Aids to Audience Decision Making 149High Contrast Between Type and Background
The greatest legibility is achieved when there is maximum contrast between the color of the type
and the color of the background the type is printed or projected on.^7 Reading times are slower for
color combinations with less tonal contrast between type and background (e.g., black type on a
dark blue background) due to the reader’s need to increase their eye-fi xation frequency and pause
duration. When the tonal shade of the background color is greater than 10%, readers have trouble
discerning black type on it.^8
Reversing the color of type (i.e., placing light-colored type against a dark background) canaffect legibility, too. When the color of the type is reversed, as it is in many slide shows, the dark
background makes the light type appear thinner than it actually is.^9 If the type color is reversed over
long passages, reading speed may be reduced by up to 15%.^10
Eleven-Point Type Size for Documents, 24 for Slides
The size of the type also affects how easily the audience can perceive letters and words. The
most legible type size for a variety of typefaces, or fonts, in documents ranges from 9 points to
12 points.^11 Audiences read fastest when type sizes are between 9 and 12 points and tend to rate
11-point type as most legible.^12
Slightly smaller type sizes may also be legible in some situations. For example, a study ofnewspaper print fi nds little difference in legibility for type sizes between 7.5 and 9 points.^13
Similarly, a study of instructional texts fi nds that type as small as 8 points is still legible for many
readers.^14 Most studies agree that type sizes below 6 points are very hard to read.^15 However,
one study fi nds the range of type sizes over which it is possible to read text at maximum speed
extends all the way from 4 to 40 points as long as the reader is at the standard distance of 16
inches from the page.^16
Twenty-four points is often the minimum type size recommended for the typical slide presenta-tion. However, type as small as 16 points on slides may be legible to many viewers depending on
the size of the screen and their distance from it.
Lower-Case Letters
Both headings and text are less legible when typed in all upper-case letters than when typed in
both upper- and lower-case letters.^17 Because the outline of a capital letter is not as distinctive as
the outline of a lower-case letter, reading speed is optimal when both upper- and lower-case letters
are used and the use of all caps is avoided.^18 Words and phrases in all capital letters, such as those
in Figure 4.1, take about 12% longer to read.^19 Headlines in all caps take between 13% and 20%
longer to read.^20
Italic type can also slow readers down.^21 Continuous prose in italic type takes readers about 5%longer to read than continuous prose in nonitalic type.^22
Legible Typeface
Typeface, or font, can also affect legibility. A series of 11 studies of the effects of typography in
which more than 11,000 readers took part fi nds readers prefer the most legible typefaces and their
preferences for typefaces are highly correlated with reading speed.^23 Serif typefaces with their