Handbook of Industrial Automation - Richard L. Shell and Ernest L. Hall Part 16 pdf

In the context of interface design, this task analysis methodology is used for checking if the information ¯ows identi®ed during the initial task analysis andtask description, is adequat

While many di€erent task analysis techniques exist

to suit the di€erent design requirements in systems, our

primary focus here is on techniques that help in

design-ing the interface The key issues involved in designdesign-ing a

human interface with automated equipment are

asses-sing what will be needed to do a job (the types of

information that human operators will need to

under-stand the current system status and requirements; the

types of output that human operators will have to

make to control the system), and deciding how this

will be provided Table 3provides a summary of the

important activities involved in the process of interface

design and the corresponding task analysis technique

to use in designing this activity We present brief maries of each of these techniques in the followingsections The reader should refer to Kirwan andAinsworth [6], or other articles on task analysis, for adetailed discussion of the di€erent task analysis tech-niques

sum-Hierarchical Task Analysis This enables the analyst

to describe tasks in terms of operations performed bythe human operator to attain speci®c goals, and

``plans'' or ``statements of conditions'' when each of

a set of operations has to be carried out to attain anoperating goal Goals are de®ned as ``desired states of

Table 2 Checklist for Task Analysis Activities

For every important task:

Intrinsics of the task What is the task?

What are the inputs and outputs for the task?

What is the transformation process (inputs to outputs)?

What are the operational procedures?

What are the operational patterns?

What are the decision points?

What problems need solving?

What planning is needed?

What is the terminology used for task speci®cation?

What is the equipment used?

Task dependency and What are the dependency relationships between the current task and the other tasks and systems?criticality What are the concurrently occurring e€ects?

What is the criticality of the task?

Current user problems What are the current user problems in performing this task?

Performance criteria What is the speed?

What is the accuracy?

What is the qualityTask criteria What is the sequence of actions?

What is the frequency of actions?

What is the importance of actions?

What are the functional relationships between actions?

What is the availability of functions?

What is the ¯exibility of operations?

User discretion Can the user control or determine pace?

Can the user control or determine priority?

Can the user control or determine procedure?

Task demands What are the physical demands?

What are the perceptual demands?

What are the cognitive demands?

What are the envirornmental demands?

What are the health and safety requirements?

Adapted from Ref 5.

tent, and actions and feedback required of the

opera-tor Once a broad list of activities and the tasks

involved have been generated using either hierarchical

task analysis or activity sampling, task decomposition

can be used to systematically expand on the task

descriptions The various steps involved in task

decom-position are presented inFig 6

Decision±Action Diagram This is one of the most

commonly used tools for decision making Figure 7

is an example of a decision±action diagram [7] The

decision±action diagram sequentially proceeds through

a series of questions (representing decisions) and

pos-sible yes/no answers (representing actions that can be

taken) The questions are represented as diamonds,

and the possible alternatives are labeled on the exit

lines from the diamond A thorough knowledge of

the system components, and the possible outcomes of

making decisions about system components is essential

for constructing complete and representative decision±

action diagrams

Table-Top Analysis As the name implies, this is a

technique through which experts knowledgeable

about a system discuss speci®c system characteristics

In the context of interface design, this task analysis

methodology is used for checking if the information

¯ows identi®ed during the initial task analysis andtask description, is adequate for successful task com-pletion Table-top analysis, hence, typically follows theinitial hierarchical or other forms of task analysiswhich yield task descriptions, and provides informa-tion input for the decomposition of the tasks A num-ber of group discussion techniques exist in practice,including the Delphi method, the group consensusapproach, the nominal group technique, etc., for con-ducting table-top analysis, each with its own meritsand demerits

Walk-Through/Talk-Through Analysis These lyses involve operators and other individuals havingoperational experience with the system, walking andtalking the analyst through observable task com-ponents of a system in real time Walk-through isnormally achieved in a completely operational system

ana-or in a simulated setting ana-or even in a mock-up setting.Talk-through can be performed even without a simula-tion of the systemÐthe only requirements are drawingand other system speci®c documentation to enable theanalysts to set system and task boundaries while per-forming the talk-through analysis For more informa-tion on walk-through and talk-through analyses, refer

to Meister [8]

Figure 5 Activities involved in activity sampling

760 Mital and Pennathur

Figure 7 (continued)

Human Interfaces for Automated Systems 761

Figure 7 (continued)

acterization exercise described in Sec 1.3.1.1; the

checklist used for user characterization can be used

for person speci®cation also One of the widely used

techniques for person speci®cation is the position

ana-lysis questionnaire Broadly, position anaana-lysis

ques-tionnaires require the operator to identify for their

speci®ed tasks andjobs, the information input, the

mental processes, the work output, the context of the

job, the relationship with other personnel in the

sys-tem, and any other relevant job characteristics Using

the responses from the operators, the skill content of

tasks and jobs can be determined, and can help in

designing personnel selection and training programs

to ensure optimal human±machine interaction

Ergonomics Checklists These checklists are generally

used to ascertain if a particular system meets

ergo-nomic principles and criteria Ergoergo-nomics checklists

can check for subjective or objective information and

can cover issues ranging from overall system design to

the design of individual equipment Checklists can also

range in detail from the broad ergonomic aspects to

the minute detail Table 4 provides an example of a

checklist for equipment operation A number of

other standard checklists have also been developed

by the ergonomics community Important among

these are the widely used and comprehensive set of

checklists for di€erent ergonomics issues by

Woodson [10,11], MIL-STD 1472C [12] which covers

equipment design (written primarily for military

equip-ment, but can be used as a guide to develop checklists),

EPRI NP-2360 [13] which is a checklist for

mainte-nance activities in any large-scale system,

NUREG-0700 [14] which is a comprehensive checklist for

con-trol room design, the HSE checklist [15] which deals

with industrial safety and human error, and the

numer-ous checklists for CRT displays and VDUs [16,17]

