Coding refers to distinctive means for highlighting different categories of displayed data for user attention.
2.6/1 Highlighting Critical Data
Provide distinctive coding to highlight important display items requiring user attention, particularly when those items are displayed infrequently.
"Example" Such items might include recently changed data, or discrepant data exceeding acceptable limits, or data failing to meet some other defined criteria.
"Comment" "Highlight" is used here in its general sense, meaning to emphasize or make prominent, and is not restricted to any particular method of display coding such as brightening or inverse video.
"Comment" Highlighting is most effective when used sparingly, adding emphasis to a display which is relatively uniform in appearance except for just a few highlighted items.
"Comment" For some purposes position coding, i.e., displaying important items consistently in a particular location, might be a sufficient means of highlighting, as when an error message appears in a space otherwise left blank. But auxiliary codes may still be needed to highlight important items, even if they are positioned consistently.
"Reference" EG 2.1.3 2.3.12 MS 5.15.3.3.1
Up to contents2.6/2 + Removing Highlighting
If highlighting is used to emphasize important display items, remove such highlighting when it no longer has meaning.
"Example" If highlighting identifies an error, remove that highlighting when the error is corrected.
Up to contents2.6/3 Coding by Data Category
Provide display coding in applications where a user must distinguish rapidly among different categories of displayed data, particularly when those data are distributed in an irregular way on the display.
Up to contents2.6/4 Meaningful Codes
Adopt meaningful or familiar codes, rather than arbitrary codes.
"Example" A three-letter mnemonic code (DIR = directory) is easier to remember than a three-digit numeric code.
"Comment" An arbitrary code, such as a Social Security Number, may eventually become familiar through frequent use.
"Reference" BB 3.6.2 MS 5.15.3.3.1
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2.6/5 + Familiar Coding Conventions
Adopt codes for display (and entry) that conform with accepted abbreviations and general user expectations.
"Example" Use M for "male", F for "female", rather than arbitrary digits 1 and 2. In color coding, use red for danger.
"Comment" If in doubt, an interface designer can survey prospective users to determine just what their expectations may be.
"See also" 2.6/32 4.0/14
Up to contents2.6/6 Definition of Display Codes
When codes are assigned special meaning in a display, provide a definition at the bottom of the display that replicates the code being defined.
"Example" The legend on a map is a common example.
"Example" For a color code each definition should be displayed in its appropriate color, as | RED = hostile | displayed in red.
"Reference" BB 7.6.1 "See also" 4.4/21
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2.6/7 Consistent Coding Across Displays
Assign consistent meanings to symbols and other codes, from one display to another.
"Comment" When coding is not consistent, the user's task of display interpretation may be made more difficult than if no auxiliary coding were used at all.
"Reference" BB 3.6.1 7.6.2 MS 5.15.3.3.1
"See also" 2.0/14 4.0/13
Up to contents2.6/8 Alphanumeric Coding
Consider alphanumeric characters for auxiliary coding in display applications such as graphics where the basic data presentation is not already alphanumeric.
"Comment" Select alphanumeric codes that are visually distinct for visual displays, and phonetically distinct for auditory displays (or in any application where displayed codes must be spoken).
"Reference" EG Table 1
"See also" 1.0/18
Up to contents2.6/9 + Consistent Case in Alphabetic Coding
For alphabetic codes display all letters consistently either in upper case or in lower case.
"Comment" For data display, upper case labels may be somewhat more legible. For data entry, computer logic should not distinguish between upper and lower case codes, because users find it hard to remember any such distinction.
"Reference" BB 1.3.3
"See also" 1.0/27
Up to contents2.6/10 + Combining Letters and Numbers
When codes combine both letters and numbers, group letters together and numbers together rather than interspersing letters with numbers.
"Example" Letter-letter-number ("HW5") will be read and remembered somewhat more accurately than letter-number-letter ("H5W").
"Comment" Unfortunately, there are common instances in which this practice has not been followed, such as the coding of British and Canadian postal zones.
