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IMEN 368 인간공학II
13. Stress and Workload
 stressors
 environmental – noise, vibration, heat, light
 psychological – anxiety, fatigue, frustration, anger
 four effects – direct or indirect (fig 13.1)
1. psychological experience – frustration or arousal
2. change in physiology
3. affect the efficiency of information processing – not always degrading
4. long-term negative consequences for health
 ENVIRONMENTAL STRESSORS
 Motion
Sustained motion (motion sickness) and cyclic motion (vibration)
High-Frequency Vibration
 high frequency; specific limb or whole body
 vibrating white finger syndrome – excessive continuous levels of high-frequency vibration
 full-body vibration – not well documented; eye-hand coordination, visual task
Low-Frequency Vibration and Motion Sickness
 lower frequency
 decoupling between the visual and vestibular inputs
 Thermal Stress
 performance degradation and health problems
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 comfort zone
 23 - 26°C in summer and 20 - 24°C in winter
 skewed -- less humidity is allowed (60%) at the upper temperature limit of 26°C
than lower limit of 20°C (85% humidity allowed)
 heat stress – performance degradation on perceptual motor task (tracking and RT);
indirect – the efficiency of information processing not the quality
 long-term consequences – dehydration, heat stroke, heat exhaustion
 heat (and humidity) influencing variables
 clothing worn, air movement, the degree of physical work (metabolic activity)
 cold stress – frostbite, hypothermia, health endangerment – disruption of coordinated
motor performance
 Air Quality
 poor air quality or pollution by smog and carbon monoxide; anoxia
 PSYCHOLOGICAL STRESSORS
 Cognitive Appraisal

1.
2.
3.
4.
differences in cognitive appraisal
may fail to perceive the circumstances of risk
may fail to understand the risk
relatively more confident or even overconfident
reference in control
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 Level of Arousal
 anxiety and danger increase in physiological arousal – HR, pupil diameter, hormonal chemistry
 inverted U function of performance (Yerkes-Dodson Law)
 trying harder, OLA (optimum level of arousal), overarousal
 criticized because it never specify exactly where the OLA is (fig 13.2)
 Performance Changes with Overarousal
 perceptual or attentional narrowing (tunneling) – cognitive tunneling
 working memory loss while LTM little hampered, even be enhanced
 strategy shifts – “do something now” -- speed-accuracy tradeoff
 Remediation of Psychological Stress
 simplification – design of displays, controls, procedures
 actions should be explicitly instructed; as compatible as possible with conventional, welllearned patterns of actions and compatible mapping of displays to controls – knowledge in the
world
 auditory alert and warnings – avoid excessively loud and stressful noise
 training
 extensive (excessive) training of emergency procedures – LTM
 generic training of emergency stress management – guidelines (inhibiting the tendency
to respond immediately), breathing control to reduce the level of arousal
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
LIFE STRESS
 financial difficulties, labor-management relations, stressful life events
 lack of attention (low motivation), distraction or diversion of attention

WORKLOAD OVERLOAD
The Time-Line Model
 workload – a ratio of time required to time available (TR/TA) – fig. 13.3
1. predict how much workload a human experiences
2. predict the extent to which performance will suffer because of overload
 as the ratio increases, the experience of workload also increases relatively continuously
 human performance decrement due to overload occur at or around TR/TA=1  spare
capacity region and overload region – fig. 13 - 4
 four challenging factors to workload estimates
1. identification of task times (covert time)
2. scheduling and prioritization
3. task resource demands and automaticity
4. multiple resources
Workload Overload Consequences
 important consequences for human performance in the overload region  something is
likely to suffer
 Edland and Svenson (1993)  more selectivity of input, more important sources of
info given more weight, decrease in accuracy, decreasing use of heavy mental
computations, locking onto a single strategy
Remediations
 task redesign – automation, display design
 training on the component tasks, task management skills, calibration on tasks
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 Mental Workload Measurement
 mental workload by the ratio of the resources required to the resources available
Primary Task Measures
 measures of system performance on the task of interest – the speed and accuracy
 Not really a workload measure per se but influenced by workload (reflect workload)
Secondary Task Methods
 measures reserve capacity
 Available resources – time estimation, memory task, mental arithmetic, etc.
