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# 3B-1 - PowerPoint PPT Presentation

3B. Chapter. Describing Data Visually (Part 2). Scatter Plots Tables Pie Charts Effective Excel Charts Maps and Pictograms Deceptive Graphs. McGraw-Hill/Irwin. © 2008 The McGraw-Hill Companies, Inc. All rights reserved. Scatter Plots.

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Chapter

### Describing Data Visually (Part 2)

Scatter Plots

Tables

Pie Charts

Effective Excel Charts

Maps and Pictograms

Deceptive Graphs

McGraw-Hill/Irwin

• 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?

• Example: Birth Rates and Life Expectancy

• Consider the following data:

• 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 cause-and-effect relationship?

• Example: Aircraft Fuel Consumption

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

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

• 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?

Strong association

Moderate association

Little or no association

Scatter Plots

• Degree of Association

• Policy Making

• 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?

In Excel, highlight the two data columns, then click on the Chart Wizard icon on the toolbar.

Select the XY (Scatter) option.

• Making a Scatter Plot in Excel

• Click Next and then click the Series tab.

• Excel assumes that the first column contain X-axis values and the second column contains Y-axis values.

• Alternatively, you can specify the data range explicitly for each variable.

Embellished graph

Scatter Plots

• Making a Scatter Plot in Excel

• Click Finish to display a plain scatter plot.

• You can embellish it using Excel’s options.

• 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).

• Example: School Expenditures

• Units of measure are stated in the footnote.

• Note merged headings to group columns.

• 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.

• 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.

• 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.

Bar Chart

Pie Charts

• An Oft-Abused Chart

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

Exploded Pie Chart

Pie Charts

• Pie Chart Options

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

• 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).

• Easily learned, Excel is widely used because of its excellent graphics capabilities.

• Chart Wizard

• Click on the Chart Wizard icon on the toolbar to open a sequence of pop-up menus to guide you through the steps of creating a chart.

• Step 1: Select the Chart type and then click Next.

• Chart Wizard

• Step 2: Add labels for years on the X-axis by selecting a data range (B4:B13). Click Next.

• Chart Wizard

• Step 3: Embellish the chart by adding a title, axis labels, adjusting the gridlines or appending a data table to the graph by clicking on the appropriate tab.

• Chart Wizard

• Step 4: Click Next to display the finished chart.

• 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).

- Display the data values (on the top of each bar).

• Embellished Charts

• Charts created in Excel can be edited to:

- Add a data table underneath the graph.

• Embellished Charts

• Charts created in Excel can be edited to:

- Change color or patterns in the plot or chart areas.

• Embellished Charts

• Charts created in Excel can be edited to:

- Format the decimals (on the axes or data labels).

- Edit the gridlines (color, dotted or solid, patterns).

• Embellished Charts

• Charts created in Excel can be edited to:

- Alter the appearance of the bars (color, pattern, gap width).

• Embellished Charts

• To alter a chart’s appearance, click on any chart object and then right-click to see a menu of properties that you can change.

• For example, right-click on the Y-axis scale and choose Format Axis.

Embellished bar chart

Effective Excel Charts

• Embellished Charts

Multiple bar chart

Effective Excel Charts

• Embellished Charts

• Excel offers many other types of specialized charts.

• Embellished Charts

• Other specialized Excel charts:

- Bubble chart displays three variables on a 2-dimensional scatter plot.

- Note: bubble size is proportional to third variable.

Data from http://peltiertech.com/Excel/ChartsHowTo/HowToBubble.html

• Embellished Charts

• Other specialized Excel charts:

- Stock chart for high/low/close stock prices.

Data from

http://finance.yahoo.com

• Embellished Charts

• Other specialized Excel charts:

- Radar (or Spider) chart compares individual performance against abenchmark.

- Caution, data may be distorted by emphasized areas.

Maps and Pictograms plot.

• 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.

Maps and Pictograms plot.

• Pictograms

• A visual display in which data values are replaced by pictures.

Maps and Pictograms plot.

• Pictograms

• Although entertaining, they can create visual distortion. What do you think?

Deceptive Graphs plot.

• Error 1: Nonzero Origin

• A nonzero origin will exaggerate the trend.

Correct

Deceptive

Deceptive Graphs plot.

• 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.

Correct

Deceptive

Deceptive Graphs plot.

• 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.

Deceptive Graphs plot.

• Error 6: 3-D and Rotated Graphs

• Can make trends appear to dwindle into the distance or loom towards you.

Correct

Deceptive

Deceptive Graphs plot.

• 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.

Deceptive Graphs plot.

• Error 10: Complex Graphs

• Avoid if possible. Keep your main objective in mind. If necessary, break graph into smaller parts.

Deceptive Graphs plot.

• 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.

Deceptive Graphs plot.

• Error 13: Area Trick

• As figure height increases, so does width, distorting the area.

Deceptive

Correct