"Reference" BB 1.5.1 MS 5.15.3.5.8
Up to contents2.6/11 + Short Codes
When arbitrary codes must be remembered by the user, ensure that they are no longer than four or five characters.
"Exception" When a code is meaningful, such as a mnemonic abbreviation or a word, it can be longer.
"Reference" BB 1.5.2 MS 5.15.3.5.8
"See also" 1.0/15
Up to contents2.6/12 Special Symbols
Consider using special symbols, such as asterisks, arrows, etc., to draw attention to selected items in alphanumeric displays.
"See also" 4.3/19
Up to contents2.6/13 + Consistent Use of Special Symbols
When using special symbols to signal critical conditions, use them only for that purpose.
"See also" 2.6/7
Up to contents2.6/14 + Markers Close to Words Marked
When a special symbol is used to mark a word, separate the symbol from the beginning of the word by a space.
"Comment" A symbol immediately adjacent to the beginning of a word will impair legibility.
"Reference" Noyes 1980
Up to contents2.6/15 Shape Coding
Consider coding with geometric shapes to help users discriminate different categories of data on graphic displays.
"Comment" Approximately 15 different shapes can be distinguished readily. If that "alphabet" is too small, it may be possible to use component shapes in combination, as in some military symbol codes.
"Reference" EG Table 1
Up to contents2.6/16 + Establishing Standards for Shape Coding
When shape coding is used, assign codes based on established standards or conventional meanings.
"Example" A number of international, national, and organizational standards for shape coding exist, and those should be followed where they apply.
"Comment" Although shape codes can often be mnemonic in form, their interpretation will generally rely on learned association as well as immediate perception. Existing user standards must be taken into account by the display designer.
"Reference" MS 5.15.3.3.6
"See also" 2.6/7 4.0/13
Up to contents2.6/17 Line Coding
For graphic displays, consider using auxiliary methods of line coding, including variation in line type (e.g., solid, dashed, dotted) and line width ("boldness").
"Comment" Perhaps three or four line types might be readily distinguished, and two or three line widths.
"Reference" EG 2.3
"See also" 2.4.3/6
Up to contents2.6/18 + Underlining for Emphasis
When a line is added simply to mark or emphasize a displayed item, place it under the designated item.
"Comment" A consistent convention is needed to prevent ambiguity in the coding of vertically arrayed items.
"Comment" For words composed from the Roman alphabet, underlining probably detracts from legibility less than would "overlining".
"Reference" MS 5.15.3.3.5
Up to contents2.6/19 + Coding by Line Length
Consider using codes with lines of varying length for applications involving spatial categorization in a single dimension.
"Example" The length of a displayed vector might be used to indicate speed.
"Comment" Perhaps four lengths can be reliably distinguished in practical use. Long lines will add clutter to a display, but may be useful in special applications.
"Reference" EG Table 1
Up to contents2.6/20 + Coding by Line Direction
Consider using codes with lines of varying direction for applications involving spatial categorization in two dimensions.
"Example" The angle of a displayed vector might be used to indicate direction, i.e., heading or bearing.
"Comment" Users can make fairly accurate estimates of angles for lines displayed at ten-degree intervals.
"Reference" Smith 1962a
"See also" 1.2
Up to contents2.6/21 Limited Use of Size Coding
Consider size coding, i.e., varying the size of displayed alphanumerics and other symbols, only for applications where displays are not crowded.
"Comment" Perhaps as many as five sizes might be used for data categorization, but two or three will probably prove the practical limit.
"Reference" EG Table 1 MS 5.15.3.3.6
Up to contents2.6/22 + Adequate Differences in Size
For size coding, a larger symbol should be at least 1.5 times the height of the next smaller symbol.
"Comment" An increase in symbol height must usually be accompanied by a proportional increase in width to preserve a constant aspect ratio and so facilitate symbol recognition.
"Reference" MS 5.15.3.3.6
Up to contents2.6/23 Limited Use of Brightness Coding
Consider coding by differences in brightness for applications that only require discrimination between two categories of displayed items; i.e., treat brightness as a two-valued code, bright and dim.