1.3.1.3 Characterization of the Situation

Apart from the user and the task variables that could

a€ect system performance, the external environment in

which the system functions can also in¯uence the

human±system interaction performance.Table 5

pro-vides a representative checklist for the most commonly

encountered situations for which the system analyst

must obtain answers, and attempt to provide for

these situations in design

1.3.2 Allocation of Functions

In designing the human±machine interface, once

com-prehensive information about the users and the

activ-ities/tasks these users will perform is known (throughthe use of tools presented in the earlier sections), thespeci®c activities and tasks need to be assigned either

to the humans or to the machines The allocation offunctions is a necessary ®rst step before any furtherdesign of the interface in the human±machine systemcan be carried out

The need for solving the function allocation blem directly stems from the need to decide the extent

pro-of automation pro-of manufacturing activities This is sobecause, in the present day manufacturing scenario,the decision to make is no longer whether or not toautomate functions in manufacturing, but to whatextent and how

The function allocation problem is perhaps as old asthe industrial revolution itself Fitts' list, conceived in

1951 (Table 6), was the ®rst major e€ort to resolve thefunction allocation problem

However, while Fitts' list provided fundamental andgeneric principles that researchers still follow forstudying function allocation problems, its failure toprovide quantitative criteria for function allocationresulted in its having little impact on engineeringdesign practices The development of practical andquantitative criteria for allocating functions is com-pounded by an important issue: unless one candescribe functions in engineering terms, it is impossible

to ascertain if a machine can perform the function;and, if one can describe human behavior in engineeringterms, it may be possible to design a machine to do thejob better (than the human) But many functions can-not be completely speci®ed in engineering (numerical)terms This implies that those functions that cannot bespeci®ed in engineering terms should be allocated tohumans, with the rest allocated to the machines Inaddition, for the practitioner, function allocation con-siderations have been limited due to the lack of [19]:

1 Systematic and step-by-step approaches to sion making during function allocation

deci-2 Systematic and concise data for addressingissues such as the capability and limitations ofhumans and automated equipment, and underwhat circumstances one option is preferableover the other

3 Methodology for symbiotic agents such as ufacturing engineers and ergonomists, to inte-grate human and machine behaviors

man-4 Uni®ed theory addressing domain issues such asroles, authorities, etc

5 Integration of other decision-making criteria(such as the economics of the situation) so

take over the function when circumstances

demand it

A number of approaches have been suggested in the

literature for solving the function allocation problem

Some of the promising approaches include function

allocation criteria based on speci®c performance

mea-sures (time required to complete tasks, for example)

[20±24], criteria based on comparison of capabilities

and limitations of humans with particular attention

given to knowledge, skills, and information sources

and channels [25±34] criteria based on economics

(allo-cate the function to the less expensive option),

[21,35,36], and criteria based on safety (to the human

operator in the system) [37±39]

Experiments with these approaches suggest that

functions that are well-proceduralized permitting

algo-rithmic analysis, and requiring little creative input, are

prime candidates for automation On the other hand,

functions requiring cognitive skills of a higher order,

such as design, planning, monitoring, exception

hand-ling, etc., are functions that are better performed by

humans The prime requirements for automating any

function are the availability of a model of the activities

necessary for that function, the ability to quantify that

model, and a clear understanding of the associated

control and information requirements Clearly, there

are some functions that should be performed by

machines because of:

1 Design accuracy and tolerance requirements

2 The nature of the activity is such that it cannot

be performed by humans

3 Speed and high production volume

require-ments

4 Size, force, weight, and volume requirement

5 Hazardous nature of the activity

Equally clearly, there are some activities that should be

performed by humans because of:

1 Information-acquisition and decision-making

needs

2 Higher level skill needs such as programming

3 Specialized manipulation, dexterity, and sensing

needs

4 Space limitations (e.g., work that must be done

in narrow and con®ned spaces)

5 Situations involving poor equipment reliability

or where equipment failure could prove

catastrophic

6 Activities for which technology is lacking

Mital et al [7] provide a generic methodology in theform of decision-making ¯owcharts for the systematicallocation of functions between humans and machines.Figure 7, presented earlier is a part of these ¯owcharts.These ¯owcharts are based on the requirements ofcomplex decision making, on a detailed safety analysis,and on a comprehensive economic analysis of the alter-natives These function allocation ¯owcharts are avail-able for di€erent manufacturing functions such asassembly, inspection, packaging, shipping, etc., andshould be consulted for a detailed analysis of the ques-tion of manufacturing function allocation