 problematic because artificial, intrusive, or both  embedded secondary task
Physiological Measures
 HR variability – mental workload
 Blink rate, pupil diameter, electroencepholography (EEG)
Subjective Measures – NASA Task Load Index (TLX)
 Do not always coincide with their performance
Workload Dissociations
 multiple measures are recommended
 FATIGUE AND SLEEP DISRUPTION
 high mental workload  performance may degrade
 Fatigue – a transition state between alertness and somnolence
 A state of muscles and the central nervous system in which prolonged physical activity
or mental processing, in the absence of sufficient rest, leads to insufficient capacity or
energy to maintain the original level of activity and/or processing
 Fatigue not only from the accumulated effects of doing too much work, but also from
prolonged periods of doing very little (vigilance)
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 Vigilance and Underarousal
Causes of the Vigilance Decrement
 Signal detection theory (hits, correct rejections, misses, false alarms) – the increased
number of misses
1. time – the longer , the more misses
2. event salience
3. signal rate – low signal expectancy (more conservative, more misses and fewer false
alarms)
4. arousal level
Vigilance Remediations
 frequent rest break
 more salient signal (signal enhancement)
 payoffs or changing the signal expectancy (false signals)
 Sustain a higher level of arousal – frequent rest breaks, external stimulation (music, noise)
 Not sleep deprived
 Sleep Disruption
 major contributor to fatigue – sleep deprivation (loss); circadian rhythm; jet lag or shift work
 Sleep Deprivation and Performance Effects
 Tasks sensitive to sleep disruption – decision making, innovation and creativity, learning or
storing new material, self-initiated cognitive activity
 long-duration missions lasting more than a day (military combat missions or long-haul truck
driving, or an airline pilot’s trip)
 the quality of sleep typically less, so a sleep debt built up
 less than adequate amount of sleep the night prior to the mission
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 Circadian Rhythms
 body temperature (fig. 13.5) – min. in the early morning and max in the late afternoon/early
evening
 Sleepiness (sleep latency test), sleep duration, performance
 Circadian Disruption
Jet Lag
 west bound (delayed shift) – adapt more rapidly, less sleep disruption
Shift Work
 assign permanently to different shifts – never fully adapted, smaller pool of owls
 a fairly continuous rotation of shifts
 alter the shift periods but to do so relatively infrequently
 delayed shifts are more effective than advanced shifts
 Longer shift
 Remediation to Sleep Disruption
 get more sleep, napping, sleep inertia, sleep credits, sleep management
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14. Safety and Accident Prevention
 Product Liability
 Injuries or death occurs in the workplace or elsewhere
 Product was somehow defective, and the defect caused injuries or death
 Design defective (inherently unsafe)
 Manufacturing defect
 Warning defect
 Defective when it “failed to perform safely as an ordinary user would expect when it was
used in an intended or reasonably foreseeable manner, or if the risks inherent in the
design outweighed the benefits of that design
 FACTORS THAT CAUSE OR CONTRIBUTE TO ACCIDENTS
 Systems approach
 accidents occur because of the interaction between system components (fig. 14.1)
 task performance -- employee-task-equipment
 Social/psychological factors – social norms
 Environmental factors – heat, noise (Table 14.1)
 Personnel Characteristics (Fig 14.2)
Age and Gender
 the most predictive factor is age (15 – 24, peak at 25)
 Physical and cognitive abilities – up for the elderly
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Job Experience
 70% of accidents within first 3 yrs, peak at about 2 – 3 months
Stress, Fatigue, Drugs, and Alcohol
 personality factors – accident prone
 Job characteristics
 high physical workload, high mental workload, other stress-inducing factors
 long work cycles and shift rotation – increase fatigue level
 Equipment
Controls and Displays
Electrical Hazards
Mechanical Hazards
Pressure and Toxic Substance Hazards
 The Physical Environment
Illumination
Noise and Vibration
Temperature and Humidity
Fire Hazards
Radiation Hazards
Falls
Exits and Emergency Evacuation
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 The Social Environment
 management practices, social norms, morale, training, incentive
