13. stress and workload

고려대학교 산업공학과

IMEN 368 인간공학 II

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 arousal2. change in physiology3. affect the efficiency of information processing – not always degrading4. 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 taskLow-Frequency Vibration and Motion Sickness lower frequency decoupling between the visual and vestibular inputs

Thermal Stress performance degradation and health problems



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); indi-

rect – 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 mo-

tor performance Air Quality

poor air quality or pollution by smog and carbon monoxide; anoxia PSYCHOLOGICAL STRESSORS

Cognitive Appraisal differences in cognitive appraisal1. may fail to perceive the circumstances of risk2. may fail to understand the risk3. relatively more confident or even overconfident4. reference in control



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, well-

learned 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



LIFE STRESS financial difficulties, labor-management relations, stressful life events lack of attention (low motivation), distraction or diversion of attention

WORKLOAD OVERLOADThe Time-Line Model workload – a ratio of time required to time available (TR/TA) – fig. 13.31. predict how much workload a human experiences2. 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 ca-

pacity region and overload region – fig. 13 - 4 four challenging factors to workload estimates1. identification of task times (covert time) 2. scheduling and prioritization3. task resource demands and automaticity 4. multiple resourcesWorkload 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



Mental Workload Measurement mental workload by the ratio of the resources required to the resources availablePrimary 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 taskPhysiological Measures HR variability – mental workload Blink rate, pupil diameter, electroencepholography (EEG)Subjective Measures – NASA Task Load Index (TLX) Do not always coincide with their performanceWorkload 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 pro-longed periods of doing very little (vigilance)



Vigilance and UnderarousalCauses of the Vigilance Decrement Signal detection theory (hits, correct rejections, misses, false alarms) – the increased num-

ber of misses1. time – the longer , the more misses2. event salience3. signal rate – low signal expectancy (more conservative, more misses and fewer false

alarms) 4. arousal levelVigilance 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



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 DisruptionJet Lag west bound (delayed shift) – adapt more rapidly, less sleep disruptionShift 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



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 de-sign 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



Job Experience 70% of accidents within first 3 yrs, peak at about 2 – 3 monthsStress, 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

EquipmentControls and DisplaysElectrical HazardsMechanical HazardsPressure and Toxic Substance Hazards

The Physical EnvironmentIlluminationNoise and VibrationTemperature and HumidityFire HazardsRadiation HazardsFallsExits and Emergency Evacuation



The Social Environment management practices, social norms, morale, training, incentive

Human Error Inappropriate human behavior that lowers levels of system effectiveness or safetyError 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 of a non-intended action)

lapses – non-intentional errors with omission – failure of prospective memoryErrors 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”



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 IdentificationPreliminary 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 cor-rective 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 14.4Fault Tree Analysis top down from an accident to possible causes with Boolean AND/OR logic Powerful method for hazard identification – fig. 14.3



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, functional-

ity) providing a barrier or safeguard (path) changing the behavior (warning and train-ing workers) administrative procedures or legislation (administrative control)

SAFETY MANAGEMENT Safety Programs

employee involvement makes a significant difference in the effectiveness of a safety pro-gram

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 fac-tors

Implementing Safety Programs the most effective means after design and guarding methods

participatory approach, training, feedbacks and incentivesMeasuring Program Effectiveness



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 behavior1. perceived severity of the hazard/injury2. the novelty of the hazard and whether exposure was voluntary3. 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 associ-

ated with the hazard, behavior needed to avoid the hazard1. gaining a person’s attention – bright orange2. legible font size, contrast, short and simple text, easily interpreted pictures and icons3. compliance by administrative controls and enforcement



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 use1. the frequency of task performance using the particular software – efficiency over

memorability2. mandatory versus discretionary use – ease of use vs. ease of learning and remem-

bering3. 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 in-terface

expert frequent users (efficiency and accuracy)



DESIGN USING THEORIES AND MODELS Seven Stages of Action

gulf of execution – the mismatch between the user’s intentions and the actions sup-ported 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 goals, operators, methods, selection rules detailed description of user tasks and specific quantitative predictions1) explicitly identify and list user’s goals and subgoals2) identify all of the alternative methods that could be used for achieving goal/subgoal3) 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



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 cate-

gories 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 fa-

miliar 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



Command Languages positive attitude, high motivation, medium- to high-level typing skills, high computer liter-

acy, and high task-application experienceFunction Keys declining; frequent users who want speeds with low-level typing skillsDirect 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 experienceNatural 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



EVALUATE WITH USABILITY TESTS AND METRICS Prototypes

low fidelity methods – index cards, stickies, paper and pen drawings, storyboards1. faster, easier, can be modified easily during UT2. more willing to change or discard ideas3. 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 but to identify specific problems with the s/w design less concerned with large sample size 5 to 6 (fig 15.3)

Pitfalls of Usability Testing Not sufficient to guarantee effective s/w fixation on the laboratory environment



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 search1. the user knows a precise label for a piece of information to be retrieved2. the user knows some general characteristics of the desired item but can identify when

he/she sees it3. the user wants to learn what exists in the database that may be of interest4. the user simply wants to understand the overall structure of the databaseMediated 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 satisfactory1. difficult to specify the queries or combinations of keywords2. 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



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 orga-

nized databases based on similarity (proximity) Different kinds of defining proximity (fig 15.4) benefits

1. layout principles of relatedness and sequence of use2. better understand the full structure of the database by examining a broad map of its

elements3. should be allowed an option to “recover”4. provide a historical record, bookmarks

costs1. getting lost – (1) consistent with user’s mental model, (2) overall map of the space2. update rate -- complex graphics, delayed travel time

Virtual and Augmented Reality1. VR interface for full immersion2. 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



Figure 13.1 A representation of stress effects



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.



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.



Figure 13.5Graph 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.



Figure 13.6Graph showing how performance on four kinds of tasks varies as a function of circadian rhythms, shown for a one day cycle.



Figure 14.1Models of causal factors in occupational injuries.



Figure 14.5Fault 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.



Figure 15.2Bridging the gulf of execution and gulf of evaluation

13. stress and workload

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