1.3.3 Information Presentation and Control1.3.3.1 The Scienti®c Basis for Information

Input and ProcessingReduced to a fundamental level, human interactionwith automation can be said to be dependent uponthe information processing ability of the human, andupon the exchange of information among the di€erentelements in a system Over the years, behavioral scien-tists have attempted to explain human informationprocessing through various conceptual models andtheories One such theory is the information theory[40] Information, according to information theory, isde®ned as the reduction of uncertainty Implicit inthis de®nition is the tenet that events that are highlycertain to occur provide little information; events thatare highly unlikely to occur, on the other hand, pro-vide more information Rather than emphasize theimportance of a message in de®ning information,information theory considers the probability of occur-rence of a certain event in determining if there is infor-mation worth considering For instance, the ``no-smoking'' sign that appears in airplanes before takeo€,while being an important message, does not conveymuch information due to the high likelihood of itsappearance every time an aircraft takes o€ On theother hand, according to information theory, messagesfrom the crew about emergency landing procedureswhen the plane is about to perform an emergency land-ing convey more information due to the small like-lihood of such an event Information is measured inbits (denoted by H) One bit is de®ned as the amount ofinformation required to decide between two equallylikely alternatives

When the di€erent alternatives all have the sameprobability, the amount of information (H) is given by

H ˆ log2N

where N is the number of alternatives For example,

when an event only has two alternatives associated

with it, and when the two alternatives are equally

likely, by the above equation, the amount of

informa-tion, in bits, is 1.0

When the alternatives are not equally likely (i.e., the

alternatives have di€erent probabilities of occurrence),

the information conveyed by an event is given by

hiˆ log2…1=pi†

where hiis the information associated with event i, and

pi is the probability of occurrence of event i

The average information (Hav† conveyed by a series

of events having di€erent probabilities is given by

HavˆXpi…log2…1=pi††

where pi is the probability of the event i

Just as a bit is the amount of information,

redun-dancy is the amount of reduction in information from

the maximum due to the unequal probabilities of

occurrence of events Redundancy is expressed as a

percentage, and is given by

% Redundancy ˆ …1 Hav=Hmax†  100

Information theory, while providing insight into

measuring information, has major limitations when

applied to human beings It is valid only for simple

situations which can split into units of information

and coded signals [41] It does not fully explain the

stimulus-carrying information in situations where

there are more than two alternatives, with di€erent

probabilities

The channel capacity theory, another theory

explain-ing information uptake by humans, is based on the

premise that human sense organs deliver a certain

quantity of information to the input end of a channel,

and that the output from the channel depends upon the

capacity of the channel It has been determined that if

the input is small, there is very little absorption of it by

the channel, but that if the input rises, it reaches the

threshold channel capacity, beyond which the output

from the channel is no longer a linear function of the

input [41] Experimental investigations have shown

that humans have a large channel capacity for

infor-mation conveyed to them through the spoken word

than through any other medium A vocabulary of

2500 words requires a channel capacity of 34 to 42

bits per second [42] Designers must keep in mind

that in this day and age of information technology,

the central nervous system of humans is subjected to

more information than the information channel can

handle, and that a considerable reduction in theamount of information must be carried out beforehumans process the information

In addition to theories such as the informationtheory and the channel capacity theory that explaininformation uptake, many conceptual models ofhuman information processing have been proposed

by researchers over the last four decades Figure 8shows one such fundamental model (most othermodels contain elements of this basic model) depictingthe stages involved in information processing [43] Thekey elements of the model are perception, memory,decision making, attention, response execution, andfeedback The following is a brief discussion of each

of these elements

Perception may involve detection (determiningwhether or not a signal is present), or identi®cationand detection (involving detection and classi®cation).The theory of signal detection [43±45] through the con-cept of noise in signals, attempts to explain the process

of perception and response to the perceived signals.Four possible outcomes are recognized in signal detec-tion theory: (1) hit (correctly concluding that there is asignal when there is one), (2) false alarm (concludingthat there is a signal when, in actuality, there is none),(3) miss (concluding that there is no signal when, inactuality, there is one and (4) correction rejection (cor-rectly concluding that there is no signal when there isnone) The fundamental postulate of signal detectiontheory is that humans tend to make decisions based oncriteria whose probabilities depend upon the probabil-ities of the outcomes above The probability of observ-ing a signal, and the costs and bene®ts associated withthe four possible outcomes above, determine theresponses of the human to the signal The resolution

of the human sensory activities (ability to separate thenoise distribution from the distribution of the signal)has also been found to a€ect the signal detection cap-ability of the human

Memory, in humans, has been conceptualized asconsisting of three processes, namely, sensory storage,working memory, and long-term memory [43].According to this conception, information from sen-sory storage must pass through working memorybefore it can be stored in long-term memory Sensorystorage refers to the short-term memory of the stimu-lus Two types of short-term memory storage are wellknownÐiconic storage associated with visual senses,and echoic storage associated with the auditory senses[46] Sensory storage or short-term memory has beenshown to be nearly automatic requiring no sustainedattention on the part of the human to retain it

tion is said to be divided (among the tasks) While

much of the theoretical base for explaining

perform-ance of tasks requiring divided attention is still

evol-ving [43,49], some guidelines for designing tasks that

require divided attention are available, and are

pro-vided in Table 10 When humans maintain attention

and remain vigilant to external stimuli over prolonged

periods of time, attention is said to be sustained

Nearly four decades of research in vigilance and

vigi-lance decrement [50±53] has provided guidelines for

improving performance in tasks requiring sustained

attention (Table 11)

