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Industrial Engineering and Ergonomics
© Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Univ.-Prof. Dr.-Ing. Dipl.-Wirt.-Ing. Christopher M. Schlick
Dr.-Ing. Dr. rer. medic. Dipl.-Inform. Alexander Mertens
Chair and Institute of Industrial Engineering and Ergonomics
RWTH Aachen University
Bergdriesch 27
52062 Aachen
phone: 0241 80 99 494
email: [email protected]
Unit 10
Ergonomic Design II:
Human information processing,
displays and manual controls
Fall Winter 2016/2017
10 - 2 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
To understand the need for ergonomic design of information
displays and manual controls
To get known the principles of human information processing
To comprehend important aspects of the human visual system
To understand advantages and disadvantages of different
information display concepts
To comprehend characteristics and suitability of different manual
controls
To learn about the interrelation between information displays and
manual controls
Learning Targets
10 - 3 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Ergonomic Design – Introduction –
displays and manual controls in planes – Concorde
Source: Wiener, Nagel 1989; Wickens 2004
10 - 4 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Information processing –
Human-Machine Interaction
Source: Schlick et al. 2010
10 - 5 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Information perception – sensory modalities
Visual
Eye
Color/Brightness
Auditory
Inner Ear
Tone pitch/loudness
Haptic - tactile
Skin (Vater-Pacinic –
Lamella and Meissner-
Receptors)
Pressure/contact/vibration
Vestibular
Organon vestibulare in the middle ear section
Linear and angular acceleration
Haptic - kinesthetic
Muscle spindle
Relative position and speed of parts of the body as well as forces
Gustatory
Blade
Taste
Olfactory
Mucosalspot in the upper nose region
Olfactory impressions
Perception of pain
All free nerve endings
Pain
Thermal
Skin (end-bulbs/ end-organs)
Temperature
10 - 6 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Information perception – attributes of sensory
modalities I
Modality Organ Stimulus Range Receptors Sensation
visual Eye Electromagn.
radiation
Wavelength
400 - 750 nm
Retinal cones and
rods
Color and
brightness
auditory Inner ear Air
pressure
fluctuations
Frequencies
20 Hz - 20 kHz
Haircells in the
organ of Corti
Tone pitch and
loudness
tactile Skin Skin deformation Male: 9 mg (lips) - 350
mg (sole of foot)
Female: 5 mg (lips) - 79
mg (sole of foot)
Vater-Pacinic-
Lamella
and Meißner-
Receptors
Pressure, contact
and vibration
kinesthetic Muscle spindle,
special section
of the joints and
ligaments
Stretch of the
muscles and
ligaments, joint
position and
movement
Different according to
joint type and ligament,
e.g. biceps (angle [α],
action force [F])
Proprioceptor:
muscle spindle,
Golgi end organs,
Ruffini-corpuscles
Body part position
to one another,
body part
movement and
acceleration
vestibular Organon
vestibulare in
the middle ear
section
Acceleration of the
human body
Fluid shifts and statoliths
(gravity)
Hair cells in the
sacculus, ultriculus
and in the
semicircular canals
Linear and angular
acceleration
10 - 7 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Information perception – attributes of sensory
modalities II
Modality Organ Stimulus Range Receptors Sensation
perception of
pain
Unspecific Impression of pain Injury and atrain Nociceptors -
mostly vacant
nerve endings from
mechanical,
chemical or thermal
sensors
Pain
olfactory Mucosalspot
in the upper nasal
cavity
Molecules in gases Depending on the
type of sub-
stance: from 1
molecule
Olfactory flagella Olfactory
impressions
gustatory Blade Molecules in fluids
Depending on the
type of substance
Gustatory papillae Taste: sweet, sour,
salty, bitter,
umami
thermal Skin Temperature Upper limit :
40°C - 47°C
(3 s exposition,
44 mm² areal)
lower limit:
-17°C
Cold: Krause end-
bulbs;
Heat: Ruffini's end
organ
Warm-cold;
for high and low
temperatures also
pain
10 - 8 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Information perception – approach of psychophysics
Stimulus Sensation
objectively measurable subjective
sound pressure loudness
physical psychological
Psychophysics
Detection of stimuli against the background noise
Distinguishing between stimuli of the same sensory modality
Scaling of sensation and formulating of laws
10 - 9 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
0. Stimulus specificity
(selective perception on the level of sensory
organs)
Only receptor adequate stimuli trigger
sensation.
1. Absolute threshold R0
(psychometric function)
Stimulation intensity, in which a sensory
signal is recognized by the person in half of
the presentations. The probability to detect a
stimulus increases monotonically with the
intensity and typically follows an S-shaped-
curve.
