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Mental Workload

DEFINITION: The feeling of mental
effort or the level of use of the human
operators limited resources.
 As task demand increases, resources left in
reserve decreases.
 When attentional resources are exceeded,
further increases in task demand will reduce
performance.
 New airplanes and military systems are
analyzed in terms of mental workload.
Measuring or Predicting
Workload
 Time-Line Analysis:
Measure amount of time
spent on task relative to amount of time
available.
○ 100% is all time available is being used.
○ Gives estimate of how time use changes during
course of task, e.g. flying.
 Dual Task Experiments
○ Primary Task measure: Change the nature of the
main task and see how performance changes.
○ Secondary Task measure: Give a secondary task.
Measure changes in its performance as the
primary task is changed in difficulty.
 Subjective Measures: self-report of users.
 These measures do not always correlate well.
Beginning a Task Analysis

All designs begin with the goals and
purposes of the overall system.
 There are different levels of goals:
○ The function: an airplane moves things great
distances in a relatively short time.
○ The job of the system: fighter vs. passenger jet.
○ The level of technology: mechanical vs. electronic
instruments.
○ The cost of the finished system.
 These types of questions provide and overview
and set up constraints.
Beginning a Task Analysis - 2

Relevant questions to keep in mind
 Kantowitz and Sorkin, 1983
 What inputs and outputs must be provided to satisfy
system goals?
 What operations are required to produced system
outputs?
 What functions should the person perform within the system?
 What are the training and skill requirements for the
operators?
 Are the tasks compatible with human capabilities?
 What equipment interfaces does the human need to
perform the job?
 Does the human help or hurt the machine, vice versa?
Analysis of an Existing Job

Process Analysis
 Derived from the beginning of industrial
engineering from the beginning of century.
 Major goal: improve efficiency.
○ issues of safety and human need are not
important here.
 Specifics:
○ Each act, movement of the human, is identified as
a therblig.
○ The therbligs necessary for the job are identified.
 This technique allows for identification of wasted
movements that can be combined or eliminated.
Analysis of an Existing Job - 2

Principles of Motion Economy
 A list of principles for job and workspace
design to increase movement efficiency.
 Examples:
○ Use of Human Body:
 Eye fixations should be as few as possible.
○ Layout of Workplace:
 Tools, materials and controls should be located close
to the point of use.
 Materials and tools should be located for best
sequence of movement.
Analysis of and Existing Job 3

Link Analysis
 Analysis of sampling pattern of displays and
controls
 Goal: to arrange elements to make moving from
one display or control to another display or
control more efficient and effective.
 Collect Sampling Data:
○ List of which elements are used or viewed in
which order
○ Frequency Data: how often each display is
viewed.
○ Conditional Probabilities: given one display is
viewed, what is the probability that another display
will be viewed next.
Analysis of an Existing Job - 4

Link Analysis (continued)
 The probabilities can assist layout.
○ If probability is high and element is important,
the display or control should be centrally
placed.
○ If a probability is high and importance is low
(that is, the designer wishes to reduce the
probability) the element should be moved to
the periphery.
○ If two displays or controls are used in
sequence, that is their conditional probabilities
are high, they should be next to each other.
Analysis of an Existing Job - 5

Critical Incident Technique
 Essentially errors or near errors are
recorded
○ journals or interviews are used.
 Since errors are often the result of design
flaws, recording the errors can help analysis.
 The record must be detailed or it will not be
useful.
 A large sample is necessary to see if errors
are systematic, indicating a design flaw, or
random, indicating human inattention.
Analysis of Job in Design
Stage

Hopkin’s Approach - a general plan
 Analyze goals of job in terms of:
○ purpose
○ assumptions and constraints
 Allocate functions
○ general refers to assigning tasks to humans
and machines
○ not easy.
 Too much to human overloads
 Too little to human bores and dehumanizes
○ Determine displays and controls
Analysis of Job in Design
Stage - 2

Timeline Analysis
 same as for determining workload.

Failure Task Analysis
 Analyze all anticipated failures to ensure
that humans will be able to handle.
 e.g. MD-11

In many cases, it is advisable to perform
more than one of these analyses.
Analysis of Job in Design
Stage - 3

Types of Design Relative to Errors
 Exclusion
○ Design to make it impossible to commit errors
○ Doubt this is really possible
 Prevention
○ Design to minimize the possibility of errors
 Fail Safe
○ Design to minimize the effects of errors
 You can combine the last two
Anthropometry

DEFINITION:
measurement of the
human body and its
biomechanical
characteristics (Adams,
1989).
 Biomechanical refers to
the mechanical (machinelike) capabilities of the
human skeleto-muscular
systems.
 Thus, this field measures
our sizes and how we
move and move easily.
Anthropometry - 2

The Need for Anthropometry
 We are not all the same size.
○ If systems were designed to fit only one person,
the few if any would be able to use the system.
 Poor design for mechanical abilities of the
human body can lead to discomfort or injury,
e.g., the height of keyboard for a computer.