In addition to the factors discussed above,

consider-able attention is being paid to the concept of mental

workload (which is but an extension of divided

atten-tion) Reviews of mental workload measurement

tech-niques are available [54±56], and should be consulted

for discussions of the methodologies involved in

men-tal workload assessment

1.3.3.2 The Scienti®c Basis for Human Control

of SystemsHumans respond to information and take controllingactions The controlling actions of the human aremediated through the motor system in the humanbody The human skeletal system, the muscles, andthe nervous system help bring into play motor skillsthat enable the human to respond to stimuli Motorskill is defned as ``ability to use the correct muscleswith the exact force necessary to perform the desiredresponse with proper sequence and timing'' [57] Inaddition, skilled performance requires adjusting tochanging environmental conditions, and acting con-sistently from situation to situation [58] A number

of di€erent types of human movements have beenrecognized in the literature [46] These include discretemovements (involving a single reaching movement to atarget that is stationary), repetitive movements (asingle movement is repeated), sequential movements

Table 7 Common Human Biases

Humans attach more importance to early information than subsequent information

Humans generally do not optimally extract information from sources

Humans do not optimally assess subjective odds of alternative scenarios

Humans have a tendency to become more con®dent in their decisions with more information, but do not necessarily becomemore accurate

Humans tend to seek more information than they can absorb

Humans generally treat all information as equally reliable

Humans seem to have a limited ability to evaluate a maximum of more than three or four hypotheses at a time

Humans tend to focus only on a few critical factors at a time and consider only a few possible choices related to these criticalfactors

Humans tend to seek information that con®rms their choice of action than information that contradicts or discon®rms theiraction

Human view a potential loss more seriously than a potential gain

Humans tend to believe that mildly positive outcomes are more likely than mildly negative or highly positive outcomes.Humans tend to believe that highly negative outcomes are less likely than mildly negative outcomes

Adapted from Ref 43.

Table 8 Recommendations for Designing Tasks Requiring Selective Attention

Use as few signal channels as possible, even if it means increasing the signal rate per channel

Inform the human the relative importance of various channels for e€ective direction of attention

Reduce stress levels on human so more channels can be monitored

Inform the human beforehand where signals will occur in future

Train the human to develop optimal scan patterns

Reduce scanning requirements on the human by putting multiple visual information sources close to each other, and by makingsure that multiple sources of auditory information do not mask each other

Provide signal for a sucient length of time for individual to respond; where possible, provide for human control of signal rate

Adapted from Ref 46.

alternative stimuli [66] Choice reaction time has been

shown to be in¯uenced by a numerous factors,

includ-ing the degree of compatibility between stimuli and

responses, practice, presence or absence of a warning

signal, the type and complexity of the movement

involved in the responses, and whether or not more

than one stimulus is present in the signal Movement

time is defned as the time from the beginning of the

response to its completion It is the time required to

physically make the response to the stimulus

Movements based on pivoting about the elbow have

been shown to take less time, and have more accuracy,

than movements based on upper-arm and shoulder

action Also, it has been determined that movement

time is a logarithmic function of distance of movement,

when target size is a constant, and further that

move-ment time is a logarithmic function of target size, when

the distance of movement is constant This ®nding is

popularly known as Fitts' law [67], and is represented

as

MT ˆ a ‡ b log2…2D=W†

where MT is the movement time, a and b are empirical

constants dependent upon the type of movement, D is

the distance of movement from start to the center of

the target, and W is the width of the target

Human response to stimuli is not only dependent

upon the speed of the response, but also on the

accuracy of the response The accuracy of the human

response assumes special importance when the

response has to be made in situations where there is

no visual feedback (a situation referred to as ``blind

positioning'') Movements that take place in a blind

positioning situation have been determined to be

more accurate when the target is located dead-ahead

than when located on the sides Also, targets below the

shoulder height and the waist level are more readily

reachable than targets located above the shoulder or

the head [68] The distance and speed of movement

have also been found to in¯uence the accuracy of the

response [69,70]

1.3.3.3 Displays

Types of Displays A display is de®ned as any indirect

means of presenting information Displays are

gener-ally one of the following four types: visual, auditory,

tactual, and olfactory The visual and the auditory

modes of displaying information are the most

common Displays based on tactile and olfactory

senses are mostly used for special task or user

situations (e.g., for the hearing impaired)

Selecting the mode of display whether it should bevisual or auditory in nature) is an important factor due

to the relative advantages and disadvantages certainmodes of display may have over other modes, for spe-ci®c types of task situations (auditory mode is betterthan visual displays in vigilance), environment (light-ing conditions), or user characteristics (person's infor-mation handling capacity) Table 12 provides generalguidelines for deciding between two common modes ofinformation presentation, namely, auditory and visual.The types of displays to use to present informationalso depend on the type of information to present.Di€erent types of information can be presented usingdisplays when the sensing mode is indirect.Information can either be dynamic or static.Dynamic information is categorized by changesoccurnng in time (e.g., fuel gage) Static information,

Table 12 Guidelines for Deciding When to Use VisualDisplays and When to Use Auditory Displays

Immediacy of actionrequirement of message

overburdened

overburdenedEnvironmental factors

too dark requiringsigni®cant adaptation

Adapted for Ref 71.