"Example" A data form might display dim labels and bright data items, in order to facilitate data scanning.
"Comment" Perhaps as many as four brightness levels might be used, but at some risk of reduced legibility for the dimmer items. It will be safer to reserve brightness as a two-valued code, particularly for displays whose overall intensity can be adjusted at the terminal by a user.
"Reference" EG 2.1.4 Table 1
"See also" 1.4/16
Up to contents2.6/24 + Brightness Inversion
When a capability for brightness inversion is available (so-called "reverse video"), where dark characters on a bright background can be changed under computer control to bright on dark, or vice versa, consider brightness inversion for highlighting critical items that require user attention.
"Comment" Brightness inversion is obviously limited to use as a two-valued code, i.e., a displayed item is either shown with standard or inverted brightness. If brightness inversion is used for alerting purposes, as recommended here, it should be reserved consistently for that purpose, and not be used for general highlighting.
"Reference" PR 3.3.4
"See also" 2.6/7
Up to contents2.6/25 Color Coding for Relative Values
When the relative rather than the absolute values of a variable are important, consider displaying gradual color changes as a tonal code to show the relative values of a single variable.
"Example" In displaying ocean depth, a saturated blue might be used to show the deepest point, with gradually desaturated blues to show decreasing depth.
"Comment" A gradual change in color might be achieved by varying saturation, starting with a saturated hue and gradually adding white. Or the change might be in intensity, starting with an intense hue and gradually adding black. Or the change might be in hue, moving gradually from red through orange, yellow, green, etc.
"Comment" People can easily make relative color comparisons when colors are displayed simultaneously. However, people find it more difficult to make absolute color judgments. Part of the problem is color naming. A particular blue-green hue might be named "green" by one person but "blue" by another. In the example above, a user could not be expected to associate any particular shade of blue with a specific ocean depth.
"Comment" Gradual color changes should not be used when absolute values are important, or to code data into discrete categories. As an example, in displaying revenues to determine the point at which a company becomes profitable, it would be more appropriate to use two distinctly different colors, such as black and red, with the color changing at the point of profitability.
"See also" 2.4.8/7
Up to contents2.6/26 Color Coding for Data Categories
When a user must distinguish rapidly among several discrete categories of data, particularly when data items are dispersed on a display, consider using a unique color to display the data in each category.
"Example" Different colors might be used effectively in a situation display to distinguish friendly, unknown, and hostile aircraft tracks, or alternatively to distinguish among aircraft in different altitude zones.
"Comment" Color is a good auxiliary code, where a multicolor display capability is available. A color code can be overlaid directly on alphanumerics and other symbols without significantly obscuring them. Color coding permits rapid scanning and perception of patterns and relationships among dispersed data items.
"Comment" Perhaps as many as 11 different colors might be reliably distinguished, or even more for trained observers. As a practical matter, however, it will prove safer to use no more than five different colors for category coding.
"Comment" With some display equipment now providing millions of different colors, designers may be tempted to exploit that capability by using many different colors for coding. The capability to display many colors may be useful for depicting complex objects, and for providing tonal codes to show the relative values of a single variable. However, such a capability is not useful for coding discrete categories, except that it may allow a designer to select more carefully the particular colors to be used as codes.
"Reference" BB 7.2 EG Table 1 MS 5.15.3.3.7 Smith 1962b Smith 1963a Smith Thomas 1964 Smith Farquhar Thomas 1965
Up to contents2.6/27 + Easily Discriminable Colors
When selecting colors for coding discrete categories of data, ensure that those colors are easily discriminable.
"Example" On a light background, a good choice of colors might be red, dark yellow, green, blue and black; on a dark background, good colors might be a somewhat desaturated red, green and blue, plus yellow and white.
"Comment" The harder it is for a user to identify a displayed color, the less useful will be the color code. If many colors are used, colors will be closer in hue and harder to discriminate. If color coding is applied to symbols that subtend small visual angles, which makes color perception difficult, there will be a special need to limit the number of colors used.
