Perception Understanding

(Mental Model)

Information

224 Chapter 8: Displays

memory, or more permanently in long-term memory, and used for diagnosis and decision making. The present chapter now focuses on displays, which are typically human-made artifacts designed to support the perception of relevant system vari-ables and facilitate the further processing of that information (Fig. 8.1). The speedometer on the car, the warning tone in the aircraft, the message on the phone-based menu system, the instruction panel on the automatic teller, the steam gauge in the industrial plant, or the fine print on the application form are all examples of displays, in various modalities, conveying various forms of infor-mation used in various tasks. In this chapter we will first describe 13 key human factors principles in the design of displays. Then the bulk of the chapter will focus on describing different categories of tasks for which displays are intended, illus-trating as we go various application of the 13 principles.

WAYS OP CLASSIFYING DISPLAYS It is possible to classify displays along at least three different dimensions. First, there are differences in the physical implementation of the display device. One may think of these as the physical tools that the designer has to work with in creating a display. For example, a display may use color or monochrome; it may use the vi-sual or auditory modality, a 3-D display may use stereo, and so on Several of these tools are listed in Table 8.1.

All of these tools will be mentioned at various points in the chapter. However, before fabricating a display the designer must first ascertain the nature of the task that display is intended to support: Is it navigating, controlling, decision making, learning, and so forth? Our chapter will, in fact, be organized around displays to support these various tasks, as we see how different display tools may be optimally

FIGURE 8.1 The figure illustrates key components in display design. A system generates information, some of which must be processed by the operator to perform a task That necessary information (but only that information) is presented on a display and formatted according to principles in such a way that it will support perception and understanding. Often this understanding is facilitated by an accurate mental model of the displayed process.

Thirteen Principles of Display Design 225

TABLE 8.1 Physical bals and Variables That the Display Detiper May Manipulate • Location

in XY space superimposed (the head-up display)

• Color (color versus monochrome) • Dimensionality

planar versus perspective mono versus stereo

• Motion what moves how it moves

• Intensity; what is bright, what is dim • Coding

what physical dimensions (i.e., color, size, shape) are assigned to variables analog versus digital, coding analog and pictures versus text

• Modality: vision versus audition • What to display: information analysis

suited for different tasks. However, we note here that defining the task is only a first step. Once the task and its goals are identified (e.g., designing a map to help a dri-ver navigate from point A to point B), one must carry out a detailed information analysis that will identify what the operator needs to know to carry out the task. Where such knowledge does not already exist "in the head of the user" in the form of skill, it is a likely candidate for the display.

Finally, and most important, the reason no single display tool is best suited for all tasks is because of characteristics of the human user who must perform those tasks. For example, a digital display may be appropriate if the task requires precise reading of the exact value of an indicator, but because of the way our visual sys-tem works, the same display is not good for assessing at a quick glance the ap-proximate rate of change and value of the indicator. As Figure 8.1 shows, the key mediating factor that determines the best mapping between the physical form of the display and the task requirements is a series of principles of human perception and information processing. These principles are grounded in the strengths and weaknesses of human perception and performance (Wickens, 1992; Boff, Kaufman, & Thomas, 1986), and it is through the careful application of these prin-ciples to the output of the information analysis that the best displays will emerge. We turn now to an overview of the principles.

RTEEN-PRINCIPLES OF DISMAY DEMON One of the basic tenants of human factors is that lists of longer than five or six items are not easily retained, unless they are provided with some organizational structure. To help retention of the otherwise daunting list of 13 principles of dis-play design presented below, we may then associate them into four distinct cate-gories: ( 1) those that directly reflect perceptual operations, (2) those that can be

(c) Redundancy Gain: The Traffic Light

(d) rity: Confusion

a x **** .• ■ ••••••• ■ ••

Position and hue are redundant

Figurf) y••.. ****** 1■ ••••

Altitude Attitude •

••••••••••••• ***** • ***** • *

226 Chapter 8: Dispi

traced to the concept of the mental model, (3) those that relate to human attention,

and (4) those that relate to human memory. Some of these principles have been

presented in previous chapters, (4, 5 and 6) and others will be discussed more fully

later in this chapter.

PfirC*PtUI P1nCIPI05 These are illustrated in Figure 8.2.

1. Avoid absolfr limits. As we noted in Chapter 4 and again in Chapter

5 when discussing alarm sounds, do not require the operator to judge the level of a

represented variable on the basis of a single sensory variable like color, size, or loud-

n • # # than five to seven possible levels. To require greater pre-

cisthn as in a color-coded map wt nine hues be to invite errors of judgment.