on the other hand, does not change with time (e.g.,

printed safety signs) A number of other types of

infor-mation are also recognized in the literature Table 13

provides a list of these types along with a brief

descrip-tion of the characteristics of these types of informadescrip-tion

In the following sections, we discuss

recommenda-tions for the design of di€erent types of visual and

auditory displays (we restrict our attention in this

chapter only to these two common modes) We ®rst

provide a brief discussion of the di€erent factors

a€ect-ing human visual and auditory capabilities We then

present speci®c display design issues and

recommenda-tions for these two broad types of displays

Visual displays: factors affecting design

Accommo-dation refers to the ability of the lens in the eye to focus

the light rays on the retina The distance (of the target

object from the eye) at which the image of the object

becomes blurred, and the eye is not able to focus the

image any further, is called the near point There is

also a far point (in®nity, in normal vision) beyond

which the eye cannot clearly focus Focal distances

are measured in diopters One diopter is 1/(distance

of the target in meters) Inadequate accommodation

capacity of the eyes result either in nearsightedness

(the far point is too close) or in farsightedness (the

near point is too close) Literature recommends an

average focusing distance of 800 mm at the resting

position of the eye (also known as the resting

accom-modation) [72] Due to changes in the iris (which

con-trols the shape of the lens), aging results in substantial

receding of the near point, the far point remaining

unchanged or becoming shorter Figure 9 shows how

the mean near point recedes with age It is

recom-mended that the designer use this information whendesigning visual displays

Visual acuity is de®ned as the ability of the eye toseparate ®ne detail The minimum separable acuityrefers to the smallest feature that the eye can detect.Visual acuity is measured by the reciprocal of thevisual angle subtended at the eye by the smallest detailthat the eye can distinguish Visual angle (for anglesless than 108) is given by

Visual angle …in minutes† ˆ …3438H†=Dwhere H is the height of the stimulus detail, and D isthe distance from the eye, both H and D measured inthe same units of distance Besides minimum separablevisual acuity, there are other types of visual acuitymeasure, such as vernier acuity (ability to di€erentiatelateral displacements), minimum perceptible acuity(ability to detect a spot from its background), andstereoscopic acuity (ability to di€erentiate depth in asingle object) In general, an individual is considered tohave normal visual acuity if he or she is able to resolve

a separation between two signs 10 of arc wide Visualacuity has been found to increase with increasing levels

of illumination Luckiesh and Moss [73] showed thatincreasing the illumination level from approximately

10 l to 100 l increased the visual acuity from 100

to 130%, and increasing the illumination level fromapproximately 10 l to 1000 l increased the visualacuity from 100 to 170% For provision of maximumvisual acuity, it is recommended that the illuminationlevel in the work area be 1000 l Providing adequatecontrast between the object being viewed and theimmediate background, and making the signs and

Table 13 Commonly Found Types of Information and Their Characteristics

Quantitative information Information on the quantitative value of a variable

Qualitative information Information on the approximate value, trend, rate of change, direction of change, or other

similar aspects of a changeable variableStatus information Information on the status of a system, information on a one of a limited number of conditions,

and information on independent conditions of some classWarning and signal Information on emergency or unsafe conditions, information on presence or absence of some

Representational Pictorial or graphic representations of objects, areas, or other con®gurations

information

Identi®cation information Information in coded form to identify static condition, situation, or object

Alphanumeric and Information of verbal, numerical, and related coded information in other forms such asSymbolic information signs, labels, placards, instructions, etc

Time-phased information Information about pulsed or time-phased signals

Adapted from Ref 46.

has been determined to be the largest when

the surrounding luminance is within the range of

70 cd/m2, and more than 1000 cd/m2 [75] Also,

Luckiesh and Moss [73] showed that increasing the

illumination level from approximately 10 l to

100 1 increased the contrast sensitivity from 100 to

280%, and increasing the illumination level from

approximately 10 l to 1000 l increased the contrast

sensitivity from 100 to 450% The literature [41] also

recommends that the background be at least 2%

brighter or darker than the target for optimal contrast

sensitivity As brie¯y described above, visual acuity

and contrast sensitivity are a€ected by a number of

factor, such as luminance level (in general, the higher

the luminance, the more the visual acuity and contrast

sensitivity), contrast, exposure time, motion of the

target, age (there is a decline in both visual acuity

and contrast sensitivity with age), and training

(through surgery of the eye or through corrective

lenses, etc.)

Adaptation is another factor that a€ects the visual

capability of the human eye It is de®ned as the

changes in the sensitivity of the eye to light A

mea-sure of adaptation is the time it takes for the eye to

adapt to light or dark It has been found that, in

general, adaptation to light occurs more quickly than

adaptation to the dark Darkness adaptation has

been found to be quick in the ®rst 5 min of

expo-sure; nearly 80% of the adaptation to darkness has

been shown to take about 25 min with full

adapta-tion taking as much as one full hour [41]

Adaptation can also be partial (depending on

whether the visual ®eld contains a dark or a bright

area), and can a€ect the sensitivity of the retina and

the vision For optimal adaptation, the overall

recommendation is to provide the same order of

brightness on all important surfaces, and provide a

stable and non¯uctuating levels of illumination It is

also important to avoid the e€ects of glare (which is

a process of overloading the adaptation processes of

the eye) This can be achieved by avoiding excessive

brightness contrasts, avoiding excessive brightness in

the light source, and providing for transient

adaptation

The ability of the eye to discriminate between

dif-ferent colors is called color discrimination Color

dis-crimination de®ciency is due to the reduced sensitivity

of the particular (to a color) cone receptors While it is

dicult to measure precisely the type and degree of a

person's color de®ciency, it is important from the

perspective of designing tasks which require perception

of colored targets for task performance

The ability to read, and the ability to perceive ing, are the other key factors that have to be accountedfor when designing visual displays