Information perception – superordinate principles I
Perception is the use of the mentioned sensory systems, including the
early processing results of neural information fusion
stimulus intensitiy R
0
0,5
1
𝐹 𝑅 = 𝑃(𝑹 ≤ 𝑅)
threshold
absolute threshold R0
probability
of detection
R0
𝑓(𝑅)
R
0.5
10 - 10 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
2. Difference threshold ΔR
(just noticeable difference)
Just noticeable difference between two
stimuli of the same sensory modality, which
is detected by the person on average in half
of the presentations.
PSE = Point of Subjective Equality
0
0,25
0,5
0,75
1
stimulus intensitiy R
𝐹 𝑅 = 𝑃(𝑹 ≤ 𝑅)
PSE
probability of
answer that
test stimulus
is stronger
than standard
stimulus
Information perception – superordinate principles II
R25% R75%
75 25
2
% %R RR
R25% R75%
𝑓𝑝(𝑅)
R
PSE
10 - 11 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Information perception – superordinate principles III
3. Difference threshold ΔR and psychophysical sensation E
Weber´s Law
Fechner´s Law
The ratio of the difference threshold to
to the standard stimulus is constant.
Weber-Fechner-Law A stimulus must grow in relation to an absolute
threshold stimulus at least logarithmically, if it has to
be precisely perceived as stronger.
k Weber factor
R intensity of instantaneous
stimulus
ΔR difference threshold
R0 absolute threshold
E psychophysical sensation
c stimulus-dependent
parameter
R
Rk
0
lnR
RcE
Source: Goldstein 2002
R
RcE
Se
nsa
tio
n E
R
10 - 12 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Information perception – superordinate principles IV
4. Psychophysical sensation E
a) Stevens' power law as a
concretization of the
Weber-Fechner-Law
Integration
The dependence of the (physically
measured) sensation E on the stimulus
intensity R based on a direct evaluation
of the sensory sensation magnitude.
k,n stimulus-dependent parameters
R absolute intensity of stimulus
R0 absolute threshold
E psychophysical sensation
5. Time referenced law of change
Sensation changes depending on the
intensity of stimulus R and its rate of
change dR / dt.
nRRkE )( 0
1 2
dRE f k R k
dt
R
Rn
E
E
10 - 13 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Information perception – superordinate principles V
Continuum n Stimulus Condition
Brightness (B) 0.33 5° Target in dark
Loudness (L) 0.6 Binaural perception
Taste (T) 0.8 Saccharine
Vibration (V) 0.95 60 Hz on finger
Continuum n Stimulus Condition
Cold (C) 1.0 Metal contact on arm
Heaviness (H) 1.45 Lifted weights
Muscle force (hand) (M) 1.7 Static contraction
Electric Shock (E) 3.5 Current through fingers
Stevens‘ Power Law:
Source: according to Stevens 1975
nRRkE )( 0
10 - 14 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Information perception – multisensory perception –
fusion of multiple sensory input I
Source: Ernst & Bülthoff 2004
It describes interactions
between sensory signals that
are not fully redundant and tries
to predict the combined effect in
terms of accuracy and
resolution
Is based on maximizing the
predictive information that is
delivered from the different
sensory modalities
Often leads to higher human
reliability and performance
Multisensory combination deals with the fusion of information from the
different sensory modalities by the central nervous system:
10 - 15 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Information perception – multisensory perception –
fusion of multiple sensory input II
Sizevisual
Sizehaptic
Σ Size
Experience the same object
property with different senses
The sensory information is
partially redundant
Combination of variances in the
sensory estimate of the property
of interest to increase resolution
Minimizing the prediction error
related to the redundant
information by averaging and
weighting the sensory signals
(weighted average)
The signal that is less noisy
receives a higher weight
Multisensory perception can be
modelled by the famous the
maximum likelihood principle
Source: Ernst & Bülthoff 2004
10 - 16 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Visual perception: Visual field and field-of-view
Visual field Field-of-view (fov) Extended fov
Fixation With recumbent
head and eyes
With recumbent head
and moving eyes
With moving head and
eyes
Horizontal,
Flash
lights
Monocular: -60 to +95°; Binocular: -60 to +60° (opt. 15°)
Monocular: –75 to +110°; Binocular: –75 to +75° (opt. 30°)
Monocular: –125 to +160°; Binocular: –125 to +125° (opt. 55°)
Horizontal,
Colour
lights
-19 to +32° green, -20 to +36° red, -27 to 47° blue/yellow
-34 to +47° green, -35 to +51° red, -42 to +62° blue/yellow
-84 to 97° green, -85 to +101° red, -92 to +112°blue/yellow
Vertical,
Flash
lights
-75 to +55° -85 to +65° -90 to +110°
Field of view airplane
Analysis of the field
of view
10 - 17 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Visual perception – field-of-view for monitoring and
detecting tasks according to DIN 894-2
Aptitude level Relevance
A: Recommendable This range should
be used
whereever it is
possible
B: Suitable This range can be
used if the
recommended
area can not be
used
C:Unsuitable This range should
not be chosen
1: Vertical field of view
2: Horizontal field of view
S: Visual axis, direction is determined by task requirements
SN: Regular visual axis, 15° to 30° under the horizontal
Field-of-view for detecting task
Field-of-view for monitoring task
10 - 18 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Visual perception – visual acuity
Source: Schlick et al. 2010
Visual acuity depending on the site of the retinal image
(under low brightness)
rods
cones
Retina
(under high brightness)
max. physiological
resolution: 0.5´- 1´
(approx.