 Human Error
 Inappropriate human behavior that lowers levels of system effectiveness or safety
Error Classification
 errors of commission – does something that should not have been done
 errors of omission -- fail to do something that should have been done
 Intended error
 mistakes – the inappropriate action was intended (Norman, 1981)
 Reason (1990) – knowledge-based mistakes and rule-based mistakes
 violation – intentionally does something inappropriate – emphasis on productivity
over safety and inadequate safety culture
 Unintended error
 slip – intention is correct but the execution is incorrect (commission errors)
 lapses – nonintentional errors with omission – failure of prospective memory
Errors and System Safety
 resident pathogens – an accident waiting to happen – safety culture
 hindsight bias or “Monday morning quarterbacking”
Error Remediation
 error containment embodied in the design of error-tolerant systems  good feedback,
give “a second chance”
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 HAZARD IDENTIFICATION AND CONTROL
 Hazard Criticality and Risk
 criticality – synonymous with risk, combination of the probability and severity
 probability – frequent, probable, occasional, remote, improbable
 severity – catastrophic (death or loss of a system), critical (severe injury or major
damage), marginal (minor injury or minor system damage), negligible (no injury or
system damage)
 Hazard Identification
Preliminary Hazards Analysis
 early in the conceptual design phase
 a list of the most obvious hazards among task actions, potential users, and environments
 cause and effect  estimate the likelihood, severity of consequences  potential
corrective measures
Failure Modes and Effects Criticality Analysis (FMECA)
 an extension of FMEA, the hazards associated with physical components of a system
 system  subassemblies  constituent components  failure modes  effects on
other components and subassemblies (hazard as well as human error)
 Extent to analysis of the human system (operator performance) – table 14.4
Fault Tree Analysis
 top down from an accident to possible causes with Boolean AND/OR logic
 Powerful method for hazard identification – fig. 14.3
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 Hazard Controls
 safety analysis -- develop a list of hazard controls
 Hazards -- criticality – controls (cost-benefit trade-offs) – relative advantage/disadvantage
column – recommended control column
 the best hazard reduction – eliminate it at once (designing out a hazard source,
functionality)  providing a barrier or safeguard (path)  changing the behavior (warning
and training workers)  administrative procedures or legislation (administrative control)
 SAFETY MANAGEMENT
 Safety Programs
 employee involvement makes a significant difference in the effectiveness of a safety
program
Identify Risks
 document analysis  interviews  facility walk-through  a list of hazards
 Reactive and proactive approach
 job safety analysis
 the heavy involvement of employees, long-term benefits, efficiency, ergonomic
factors
Implementing Safety Programs
 the most effective means after design and guarding methods
 participatory approach, training, feedbacks and incentives
Measuring Program Effectiveness
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 RISK-TAKING AND WARNINGS
 Risk-Taking as a Decision Process
 the choice between safe and unsafe behavior is initially knowledge based decision process
to rule based or simply automatic
 Diagnosis  generation of alternative actions  evaluate alternative actions  decision
based on simplifying heuristics (satisfying)
 three psychological components affecting safe behavior
1. perceived severity of the hazard/injury
2. the novelty of the hazard and whether exposure was voluntary
3. familiarity
 Choice to act safely  action selection process as involving two cognitive stage
 risk perception – availability of risk in memory
 action choice -- cost of compliances
 Written Warnings and Warning Labels
 warnings are the easiest and cheapest means of protecting from product liability suits
 signal word (danger, warning, or caution), description of the hazard, consequences
associated with the hazard, behavior needed to avoid the hazard
1. gaining a person’s attention – bright orange
2. legible font size, contrast, short and simple text, easily interpreted pictures and icons
3. compliance by administrative controls and enforcement
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15. Human-Computer Interaction