(a) Absolute Judgement;

If the light is am be' prced with caution.* Amber light is one of 6 possible hues

(b) Top-Down Processing; A Checklist

A should be on

B should be on

C should be on

D should be off

1100111.1110111.101111.111111111,

FIGURE 8.2 Some examples of four perceptual principles of display design, described in the text. (a)

Absolute judgment; (b) Top-down processing (a tendency to perceive as "D should be on");

(c) Redundancy gain; and (d) Similarity -4 confusion.

liirteen PrEnciples of Display Design 227

(4,

2. -ownT0424 ..ocies will erceive and tjpj si in accor- dance with what rice. If a sign is presented that is contrary to expectations, like the warning or alarm for an unlikely event, then more physical evidence of that signal must be presented to guarantee that it will be interpreted correctly.

3. Redundancy gain. When the same message is expressed more than once, it will be more likely to be interpreted correctly. This will be particularly true if the same message is presented in alternative physical forms (e.g., tone and voice, voice anAarint,the Itandnctes, .c.9.1or and slur); that is, redundancy is not simply the same as repetition. When alternative physical forms are used, there is a greater chance that the factors that might degrade one form (e.g., noise degrading an au-ditory message) will not d ade the other (e printed text).

4 DiscriMinability imila _age .; s _141 Similar appearing signals will be likely to be confused, either at the time they are perceived or after some delay if the signals must be retained in working memory before action is taken. What causes two signals to be similar is the mai() of similar features to dissimilar ones (Tversky, 1977). Thus, AJB648 is more similar to A113658 than is 48 similar to 58, even though in both cases only a single digit is different. Whereconfision could be serious, the designer should delete unnecessary similar features and highlight dissimilar (different) ones in order to create du* tinctiveness. Note, for example, the high degree of confusabil-ity of the two captions in Figure 8.2d. In Figure 4.11 we illustrated another exam-ples of the danger of similarity and confusion in visual information.

Mental Model MultiOlee When operators perceive a display, they often interpret what the display looks like and how it moves in terms of their expectations or mental model of the system being displayed, a concept that was discussed in Chapter 6 (Norman, 1988, Johnson-Laird, 1983; Gentner & Stevens, 1983). The information presented to our system monitor in the opening story was not consistent with the mental model of the operator. Hence, it is appropriate for the format of the display to capture aspects of that mental model, based on user's experience of the system whose information is being displayed. Principles 5, 6, and 7 illustrate how this can be achieved.

5.Principle of pictorial realism (Roscoe, 1968).A display should look like (i.e., be a pictueraitabirthalitzpr_ssegil Thus ifwe think of temperature as having a hjit_a_rid lowyalug, a thermometer should be oriented vertically. If the display contains multiple elements, then these elements can be configured in a manner that looks like how they are configured in the environment that is repre-sented (or how the operator conceptualizes that environment). In this instance we can define a variant of the principle of pictorial realism as the principle of config-teal displays (Sanderson et al., 1989).

6.P!:ithe moving art (Roscoe, 1968). The moving element(s) of any display of dynamic 7"---mation should move in a spatial pattern and direction that is compatible with the user's mental model of how the represented element moves. Thus, if a pilot thinks that the aircnift moves upward when altitude is

1C 5

228 Chapter 8: Displays

gained, the moving element on an altimeter should also move upward with in-creasing altitude.

7.Ecolo *cal in design. Collectively, adherence to the principle of picto- rial realism and the princip e of the moving part can create displays that have a close correspondence with the environment that is being diiplayed. Because of this adherence to the ecology of the displayed world, these types of displays have been recently referred to as Ecological Interfaces (Vicente & Rasmussen, 1992; Bennett, Toms, & Woods, 1993; Rasmussen, Pejtersen, & Goodstein, 1995).

Principles Based on MUM:kin Complex multielement displays require three components of attention to process ( Parasuraman, )avies, & Beatty, 1984). Selective attention may be necessary to choose the displayed information sources necessary for a given task. cured at-tention allows those sources to be perceived without distraction from neighboring sources, and 4iyjde attention may allow parallel processing of two (or more) sources of information if a task requires it. All four of the attentional principles de-scribed below characterize ways of capitalizing on attentional strengths or mini-mizing their weaknesses.

8. Minimizrmation access cos!. There is typically a cost in time or effort to "move" selective attention from one display location to another to access in-formation. Our display monitor in the opening story wasted valuable time going from one page to the next in the book and visually scanning from there to the in-strument panel. The information access cost may also include the time to key through a computer menu structure to find the correct "page" Thus, good designs will be those that can minimize the net cost by keeping frequently accessed sources in such a location that the cost of traveling between them is small. This principle was not supported in the maintenance manual in the episode at the beginning of the chapter. We discuss it again in Chapter 10.