"Comment" Varying saturation and intensity in addition to hue may increase the discriminability of colors. For instance, on a dark background a designer might select a saturated yellow but a desaturated red.
"Comment" Colors selected for coding should be tested with users to ensure that they are easily discriminable. Testing should be conducted under realistic conditions, since such factors as display type and ambient room lighting will affect color perception. If colors will be used for displaying text, care should be taken to ensure that colored letters are legible as well as discriminable.
Up to contents2.6/28 + Conservative Use of Color
Employ color coding conservatively, using relatively few colors and only to designate critical categories of displayed data.
"Comment" Casual, arbitrary use of colors on every display may cause displays to appear "busy" or cluttered. Casual use of color will also reduce the likelihood that significant color coding on particular displays will be interpreted appropriately and quickly by a user.
"Reference" BB 7.1
Up to contents2.6/29 + Adding Color to Formatted Displays
Add color coding after displays have already been designed as effectively as possible in a monochrome format.
"Comment" Do not use color coding in an attempt to compensate for poor display format. Redesign the display instead.
"Reference" BB 7.3
Up to contents2.6/30 + Redundant Color Coding
Make color coding redundant with some other display feature such as symbology; do not code only by color.
"Comment" Displayed data should provide necessary information even when viewed on a monochromatic display terminal or hard-copy printout, or when viewed by a user with defective color vision.
"Reference" BB 7.4 7.6.3 MS 5.15.3.3.7
Up to contents2.6/31 + Unique Assignment of Color Codes
When color coding is used, ensure that each color represents only one category of displayed data.
"Comment" Color will prove the dominant coding dimension on a display. If several different categories of data are displayed in red, say, they will have an unwanted visual coherence which may hinder proper assimilation of information by a user.
"Reference" BB 7.6.1 Smith Thomas 1964
Up to contents2.6/32 + Conventional Assignment of Color Codes
Choose colors for coding based on conventional associations with particular colors.
"Example" In a display of accounting data, negative numbers might be shown as red, corresponding to past use of red ink for that purpose.
"Example" Red is associated with danger in our society, and is an appropriate color for alarm conditions. Yellow is associated with caution, and might be used for alerting messages or to denote changed data. Green is associated with normal "go ahead" conditions, and might be used for routine data display. White is a color with neutral association, which might be used for general data display purposes.
"Comment" Other associations can be learned by a user if color coding is applied consistently.
"Reference" BB 7.7.1 7.7.2 7.7.3 MS 5.15.4.6.1.f
"See also" 2.6/5 4.0/13 4.0/14 4.3/19
Up to contents2.6/33 + Brightness and Saturation to Draw Attention
Use brighter and/or more saturated colors when it is necessary to draw a user's attention to critical data.
"Comment" On some display equipment, designers can vary the intensity as well as the saturation for individual hues. On those displays, both intensity and saturation can be used to draw a user's attention to critical data.
"Comment" Although saturated and/or intense hues are useful for drawing a user's attention, their overuse will result in a display which is garish and difficult to view for long periods.
"Comment" When deciding the desired saturation of any given display color, consider the ambient lighting under which the display will be viewed. Colors that appear highly saturated in a darkened room will appear less saturated when viewed under high ambient illumination.
Up to contents2.6/34 + Saturated Blue for Background Color
Use saturated blue only for background features in a display, and not for critical data.
"Example" Saturated blue might be used for shading background areas in graphic displays, where its lower apparent brightness could possibly be of benefit. Or saturated blue might be used to display a background grid or familiar scale on a graphic display.
"Comment" The human eye is not equally sensitive to all colors, nor are its optics color-corrected. Blue symbols appear dimmer than others, and are more difficult to focus.
"Comment" If blue must be used for displayed data, use a desaturated blue or cyan in order to make the data more legible.
"Reference" BB 7.6 7.7.5 Weitzman 1985
Up to contents2.6/35 Blink Coding
Consider blink coding when a displayed item implies an urgent need for user attention.
