A large number of software packages are available that can help make tables, graphs, and charts. Most of these packages enable the user to redraw a graph with only a few keystrokes.
On the other hand, allowing the software to dictate the graph can be tempting (Tables 4.6 and 4.7). For example, many packages can draw bar charts and pie charts that appear three-dimensional. Does this mean a three-dimensional chart is appropriate? Also, a problem common to three- dimensional bar charts is that a bar in the front row can block a bar in the back row. Remember that the purpose of a graph is to simply and effectively information simply and effectively. Determine whether a three-dimensional chart presents the information better than a two-dimensional chart. If the aim is to compare trends over time for confirmed and reported cases, perhaps a three- dimensional bar chart is preferable. However, an arithmetic scale line graph with two lines would be more than adequate.
For pie charts, the same question of usefulness needs to be asked. Often a three-dimensional pie chart will skew the relative sizes of the individual pie slices, making it difficult to see which slice is bigger—and size is the purpose of using a pie chart.
Technology is often misused in selecting color, particularly for slides that accompany oral presentations. If colors are to be used, follow these recommendations:
• Select colors so that all components of the graph—title, axes, data plots, and legends—stand out clearly from the background and each plotted series of data can be distinguished from the others.
• Avoid contrasting red and green because up to 10% of men in the audience can have some degree of color blindness.
• When possible, select colors that aid in communicating the information. For example, consider an area map in which states are divided into four groups according to rates for a particular disease. Rather than choosing colors solely for appearance, use a lighter color or shade for states with the lowest rates and progressively darker colors or shades for states with increasingly higher rates so that the colors contribute to, rather than distort or distract from, the information. Finally, some software packages do not enable you to produce some of the types of graphs covered in this section. In particular, some software packages cannot create a histogram; instead they produce a bar chart (although some software programs enable you to adjust the “gap width” between the bars to zero, essentially allowing you to display a histogram). The data and the relationships they visually communicate, not the technology, should dictate the graphs used. If the software cannot accommodate the data, don’t compromise the integrity of the data or its presentation. Use different software!
Table 4.6 Guide for selecting data graphics
TYPE OF GRAPH OR CHART WHEN TO USE
LINE GRAPHS
Arithmetic scale line graph Trends in numbers or rates over time
Histogram • Frequency distribution of continuous variable
• Number of cases during epidemic (i.e., epidemic curve) or over time
Frequency polygon Frequency distribution of continuous variable, especially to show components
BAR AND PIE CHARTS
Simple bar chart Comparison of size or frequency of different categories of single variable
Grouped bar chart Comparison of size or frequency of different categories of two to four series of data
Stacked bar chart Comparison of totals and depiction of component parts of the total among different groups
Deviation bar chart Illustration of differences, both positive and negative, from baseline
Pie chart Components of a whole
MAPS
Area map Display of events or rates geographically
IF DATA ARE AND THESE CONDITIONS APPLY THEN CHOOSE
Time series Numbers of cases (epidemic or secular trend)
One set Histogram Time series Numbers of cases
(epidemic or secular trend)
Two or more
sets Arithmetic scale line graph Time series Rates Range of values
≤2 order of magnitude
Arithmetic scale line graph
Continuous data other than time series Frequency distribution Histogram or frequency polygon Discrete categories (other than place) Bar chart or pie chart Place No. cases Not readily identified on a map Bar chart
Place No. cases Not readily identified on a map
Specific site
unimportant Area map
Place Rates Area map
Source: U.S. Department of Health and Human Services, Principles of Epidemiology. 2nd ed. (CDC Self Study Course #3030-G): p. 263–4.
C H E C K L I S T F O R C O N S T R U C T I N G TA B L E S , G R A P H S , C H A R T S , A N D O T H E R V I S U A L S
Checklist for Tables• Title
▶ Does the table have a title?
▶ Does the title describe the content, including subject, person, place, and time?
▶ When appropriate (e.g., for publications, reports), is the title preceded by the designation “Table #”? (“Table” is used for typed text; “Figure” is used for graphs, charts, and maps. Separate numerical sequences are used for tables and figures in the same document [e.g., Table 1, Table 2, Figure 1, and Figure 2]).