mean-Design recommendations for visual displays Asalready mentioned, visual displays are classi®ed onthe basis of the type of information they present tothe user Information presented to the user can bestatic or dynamic in nature Display of dynamic infor-mation will require capture of the changing nature ofthe information (for example, continuous changes inspeed indicated by the tachometer in the car) Staticdisplays do not display, in real time, the changes in theinformation content in time (Note that, in static dis-plays, the displays themselves do not change with time.However, static displays can be used to present, in theform of graphs, for example, changes in informationcontent over time, after the event has occurred; staticdisplays do not provide information in real time.)Almost all dynamic visual displays contain elements

of one of the more fundamental forms of static mation displays, namely, textual information, informa-tion in the form of graphical displays, information insome coded form, or symbolic information In the fol-lowing sections, we ®rst brie¯y present recommenda-tions on design of these four forms of static visualdisplays We then provide guidelines on designingdynamic information displays

infor-Static visual displays The literature distinguishesbetween two forms of textual displaysÐtextualdisplays in hardcopy format, and textual displays invisual display terminals or computer screens [46].While there are differences in performance based onwhether the display is in hardcopy form or in a visualdisplay unit, there are three essential characteristics ofany display in the form of text; the textual displayshould be visible, legible, and readable Visibility ofthe text refers to the characteristic that makes acharacter or a symbol distinguishable and separatefrom its surroundings Legibility of the text refers tothe characteristic of alphanumeric characters thatmakes it possible to identify one character from theother The stroke width, the character format, con-trast, illumination etc., in¯uence the legibility of thetext Readability of the text refers to the characteristic

of alphanumeric characters that enables organization

of the content into meaningful groups (of information)such as words and sentences

Various factors in¯uence the visibility, the legibility,and the readability of textual information presented inhardcopy form They are typography, size, case,layout, and reading ease

The typography has been found to be especially

important when the viewing conditions are

unfavor-able, or when the information is critical (such as a

sign warning of danger) Typography depends on

factors such as the stroke width of the alphanumeric

character (ratio of thickness of the stroke to height of

the character), the width-to-height ratio of the

character, and the type style

Table 14 also provides accepted guidelines, based on

research, for size of characters, case, layout of

characters, and for reading ease of alphanumeric

characters Some examples of type style and other

aspects in typography of text are given inFig 11

Numerical text can also be represented in graphicalforms Graphs can be in di€erent forms such as linegraphs, bar and column graphs, pie charts, etc.Pictorial information such as in the form of graphsimproves the speed of reading, but the general recom-mendation in the literature is to combine pictorialinformation with information in the form of plaintext, improve the accuracy of the information pre-sented [76,77]

The visibility, readability, and legibility of display-terminal-based text has been found to dependupon the typography, the reading distance, the size ofcharacters, and hardware considerations, such as the

Table 14 Recommendations for Design of Hardcopy Text

Typography

Stroke width When the illumination is reasonable, use 1 : 6 to 1 : 8 for black on white and 1 : 8 to 1 : 10 for

white on blackWhen the illumination is reduced, use thick letters than thin letters for greater readabilityWhen illumination is low or with low background contrast, use boldface characters with a lowstroke width±height ratio

When letters are highly luminous, use 1 : 12 to 1 : 20 ratioWhen letters are black on a highly luminous background, use thick strokesWidth±height ratio Use a 3 : 5 ratio for most practical applications; for transluminated or engraved legends, use 1:1Size of character

For close reading When the reading distance is 12 to 16 in.:

Use 0.09±0.11 in or 22 to 270 of visual angle for normal use of alphanumeric charactersWhen the viewing distance is 28 in.:

For critical use under 0.03 fL luminance, and variable position of character, use 020±0.30 in.height

For critical use over 1.0 fL luminance, and variable position of character, use 0.12±0.20 in.height

For critical use under 0.03 fL luminance, and ®xed position of character, use 0.15 to 0.30 in.height

For critical use over 1.0 fL luminance, and ®xed position of character, use 0.10±0.20 in.height

For noncritical use 0.05±0.20 in heightFor distant reading Use Wsˆ 1:45  10 5 S  d, and HLˆ Ws=R, where Wsis the stroke width, S is the

denominator of the Snellen acuity score (20/20, 20/40 etc.), d is the reading distance, HListhe height of the letter, and R is the stroke width-to-height ratio of the font

Case In general, use lowercase letters than uppercase letters for better readability

Use initial uppercase for search tasksLayout

Interletter spacing Provide close-set type than regular-set type where possible for easier readability

Interline spacing Increase spacing between lines for better clarity

Reading ease

Type of sentence Use simple, armative, active sentences where possible

Order of words Match order of words in sentence to the order of actions to be taken

Adapted from Ref 46.

respectively, are said to be dependent on the relative

(comparing more than one stimulus) and the absolute

judgments (identi®cation of stimulus without the

opportunity to compare) of people It has also been

shown that humans, in general, have the ability to

make better relative judgments than absolute

judg-ments [47,78] This being the case, the orthogonality

or independence of the coding schemes determines how

unique the information provided by a code is, and

results in an increase in the number of stimuli that

can be identifed on an absolute basis [46]; for example,

if size (large and small) and color (black and white)

were orthogonal dimensions of coding, then each of

the possible codes namely, large-black, large-white,

small-black, and small-white, would provide unique

information A number of di€erent guidelines also

exist in the literature that can help good coding

prac-tices Table 15 summarizes general guidelines for

designing a good visual coding system In addition,

di€erent visual coding methods have their own speci®c

design features that can be exploited by the designer

for speci®c tasks and work situations Using

alpha-numeric characters (which are 0, 1, 2, 3; ; 9

and a; b; c; ; z in the English language), singly and

in combination, for instance, has been found to be

useful for identi®cation purposes, and for situations

with space constraints Color coding of surfaces (24

or more combinations of hues, saturation, and

brightness are possible, though research recommends

use of no more than nine combinations) are useful

for industrial tasks requiring searching and counting

Color-coding surfaces can, however, be ine€ective if

the worker population is color de®cient [79,80] Color

coding any lights used in the workplace has been

shown to be e€ective for qualitative reading [81]