"Comment" If used sparingly, blinking symbols are effective in calling a user's attention to displayed items of unusual significance. Blinking characters may have somewhat reduced legibility, and may cause visual fatigue if used too much.
"Comment" Perhaps three or four blink rates might be reliably distinguished, but it will probably prove safer to use blinking as a two-level code, blinking versus nonblinking.
"Reference" BB 1.10.2 1.10.3 EG Table 1 MS 5.15.3.3.2 Smith Goodwin 1971b Smith Goodwin 1972
"See also" 4.3/19
Up to contents2.6/36 + Blinking Marker Symbols
When a user must read a displayed item that is blink coded, consider adding an extra symbol such as an asterisk to mark the item, and then blinking that marker symbol rather than blinking the item itself.
"Comment" This practice will draw attention to an item without detracting from its legibility.
"Reference" BB 1.10.3 Smith Goodwin 1971b
Up to contents2.6/37 + Optimal Blink Rate
When blink coding is used, select a blink rate in the range from 2 to 5 Hz, with a minimum duty cycle (ON interval) of 50 percent.
"Comment" Although equal ON and OFF intervals are often specified, an effective code can probably be provided even when the OFF interval is considerably shorter than the ON (a "wink" rather than a blink), as in occulting lights used for Navy signaling.
"Reference" BB 1.10.4 MS 5.15.3.3.2
Up to contents2.6/38 Coding with Texture, Focus, Motion
Consider other visual coding dimensions for special display applications, including variation in texture, focus, and motion.
"Comment" Texture can be useful for area coding in graphic displays. Only two levels of focus are feasible, clear and blurred, with the risk that blurred items will be illegible. Perhaps 2 to 10 degrees of motion might be distinguished, in applications where motion is an appropriate and feasible means of display coding.
"Reference" EG 2.3
"See also" 2.4
Up to contents2.6/39 Auditory Coding
Consider auditory displays as a means of supplementing visual display, or as an alternative means of data output in applications where visual displays are not feasible.
"Example" Auditory signals may be helpful in alerting users to critical changes in a visual display.
"Example" Auditory output might be used to permit telephone access to computer-stored data.
"Exception" Auditory display may be impractical in situations where high ambient noise prevents accurate listening.
"Comment" As compared with visual displays, an auditory display offers a potential advantage in attracting a user's attention; a user does not have to "listen at" an auditory display in order to hear it. On the other hand, auditory displays suffer from a number of comparative disadvantages. Auditory displays generally do not offer as great a range of coding options. Auditory displays do not permit easy scanning to discern critical data items, or items that may have been missed at first listening. For human listeners with normal vision, auditory displays do not provide a natural representation of spatial relations.
"Reference" MS 5.15.3.9.1
"See also" 1.3/30 4.0/26 4.0/27 4.0/28 4.0/29
Up to contents2.6/40 + Distinctive Auditory Coding
For auditory displays, employ distinctive sounds to code items requiring special user attention.
"Example" A variety of signals may be available, including sirens, bells, chimes, buzzers, and tones of different frequency.
"Comment" Tones may be presented in sequence to enlarge the signal repertoire.
"Reference" Smith Goodwin 1970
"See also" 4.3/19
Up to contents2.6/41 + Voice Coding
For auditory displays with voice output, consider using different voices to distinguish different categories of data.
"Comment" At least two voices, male and female, could be readily distinguished, and perhaps more depending upon fidelity of auditory output, and listening conditions.
"Reference" Simpson McCauley Roland Ruth Williges 1985 Smith Goodwin 1970
Up to contents2.6/42 + Coding Synthesized Voice Alarms
If computer-generated speech output is used for auditory display, add a special alerting signal to introduce voice alarms and warning messages in order to distinguish them from routine advisory messages.
"Reference" Hakkinen Williges 1984 Simpson Williams 1980
"See also" 4.0/26 4.0/27 4.0/28 4.0/29
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