A good visual aid can do in five seconds what words cannot do in five minutes: show the shape of a coastline, the rise of a fever curve, the crack in a bridge support. A bad one turns a speech into a reading exercise.
The basic test
Before making any visual, ask what job it does. A visual aid should do something that words alone cannot do well. Showing a map of flood zones passes the test. Projecting the sentence you are about to say does not. If a visual simply repeats your words, it competes with you rather than helping.
Kinds of visual aids
Objects and models let listeners see the real thing or a scaled version of it. Photographs show what something looks like. Diagrams show how parts fit together. Charts show patterns in numbers, and each kind suits a different job. A line graph shows change over time, such as a city's population across a century. A bar graph compares amounts across categories, such as spending by department. A pie chart shows parts of a whole, though it becomes hard to read with more than a few slices.
Designing slides
Slides are the most common visual aid and the most commonly misused. A few principles cover most cases. Put one idea on each slide. Use few words and large type that can be read from the back of the room. Use high contrast, such as dark text on a light background. Replace topic labels with point statements: a slide headline should be a full sentence stating the point, such as "Asthma visits doubled after the plant opened," rather than "Asthma data." Then the audience knows what to look for in the chart below it.
The redundancy trap
The most common slide mistake is filling the screen with sentences and then reading them aloud. Research on multimedia learning, especially by the psychologist Richard Mayer, suggests that reading identical text aloud while it is on the screen tends to hurt understanding rather than help it, because listeners try to read and listen at once and do neither well. The speaker is the presentation. Slides should support the speaker, not replace the speech.
A warning from space
In 2003 the space shuttle Columbia broke apart while returning to Earth, killing all seven astronauts. During the flight, engineers had briefed managers on whether a piece of foam that struck the wing at launch could have caused serious damage. The investigation board later noted that NASA had come to rely on slide briefings in place of technical papers, and it found that important qualifications were buried in dense bullet points. The designer and writer Edward Tufte analyzed one such slide, showing how a crucial warning, that the test data came from pieces of foam far smaller than the one that hit Columbia, sat in small type deep in the bullet list beneath a reassuring headline. Design did not cause the disaster, but it helped hide a warning.
Handling visuals
Show a visual only while you are discussing it. When you are not discussing a visual, blank the screen or cover the object, so attention returns to you; many presentation programs blank the screen with a single key. Face the audience, not the screen. Arrive early to test the equipment, and know what you will do if it fails. Paper handouts are usually best given out after a talk, since listeners will read them instead of listening if they arrive at the start, unless the audience needs them to follow along.
Visuals everyone can use
Some listeners cannot see a screen well, and some have color vision differences; red and green are the pair most often confused, which matters for charts that use them to mean good and bad. Describe important visuals aloud, as in "this line climbs steeply after 2010," so that no one depends on sight alone. Caption video where possible, and use labels and patterns as well as color.
Worked example: the water slide
Imagine a parent preparing a five-minute talk for a county school board about lead in school drinking fountains. The first draft has one slide titled "Water Testing Results," holding a table of forty numbers and nine bullet points. The plan is to put it up and talk through it.
Apply the principles. What does the board need to see? Not forty numbers but one fact: how many schools exceeded the state limit, and which ones. The redesign uses three slides. The first has the headline "Seven of twelve schools tested above the state limit" over a simple bar chart, with those seven bars in a dark color and the rest in light gray, and a line marking the limit. The second shows a map of the district with the seven schools marked. The third states the request: "Replace the fountains at these seven schools before spring."
The forty-number table goes into a printed handout given to board members afterward, so anyone can check the data. During the talk, the parent describes each chart in a sentence, blanks the screen between points, and speaks to the board members, not to the slides. The evidence is the same. Now the board can actually see it.