 THE TROUBLE WITH COMPUTERS AND SOFTWARE DESIGN
 increased computer technology does not guarantee increased productivity
 well-designed software interface  impact on learning time, performance speed,
error rate, and user satisfaction
 Design Criteria for Usable Software
 efficiency, accuracy, learnability, memorability and satisfaction

SOFTWARE DESIGN CYCLE: UNDERSTAND, DESIGN AND EVALUATE (Fig 15.1)
 user-centered design, participatory design, iterative design

UNDERSTAND SYSTEM AND USER CHARACTERISTICS
 creeping featurism, balance between functionality and ease of use
1. the frequency of task performance using the particular software – efficiency over
memorability
2. mandatory versus discretionary use – ease of use vs. ease of learning and
remembering
3. the knowledge level of the user
 novice users – ease of learning, low reliance on memory – “walk up and use”,
GUI (recognition over recall)
 knowledgeable intermittent users – reducing the load on memory, adaptive
interface
 expert frequent users (efficiency and accuracy)
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 DESIGN USING THEORIES AND MODELS
 Seven Stages of Action
 gulf of execution – the mismatch between the user’s intentions and the actions
supported by the software – good, well-human factored controls
 gulf of evaluation – the mismatch between the user’s expectations and the system
state – food, dynamic info in interpretable displays
 Models of User Performance for Design: GOMS


1)
2)
3)
goals, operators, methods, selection rules
detailed description of user tasks and specific quantitative predictions
explicitly identify and list user’s goals and subgoals
identify all of the alternative methods that could be used for achieving goal/subgoal
write selection rules
 DESIGN TO SUPPORT MENTAL MODELS WITH CONCEPTUAL MODELS
AND METAPHORS
 Mental model – a set of expectancies, relatively complete and accurate
 conceptual model – the general conceptual framework through which the functionality
is presented
 making invisible parts and processes visible to user
 providing feedback
 building in consistency
 presenting functionality through a familiar metaphor
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 Metaphors to develop an effective mental model – supports the transfer of knowledge
 Overlook powerful capabilities available in the computer (not exist in the real world)
 Cause errors or gaps because the differences btn the metaphorical world and the S/W system
 DESIGN USING PRINCIPLES AND GUIDELINES
 General Usability Guidelines
 general usability principles – table 15.1
 Basic Screen Design
 Mayhew (1992) - general layout, text, numbers, coding techniques, color
 Dialog Styles
Menus
 pull down or multiple hierarchical menus
 should be used as a dialog style with negative attitudes, low motivation, poor typing skills, little
computer or task experience
 each menu should be limited to between four and six items – increased by grouping into
categories and separating them with a simple dividing line
 ‘broader & shallow’ vs. ‘narrow & deep’
Fill-in Forms
 negative to neutral attitude, low motivation, little system experience but good typist and be
familiar with the task
Question-Answer
 negative attitude, low motivation, little system experience, relatively good typing skills
 tasks for low frequency of use, discretionary use, and low importance
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Command Languages
 positive attitude, high motivation, medium- to high-level typing skills, high computer literacy,
and high task-application experience
Function Keys
 declining; frequent users who want speeds with low-level typing skills
Direct Manipulation
 map well onto a user’s mental model, easy to remember, no typing skills
 negative to moderate attitude, low motivation, low-level typing skills, moderate to high task
experience
Natural Language
 DESIGN OF USER SUPPORT
 Software Manuals
 should have well-designed, task-oriented, search tools
 search words based on their goals and tasks, not on system components or names
 standard human factors principles and guidelines (table 15.2)
 Online Help Systems
 search effectiveness and efficiency is a general difficulty
 Egan and colleagues (1989) – 50% longer search time than a hardcopy manual
 EVALUATE WITH USABILITY HEURISTICS
 less expensive and less time consuming than UT
 identifies the most relevant interface design principles and guidelines – 2 to 4 experts
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 EVALUATE WITH USABILITY TESTS AND METRICS
 Prototypes

1.