Sources of Anthropometry Data
 Text
 Tables in books on reserve
○ McCormick and Sanders
○ Kantowitz and Sorkin
Examples of Anthropometric
Data

Static Measures
 Passive measures of the dimensions of the
human body.
 These measures are used to determine size
and spacing requirements of work space.
 Example Measures
○ height
○ weight
○ wing span
○ seat to elbow height.
Examples of Anthropometric
Data - 2

Dynamic Measures
 Measures of the dynamic properties of the
human body, such as strength and
endurance.
 These measures are used to match the
dynamic characteristics of controls to user.
 Measures
○ range of motion for various joints
○ force of leg pushes
○ strength of fingers
How to Use Anthropometric
Data

Know your population
 If your measures are from a different group than
your users are from problems could result.
○ Women are different from men.
○ Asians are different from Americans.

Use Recent Data
 Changes in diet and habit lead to changes in
size and fitness of population
○ Try on old suit of armor

Most size measures are done on nudes.
 clothes change our sizes
○ think of seat belts in summer vs. winter.
How to Use Anthropometric
Data - 2

Techniques of Design
 Design for extremes
○ A standard is to design to fit 5th to 95th percentiles
of the population
○ The chair is a common example.
 Make the design adjustable
○ Ideally this is optimal, especially if adjustment only
needs to be done once.
○ If the adjustment is hard or must be done
frequently, the user may not do them.
○ Airline pilots are require to do this for cockpit.
 The techniques can be combined

Always test the design
Requirements for Task
Analysis

Background Research
 Determine Goals
 Interview potential users to gain insight into their
needs
 Library Research on Design Issues

Structure of Task Analysis
 Determine flow of activity
 Determine Possible Sources of Error/Frustration

Paper
 Background and Task Analysis
 Give Citations
Human Computer Interaction

Issues to Cover
 The Workstation
 The Dialogue
 The User Environment
The Workstation

Perceptual Issues (The Visual Display Unit
or VDU)
Luminance (Foreground - 35 cd/m2)
Refresh Rate (Flicker)
Contrast (Minimum ratio of 3/1)
Legibility (Size, height/width ratio, and stroke
width)
 Polarity (Dark on Light is recognized easier,
Bauer and Cavonius, 1980)
 Chromatic Discrimination
 Visual Fatigue (e.g., due to blurry letters)




The Workstation

Antrhopometric issues
 Display level
○ We like to look down a little
○ If we don’t, we can get neck strain and back strain.
 Keyboard layout, level, and shape
○ Stuck with QWERTY
○ Angle of hands can cause problems. It is worse if
the keyboard is too high
○ Split can help. Angle each of the hands out.
 Mouse (and other related pointing devices
○ Gain (movement of mouse relative to movement
on screen)
 Work area (e.g., place for papers books)
The Dialogue

Mental representation of action of
computer
 from rules to models
 from specific steps to general expectations of
behavior

User Interface
 commands vs. menus
 recall vs. recognition
 Menus
○ Chunking and organization
○ More organization vs. fewer substeps
○ Flexability of use
The Dialogue - 2

Help and aiding interaction
 Help vs. tutorials
 During use vs. before
 Online analysis of use with hints

Skill of User
 Menus, help, etc. are slow and relatively inflexible but easy
to learn
○ Menus vs Ribbons vs Breadcrumbs
○ Image (Icon) vs. Text
 Commands (shortcut keys) are faster and more flexible but
harder to learn
 Thus skill level interacts with interface
Hypertext - Embedded Commands
 Icons

The User Environment

Lighting (internal and external)
 Glare, and contrast and saturation reduction
 Adaptation related problems

Noise (from computer and others)
 Loss of concentration or disruption o f others
 Masking of computer sounds

Support of use
 Training and innovation

Unwilling users
 Assuming the environment is dependent on
computer skill and use
Influence of the Web

Factors to consider
 Similar to All programs
○ Distance
 How provide help?
○ Variation of environment
 Room and other conditions very
 Unique to Web
○ Variation of Setup, e.g. different browsers
○ Limitations in Interface
○ Loss of standardization