The recommendation is to limit the number of lights

coded to three Coding using geomerical shapes

(there are a total of 15 or more geometrical shapes),has been found to be useful in situations using sym-bolic representation of an action or an event Theliterature recommends the use of no more than ®vegeometrical shapes, as using more than ®ve will lead

to diculty in discrimination of the di€erent shapes[81] While a total of 24 di€erent angles of inclination(of characters) are available if coding is to be done

by using angles of inclination, the recommended limit

is 12 [82] Using this form of coding has been found

to be useful for indicating direction, angle, or tion on round instruments Other commonly usedforms of visual coding include di€ering brightness

posi-of lights (recommended limit is two levels) [81], anddi€ering ¯ash rates of lights (recommended limit istwo levels)

Using symbols for coding information is anotherimportant means of representing visual information.The e€ectiveness of symbolic coding depends on howstrongly the symbol is associated with the concept orobjects it is intended to represent The strength of thisassociation has been shown to depend on any existingand established association [83], and on the ease ofleaming any new associations The normal procedure

in setting up symbolic coding systems in the workplaceshould involve considerable experimentation withexisting and any new proposed symbolic codes Theexperimentation should involve the worker populationfor which the symbolic coding system is intended, andcoding system should be evaluated on the basis of theease of recognition, on matching symbols with whatthey represent (based on reaction time of participants),and based on the preferences and opinions of the users.Figure 12 provides examples of good and bad symboldesigns The symbol labeled ``bad design'' in the ®gurehas too much detail and is not simple in design Thesymbol labeled ``good design'' in the ®gure has all the

Table 15 General Recommendations for Designing a Good

Coding System

Make codes detectable by the human sensory mechanisms

under the given environmental conditions,

Make codes discriminable from each other by providing for a

di€erence threshold or a just-noticeable di€erence

Make codes meaningful to the user by providing for

conceptual compatibility

Where possible, standardize codes from situation to

situation

Use multidimensional codes to increase the number and

discriminability of coding stimuli used

Adapted from Ref 46. Figure 12 Examples of good and bad symbol design

details identifying the symbol within the boundary of

the symbol, and not outside the symbol

Dynamic information displays These displays are

used to present information about variables that are

subject to change in time Depending on the type of

information presented by the display, dynamic displays

can provide quantitative information, qualitative

information, check readings, and information on

situa-tion awareness measures

Quantitative visual displays provide informationabout the quantitative value of a variable of interest.The conventional types of displays used to conveyquantitative information include analog displays(®xed scale and moving pointer, moving scale and

®xed pointer) and digital displays (mechanical typecounters) Figure 13 provides some examples of thethree conventional types of displays Research in ana-log displays has provided certain general guidelines for

Figure 13 Commonly used quantitative displays

designing such displays [84] Fixed scale and moving

pointer displays are preferable to moving scale and

®xed pointer displays in most cases This is more so

especially when manual control is used to control the

moving element in the display (since it is better to

control the pointer rather than the scale) Also, any

small variations are better apparent when using a

moving pointer, ®xed scale device However, when

the range of numerical values is too large to be

accom-modated within the scale, the recommendation is to

use a ®xed pointer, moving scale display with

rectan-gular open windows in the scale for easier reference In

general, it has been determined that digital displays

perform better than analog displays where precise

numerical values are needed, and when the presented

numerical values are not continuously changing

In addition to these guidelines for the design of

quantitative displays, research has identi®ed numerous

characteristics that contribute towards making design

of quantitative displays e€ective and ecient Some of

these characteristics include the design of the scale

range (di€erence between the largest and the smallest

scale values), the design of the numbered interval in the

scale (numerical di€erence between adj acent scale

numbers), the design of the graduation interval (the

di€erence between the smallest scale points), the design

of the scale unit (smallest unit to which the scale can be

read), the numerical progressions used in scales, the

design of scale markers, and the design of scale

pointers [46] The numerical progression by 1's

(0; 1; 2; 3; † has been found to be the easiest to use

Decimals in scales, and scales with unusual numerical

progressions such as by 6's and 7's are discouraged

The most common recommendation for the length of

the scale unit is to use values ranging from 0.05 to

0.07 in The key factor in deciding the length of the

scale unit is that the values should be as distinct as

possible to permit easy human reading

Recommendations [81] are also available for design

of scale markers (see Fig 14 for a summary of these

recommendations) Some common recommendations

for design of pointers include having a pointed

(about 208 tip angle) pointers, and having the tip of

the pointer meet the smallest of the scale markers in the

scale Also, to avoid parallax between the scale and the

pointer, it is recommended to have the pointer as close

as possible to the surface of the scale [46]

