Note merged headings to group columns.

Example: School ExpendituresExample: School Expenditures

TablesTablesTablesTables

3B-13

• A A pie chartpie chart can only convey a general idea of the data. 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 Pie charts should be used to portray data which sum to a total (e.g., percent market shares).market shares).

• A pie chart should only have a few (i.e., 2 or 3) slices.A pie chart should only have a few (i.e., 2 or 3) slices.

• Each slice should be labeled with data values or percents.Each slice should be labeled with data values or percents.

An Oft-Abused ChartAn Oft-Abused Chart

Pie ChartsPie ChartsPie ChartsPie Charts

3B-14

• Consider the following charts used to illustrate an article from the Wall Street Journal. Consider the following charts used to illustrate an article from the Wall Street Journal. Which type is better?Which type is better?

An Oft-Abused ChartAn Oft-Abused Chart


2-D Pie Chart2-D Pie Chart Bar ChartBar Chart

3B-15

• ExplodedExploded and and 3-D pie charts3-D pie charts add strong visual impact but slices are hard to add strong visual impact but slices are hard to assess.assess.

Pie Chart OptionsPie Chart Options


Exploded Pie ChartExploded Pie Chart Exploded 3-D Pie ChartExploded 3-D Pie Chart

3B-16

• Pie charts can only convey a general idea of the data values.Pie charts can only convey a general idea of the data values.

• Pie charts are ineffective when they have too many slices.Pie charts are ineffective when they have too many slices.

• Pie chart data must represent Pie chart data must represent parts of a wholeparts of a whole (e.g., percent market share). (e.g., percent market share).

Common Errors in Pie Chart UsageCommon Errors in Pie Chart Usage


3B-17

• A nonzero origin will exaggerate the trend.A nonzero origin will exaggerate the trend.

DeceptiveDeceptive CorrectCorrect

Deceptive GraphsDeceptive GraphsDeceptive GraphsDeceptive Graphs

Error 1: Nonzero OriginError 1: Nonzero Origin

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