• Rows and columns
▶ Are each row and each column labeled clearly and concisely?
▶ Are the specific units of (e.g., years, mm Hg, mg/dL, rate per 100,000) shown?
▶ Are the categories appropriate for the data?
▶ Are the row and column totals provided? • Footnotes
▶ Are all codes, abbreviations, and symbols explained?
▶ Are all exclusions noted?
▶ If the data are not original, is the source cited? Checklist for Graphs and Charts
• Title
▶ Does the graph or chart have a title?
▶ Does the title describe the content, including subject, person, place, and time?
▶ When appropriate (e.g., for publications, reports), is the title preceded by the designation ‘Figure #”? (“Table” is used for typed text; “Figure” for graphs, charts, and maps. Separate numerical sequences are used for tables and figures in the same document [e.g., Table 1, Table 2, Figure 1, and Figure 2]).
• Axes
▶ Is each axis labeled clearly and concisely?
▶ Are the specific units of measurement (e.g., years, mm Hg, mg/dL, rate per 100,000) included as part of the label?
▶ Are the scale divisions on the axes clearly indicated?
▶ Are the scales for each axis appropriate for the data?
▶ Does the y-axis start at zero?
▶ If a scale break is used with a scale line graph, is it clearly identified?
▶ Has a scale break been used with a histogram, frequency polygon, or bar chart? (Answer should be no!)
▶ Are the axes drawn heavier than the other coordinate lines?
• Coordinate lines: Does the figure include only as many coordinate lines as are necessary to guide the eye? (Often, these are unnecessary.)
• Data plots
▶ Are the plots drawn clearly?
▶ If more than one series of data or components are shown, are they clearly distinguishable on the graph?
▶ Is each series or component labeled on the graph or in a legend or key?
▶ If color or shading is used on an area map, does an increase in color or shading correspond to an increase in the variable being shown?
• Footnotes
▶ Are all codes, abbreviations, and symbols explained?
▶ Are all exclusions noted?
▶ If the data are not original, is the source cited? • Visual display
▶ Does the figure include any information that is not necessary?
▶ Is the figure positioned on the page for optimal readability?
▶ Do font sizes and colors improve readability? Checklist for Effective Visuals
• Legibility
▶ Can your overhead transparencies be read easily from 6 feet when not projected?
▶ When projected, can your visuals be read from the farthest parts of the room? • Simplicity
▶ Have you used plain words?
▶ Is the information presented in the language of the audience?
▶ Have you used only “key” words?
▶ Is each visual limited to only one major idea, concept, or theme?
▶ Does each visual have no more than three colors?
▶ Have you minimized letters and numbers on each visual?
▶ Have you minimized lines of narration and words per line? • Colorfulness
▶ Have you selected appropriate colors for your visuals (Table 4.8)? Use warm/hot colors to emphasize, to highlight, to focus, or to reinforce key concepts. Use cool/cold colors for background or to separate items.
Table 4.8 Selecting colors for graphs, charts, and other visuals
HOT WARM COOL COLD
COLOR Reds Light orange Light blue Dark blue
Bright orange Light yellow light green Dark green Bright yellow Light gold Light purple Dark purple Bright gold Browns Light gray Dark gray
• Are you using the best color combinations? The most important item should be in the most important color and have the greatest contrast with its background. The most legible color combinations are
▶ Black on yellow
▶ Black on white
▶ Dark green on white
▶ Dark blue on white
▶ White on dark blue
• Accuracy: Has someone who has not previously seen the visual checked for typos, inaccuracies, and errors in general?
• Durability:
▶ Have you backed up your visuals? Keep backup copies of PowerPoint slide presentations in various locations on a protected computer network.
▶ Transparencies are fairly durable visual aids but require some protection from scratches from a clear sheet of acetate or Mylar. Transparencies are not easily modified without changing the source computer file and reprinting.
Source: U.S. Department of Health and Human Services, Principles of Epidemiology, 2nd ed. (CDC Self Study Course #3030-G); p. 263–4.