Describing Data Visually (Part 2)

2. 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.

3. 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?

4. Scatter Plots • Example: Birth Rates and Life Expectancy • Consider the following data:

5. 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 cause-and-effect relationship?

6. 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 is assumed since a longer flight would consume more fuel.

7. 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?

8. Very strong association Strong association Moderate association Little or no association Scatter Plots • Degree of Association

9. Scatter Plots • 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?

11. In Excel, highlight the two data columns, then click on the Chart Wizard icon on the toolbar. Select the XY (Scatter) option.

12. Scatter Plots • 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.

13. Default graph 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.

14. 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).

15. Tables • Example: School Expenditures • Units of measure are stated in the footnote. • Note merged headings to group columns.

16. 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.

17. 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.

18. 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.

19. 2-D Pie Chart 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?

20. Exploded 3-D Pie Chart Exploded Pie Chart Pie Charts • Pie Chart Options • Exploded and 3-D pie charts add strong visual impact but slices are hard to assess.

21. 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).

22. 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.

23. Use the mouse to select (highlight) the data you want to plot.

24. Effective Excel Charts • 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.

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

26. Effective Excel Charts • 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.

27. Effective Excel Charts • Chart Wizard • Step 4: Click Next to display the finished chart.

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

29. Effective Excel Charts • Embellished Charts • Charts created in Excel can be edited to: - Add a data table underneath the graph.

30. Effective Excel Charts • Embellished Charts • Charts created in Excel can be edited to: - Change color or patterns in the plot or chart areas.

31. Effective Excel Charts • 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).

32. Effective Excel Charts • Embellished Charts • Charts created in Excel can be edited to: - Alter the appearance of the bars (color, pattern, gap width).

33. Effective Excel Charts • 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.

34. Over-embellished chart Embellished bar chart Effective Excel Charts • Embellished Charts • Be careful about over-embellishing your charts.

35. Area (mountain) chart Multiple bar chart Effective Excel Charts • Embellished Charts • Excel offers many other types of specialized charts.

36. Effective Excel 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

37. Effective Excel Charts • Embellished Charts • Other specialized Excel charts: - Stock chart for high/low/close stock prices. Data from http://finance.yahoo.com

38. Effective Excel Charts • Embellished Charts • Other specialized Excel charts: - Radar (or Spider) chart compares individual performance against abenchmark. - Caution, data may be distorted by emphasized areas.

39. 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.

40. U.S. population change by county, 1990/2000

41. Maps and Pictograms • Pictograms • A visual display in which data values are replaced by pictures.

42. Maps and Pictograms • Pictograms • Although entertaining, they can create visual distortion. What do you think?

43. Deceptive Graphs • Error 1: Nonzero Origin • A nonzero origin will exaggerate the trend. Correct Deceptive

44. 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. Correct Deceptive

45. 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.

46. Deceptive Graphs • Error 6: 3-D and Rotated Graphs • Can make trends appear to dwindle into the distance or loom towards you. Correct Deceptive

47. 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.

48. Deceptive Graphs • Error 10: Complex Graphs • Avoid if possible. Keep your main objective in mind. If necessary, break graph into smaller parts.

49. 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.

50. Deceptive Graphs • Error 13: Area Trick • As figure height increases, so does width, distorting the area. Deceptive Correct