2.
3.

low fidelity methods – index cards, stickies, paper and pen drawings, storyboards
faster, easier, can be modified easily during UT
more willing to change or discard ideas
more substantive feedback to the functionality of prototypes
high fidelity methods include fully interactive screens with the look and feel
 Usability Metrics
 in the conceptual design phase  qualitative assessment of general usability and user
satisfaction (low fidelity prototypes)
 later stages of UT  quantitative measures (table 15.3)  fully functioning prototype
 think aloud, interviews, observations
 what was observed and why such behavior was observed
 Number of Users and Data Interpretation
 not a research experiment – less concerned with large sample size  5 to 6 (fig 15.3)
 Pitfalls of Usability Testing
 understanding the users and their tasks
 fixation on the laboratory environment
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 INFORMATION TECHNOLOGY
 Hypertext, Hypermedia, and the Internet
 hypertext – linking chunks of information (nodes) in a network
 weblication – software delivered as a service over the Web
 Information Database Access
 four types of search
1. the user knows a precise label for a piece of information to be retrieved
2. the user knows some general characteristics of the desired item but can identify when
he/she sees it
3. the user wants to learn what exists in the database that may be of interest
4. the user simply wants to understand the overall structure of the database
Mediated Retrieval
 direct retrieval systems – label the index or keyword terms due to standard conventions
 for multiple classes of users – multiple routes to access the same entities  keyword
searches are not always satisfactory
1. difficult to specify the queries or combinations of keywords
2. users are not always fully satisfied with the keyword search results
Intelligent Agents
 a helper acts as an interface agent between the user and the information database
 provide expert assistant to users
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Spatially Organized Databases
 rely on spatial representation of the information space to support search processes
 navigation or travel through information space rather than direct retrieval spatially
organized databases based on similarity (proximity)
 Different kinds of defining proximity (fig 15.4)
 benefits
1. layout principles of relatedness and sequence of use
2. better understand the full structure of the database by examining a broad map of its
elements
3. should be allowed an option to “recover”
4. provide a historical record, bookmarks
 costs
1. getting lost – (1) consistent with user’s mental model, (2) overall map of the space
2. update rate -- complex graphics, delayed travel time
 Virtual and Augmented Reality
1. VR interface for full immersion
2. negative effects of delayed updates
 Affective Computing
 people with similar personality characteristics will be attracted to each other
 the role of trust in Internet-based interactions – “real world feel” (speed of response, listing
a physical address, photos of the organization
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Figure 13.1 A representation of stress effects
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Figure 13.2
The Yerkes-Dodson law showing the relationship between level of arousal (induced by stress)
and performance. The OLA is shown to be higher for less complex tasks.
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Figure 13.3
Timeline analysis. The percentage of workload at each point is computed as the
average number of tasks per unit time, within each window. Shown at the bottom of the
figure is the computed workload value TR/TA.
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Figure 13.5
Graph plotting mean sleep latency (top), circadian rhythms (body temperature), and sleep
duration (Bottom) against time for two day-night cycles. The bars around sleep duration
represent the variability.
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Figure 13.6
Graph showing how performance on four kinds of tasks varies as a function of circadian
rhythms, shown for a one day cycle.
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Figure 14.1
Models of causal factors in occupational injuries.
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Figure 14.5
Fault tree analysis showing the causes of an accident. The unsafe act must be committed at a time when the
system is vulnerable (thus, the and gate). The unsafe might be committed when its safety implications are
understood but dismissed either because the cost of compliance is too high or for other intentional reasons.
Alternatively, the safety implications may not be known, as a result of a series of possible breakdowns in the
effectiveness of warnings, as described in the text .
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Figure 15.2
Bridging the gulf of execution and gulf of evaluation
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