Qualitative visual displays are used to present

infor-mation on a changing variable based on quantitative

information about a variable The information

presented could be indicative of a trend in the variable,

or a rate of change of the variable Also, qualitative

displays can be used to determine the status of avariable in terms of predetermined ranges (whetherthe fuel tank is empty, full, or half-full), or for main-taining a desirable range of values of a variable (such

as speed) The most common forms of presentingqualitative information through displays is by colorcoding or by using shapes (or areas to representvariables of speci®c interest, such as ``danger'') tocode the information Figure 15 provides an example

of a color- and area-coded qualitative display.Research [85] on check-reading displays (used todetermine if a particular reading is normal or not)has provided the following conclusions about thedesign of such displays:

1 In general, males make fewer errors in reading tasks than females

check-2 The accuracy of check reading is a function ofviewing time; fewer errors will be made if theexposure time is relatively long (greater than0.5 sec); also, check-reading performance di€er-ences between males and females become in-signi®cant when exposure time is increased to0.75 sec

3 The selection of background color is importantfor check-reading tasks; for exposure time lessthan 0.75 sec, black dials and pointers with awhite background lead to fewer errors incheck reading than with white dials andpointers on a black background; however, forexposure times greater than 0.75 sec, fewererrors result with a black dial background; the

®nal selection of the background color should

be based on the time routinely available forcheck reading (Fig 16)

4 Both the 9 o'clock and the 12 o'clock pointerpositions in the dial yield acceptableperformances; the actual design has then to bebased on user preferences

5 Check-reading performance is not a€ected bythe presence of between 1% and 3% deviantdials

6 The normal reading must be coded clearly; ifmany instruments are used in concert, the dis-plays must be con®gured clearly so that thedeviant reading stands out Figure 16 providesexamples of good and bad check reading dis-plays

One other type of qualitative display is statusindicator displays These indicators are usuallyrepresentative of discrete pieces of information such

as whether the condition is normal or dangerous, or

signal The standard recommendation is that these

signals should be at least 500 ms in duration; if they

are shorter than this, the recommendation is to

increase the intensity of the signal Di€erent

recom-mendations have been made by researchers to improve

the detectability of auditory signals A summary of

these recommendations is provided in Table 17

The relative discriminability of auditory signals also

depends upon the intensity and the frequency of

sound, and the interaction between these two factors

Relative discriminability is usually measured in terms

of the just-noticeable di€erence, which is the smallest

change in the intensity or frequency that can be noticed

by humans 50% of the time The smaller the noticeable di€erence, the easier it is to detect thedi€erences in either intensity or frequency of sound.Research has shown that it is easier to detect thesmallest di€erences when the intensity of sound ishigher (at least 60 dB above the threshold level).Also, with respect to frequency, it is recommendedthat signals use lower frequencies for higher discrimin-ability However, since ambient noise is also a low-frequency sound, it is advisable to use signals in the500±1000 Hz range Also, it is good to keep signals

just-30 dB or more above the threshold level for ecientfrequency discrimination

It has also been determined that, on an absolutebasis (identi®cation of an individual stimulus presented

by itself), it is possible for the human ear to identifyfour to ®ve levels of intensity, four to seven levels offrequency, two to three levels of duration, and aboutnine levels of intensity and frequency combined.Sound localization is the ability to determine andlocalize the direction of the sound The di€erences inthe intensity of sounds, and the di€erences in the phase

of sounds are the primary measures by which thehuman auditory system determines the direction ofthe sound source It has been shown that for frequen-cies below 1500 Hz, if the source of the auditory signal

is directly to one side of the head, the signal reaches thenearer ear approximately 0.8 msec before it reaches theother ear Also, localization is dicult at lowfrequencies, since there is very little di€erence in thetime it takes for the signal to reach both earssimultaneously However, at high frequencies(generally above 3000 Hz), the presence of the headbetween the ears makes intensity di€erences morepronounced resulting in e€ective localization of thesound source

Design recommendations for auditory displays Asummary of recommendations for the design of audi-tory displays is provided in Table 18 This is inaddition to the recommendations in the table onwhen to use auditory displays, as opposed to visualdisplays

1.3.3.4 ControlsGeneral Considerations in Control Design Controlsare the primary means of transmitting the controllingaction to devices and systems Numerous factors affectthe design of control devices These factors include theease of identi®cation, the size of the control, control±response ratio, resistance of the control, lag, backlash,deadspace, and location In the following paragraphs,

Table 16 Recommendations for Design of Signal and

Warning Lights

Use signal and warning lights to warn of an actual or

potential danger

Use only one light in normal circumstances; if several lights

are used, have a master warning light to indicate speci®c

danger

For commonly encountered danger or warning situations, do

not use a ¯ashing light; use only a steady light For

situations that are new or occasional, use ¯ashing warning

lights

Use four ¯ashes per second when using ¯ashing warning

lights When using di€erent ¯ashing rates to indicate

di€erent levels of some variable, do not use more than

three such rates with one light

Have the signal or warning light at least twice as bright as the

background

Use red color for these lights and di€erentiate danger lights

from other signal lights in the immediate environment

Ensure that the warning lights subtend at least a visual angle

of 18

Adapted from Ref 84.

Table 17 Recommendations for Increasing the Detectability

of Auditory Signals

Reduce the intensity of noise near the frequency of the signal

of interest

Increase the intensity of the signal

Present the signal for at least 0.5±1 sec

Determine the frequency where noise is low, and change the

signal frequency to correspond this frequency

Present noise to both ears and the signal to one ear only

Introduce a phase shift in the signal and present the unshifted

signal to one ear and the shifted signal to the other

Adapted from Ref 86.