9. Proximity compatibility rin le (Wickens & Carswell, 1995). Sometimes two or more sources of information are related to the same task and must be men-tally integrated to complete the task (e.g., ajudi line and ittIsLend or the...Elam lay-out and the warabignliatemmskiLl san ip_sir,,9211Sory); that is, divided attention between the two sources for the one task is desirable. These information sources are said to have dose "mental proximity." As described in principle 8, good display design should then provide them with close "display proximity" by display-ing them close together so that their information access cost will be low (Wickens & Carswell, 1995). However, close display proximity can also be obtainecjAy_dis-pla*g them in a common color by linking, them er with lines, or br,:_con-

pattern,.asTucu in ptincipks 7 above (Fig. 8.3a). But, as Figure 8.3b shows, too much close display proximity is not always good, partic-ularly if one of the elements must be the subject of focused attention. In this case of focused attention, close proximity may be harmful, and it is better for the sources to be more separated. The "low mental proximity" of the focused attention task is

Thirteen Principles of Display Design 229

40 VS, 40 0

VS. IL 7.2

0 0

vs.

(a) (b)

FIGURE 8.3 The proximity compatibility 'pistil*. (a) Five examples of "close" display proximity (on the left) that will be helpful for tasks requiring integration of information in the two sources shown. (b) Two examples of close proximity that will hurt the ability to focus on one indicator and ignore the other.

then best served by the "low display proximity" of separation. Thus, the two types of proximity, display and mental, are "compatibly related."

10.Princip le of multiple resources. As we noted in Chapter 6, sometimes pro-cessing oT a lot of in ormation can be facilitated by dividing that information across resources—presenting visual and auditory information concurrently, for example—rather than presenting all information visualFor all auditorily.

Memory Principles As we learned in Chapter 6, human memory is vulnerable. Working memory is vulnerable because of its limited capacity: We can only keep a small iumber of "mental balls" in the air at one time and so, for example, may easily forget the phone number before we have had a chance to dial it or write it down. Our oper-ator in the opening story had a hard time remembering information on one page of the manual while he was accessing or reading the other. Our long-term mem-ory is vulnerable because we forget certain things or, sometimes, because we re-member other things too well and persist in doing them when we should not. The final three principles address different aspects of these memory processes.

11.Diruzild .gAikt,rgaiding. Humans are not very good at predicting the future. In large part this limitation results because prediction is a difficult cogni-tive task, depending heavily on working memory. We need to think about current conditions, possible future conditions, and the rules by which the former may gen-erate the latter. When our mental resources are consumed with other tasks, pre-diction falls apart, and we become reactive, responding to what has already happened, rather than proactive, responding in anticipation of the future. Since

220 Chapter 8: Displays

proactive behavior is usually more effective than reactive, it stands to reason that displays that can explicitly predict what will (or is likely to) happen will generally be quite effective in human performance. A predictive display removes a resource demanding cognitive task, and replaces it with a simpler perceptual one. Figure 8.4 shows some examples of effective predictor displays.

12.Princi k o * Id. D. Nonnan (1988) has written elo- quently a s out two kinds of knowledge that support people's interactions with systems. Knowledge in the head, on the one hand, is what we typically think of when we think of knowledge. It is remembering what needs to be done when, which is a pretty good memory system for routine tasks but not so good for tasks that are complex, recently learned, or poorly explained. Knowledge in the world, on the other hand, involves placing explicit visible reminders or state-ments of what is to be done at the time and place that will trigger the appro-priate action. A pilot's checklist is a good example of knowledge in the world (Degani & Wiener, 1990). So too wolaksas_mp. -___utet n that, at each step,

provides the ussryith a.mpJt.ehst ofaktb.Moibluptiotts.szthatthg.42- proppate actionsaiLbe easiLy-recognize.d. Clearly, when knowledge is put in the world, it will not be forgotten, whereas when it must be accessed only in the head, forgetting is possible.

Of course sometimes too much knowledge in the world can lead to clutter problems, and systems designed to rely on knowledge in the head are not neces-sarily bad. For example in using computer systems, experts might like to be able to retrieve information by direct commands (knowledge in the head) rather than stepping through a menu (knowledge in the world) (see Chapter 15). Good design must balance the two kinds of knowledge.

13.Pins icy. When our memory works too well, it may continue to trigger actions that are no longer appropriate, and this is a pretty instinctive and automatic human tendency. Old habits die hard. Because there is no way of avoid-ing this, good designs should try to accept it and "go with the flow" by designing displays in a manner that is consistent with other displays that the user may be per-ceiving concurrently (e.g., a user alternating between two computer systems) or may have perceived in the recent past. Hence, the old habits from those other dis-plays will transfer positively to support processing of the new displays. Thus, color coding should be consistent across a set of displays so that, for =air pie, red always meanss the a_mking. As another example, a set of different display panels should be consistently organized, thus, through learning, reducing information access cost each time a new set is encountered.

Conclusion In concluding our discussion of principles, it should be immediately apparent that principles sometimes conflict or "collide." Making all displays consistent, for ex-ample, may sometimes cause certain displays to be less compatible than others, just as making all displays optimally compatible may make them inconsistent. Correspondingly, putting too much knowledge in the world or incorporating too much redundancy can create very cluttered displays, thereby making focused at-tention more difficult. Alas, there is no easy solution to say what principles are

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