1 Charts and graphs

2 Agenda value and limits of graphical analysis how to create and read and interpret graphs basic types of graphs and conditions under which should be used

3 Basic principle: Charts and graphs, like all numerical information, have one function: To communicate information to an audience in support of a thesis or claim.

4 Main benefit of graphing: Eases interpretation of data. –Visual representation draws attention to relative amounts. –Easier to see all data simultaneously.

5 Main drawbacks to graphs: 1.Lose sight of actual amounts. 2.Easier to use for deception.

6 Interpretation Text explains meaning and significance of results Two elements: Chart reading and chart interpretation

7 Chart reading Verbal expression of main features of the pattern What do the data show? Forest, not trees

8 Chart interpretation: Commentary or analysis of appropriate conclusions or questions What do the data mean?

9 Qualities of good graph 1.Appropriate to the data 2.Self-explanatory: effective title and axis labels 3. Simple and uncluttered 4.Not misleading

10 Appropriate to data

11 Pie charts 1.Slices represent shares of a whole. 2.The categories that comprise the variable must be qualitatively different or crude rankings. 3.The slices/categories must exhaust the possibilities. 4.The number of slices must be small. Avoid having many narrow slices. 5.Must include a legend.

12 Slices = shares of wholes Abstainer19 Non-binge drinker 37 Occasional binge drinker 21 Frequent binge drinker 23

13 Slices need not be percentages Ray$6000 Bob$4500 Bill$4000 Jim$3000

14 “Total” never a slice Ray$6000 Bob$4500 Bill$4000 Jim$3000 Total$17,500

15 Pie chart: Appropriate data Categories represent different qualities –Sex –Religion –Race/ethnicity Categories represent crude quantitative differences –Large, small –High, medium, low –Strongly agree, agree, disagree, strongly disagree –Freshman, sophomore, junior, senior

16 Pie chart: Inappropriate data Precise quantities –Age –GPA –# of credits

17 Exhaustive categories Non-binge drinker 37 Occasional binge drinker 21 Frequent binge drinker 23

18 Bar and column graphs Interchangeable. Can be used with any kind of frequency count data. Can be used to show multiple series of data. Legend required if using more than one series.

19 Line graphs: Don’t create them using the “line graph” option in Excel.... The results are funky. (A technical term) Use “xy scatter” with connection of points instead.

20 xy scatter graphs Can only be used when both variables are quantitative. Show the relationship between two variables. The causal variable goes on the x (horizontal) axis.

21 Self-explanatory Key is title and labels 1.Title should be clear, concise, complete. 2.All variables must be named. 3.Measurement units must be properly stated. 4.Time period must be stated. 5.Spatial or geographic domain must be specified.

22 Simple and uncluttered 1.Limited number of variables 2.No unnecessary legends. 3.No distracting colors and shading. 4.Consistent colors and shading. 5.Use colors or patterns that will be distinct if printed in black & white.

23 Misleading graphs and charts. Truncated y-axis Unlabeled axes Arbitrary axis dimensions 3-dimensional pictograms 3-dimensional pie charts Line graphs with data gaps

24 Truncated vertical [y] axis

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28 Arbitrary dimensions

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32 3-dimensional pictograms

33 Rising costs of prescriptions

34 Deceptive pictogram Problem is that changing one dimension while keeping proportions changes both dimension, leading to misleading greater change in volume.

35 Pictograms

36 3-dimensional pie charts

37 Three-dimensional pie graphs

38 Line graphs with data gaps