1 mm at 3-6 m)
10 - 19 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Visual perception – visual acuity –
Range of accommodation
Source: Herczeg 2003
Range of accommodation and near point dependent on age
1
Df
: focal length of the lense in [m]
f
: optical power of the lense in [dpt or m-1] D 𝒇
10 - 20 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Visual perception – color vision I
Color sensitivity as a function of the adaptation state of the retina
Source: WALD (1964)
10 - 21 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Visual perception – application I
Small objects are best seen in black and white contrast
In low ambient light intensity (night), the eye is most sensitive to blue and green
Black and white contrast Black and red contrast
TOMTOM - nightcolors
10 - 22 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Visual perception – application II
Lancia Ypsilon: The instruments are applied exentric,
outside the driver‘s optimal field-of-view
Sidewards head and eye movements are necessary
Distraction from the driving task
10 - 23 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Visual perception – color vision II
The red-green-amblyopia appears in 8-9% of the male population
The blue-yellow-amblyopia appears in about 1% of the entire population
Complete color blindness only occurs in about 0.001% of the population
Thus about 10% of the male population have a color vision deficiency and 5%
of the entire population.
Color vision deficiency
Regular color perception Red-green-amblyopia
(dyschromatopsia)
Blue-yellow-amblyopia
(tritanopie)
10 - 24 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Information displays – display types I
Analog displays
Continuous presentation of state variables
Moving scale or moving needle (pointer)
Disadvantage: Interpolation of intermediate values
Round scale with
moving scale
Round scale with
moving pointer
Sector scale Quadrant scale
Horizontal scale with moving scale Longitudinal scale with
moving pointer
10 - 25 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Information displays – display types II
Digital displays
Transmission of discrete (i.e. categorial) state information
Binary displays, 7-segment displays
Alphanumeric displays
Hybrid displays
Representation of the magnitude of a state variable and its rate of change with two separate elements
Electronic information displays
Software-based implementation of display elements
10 - 26 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Information displays – applications
Display type Perceptual task
Quantitativ
reading
Qualitative reading
Adjusting values
Monitoring and control
Digital display Good Unfavorable
Numbers must be
individually read and
interpreted.
Changes are poorly
noticeable.
Good
Very accurate but at fast
settings hard to read.
Relationship between control
paramater and display part
unclear.
Unfavorable
Changes are poorly noticable.
Rapid changes are barely
legible.
Relationship between control
parameter and display unclear.
Analog display
Moving pointer
Moderate Good
Direction and magnitude
are easy identifiable due
to the pointer position.
Good
Quickly adjustable.
Good manual control by
pointer position.
Unmistakable relationship
between control parameter
and display.
Good
Pointer position is easy to
monitor and regulate.
The relationship between
control parameter and display is
easy to understand.
Moving scale
Moderate Unfavorable
Direction and magnitude
of the deviation are not
recognizable without
reading the scale values.
Moderate
At fast settings hard to read.
Relationship between control
parameter and display possibly
misleading.
Moderate
Changes are poorly noticable.
Rapid changes are barely
legible.
Relationship between control
parameter and display may be
misunderstood eventually.
10 - 27 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Information displays – analog displays –
design guidelines (DIN 894-2)
Labeling and scale arrangement
Source: Schlick et al. 2010
Left:
Masking effect by badly
coordinated pointer shape and
inner label
Right:
Well-designed pointer shape
and labeling
Not more than three division
levels: Long, medium, short
Not more than four medium tick
marks between two long lines,
not more than four short tick
marks between two middle lines
Measured values between tick
marks should account to 1, 2 or
5 or multiples thereof
10 - 28 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Information release – motor function – correlations
between the body forces according to DIN 33411-1
body forces
muscle and inertia forces
(acting in the body system)
action forces
(acting outwardly from the body)
form of the
exercising forces
cause of
the force
appearance of the force function of the force direction
of the
force
force
releasing
body part
active
dynamic
muscle
activity
dynamic
muscle
force
shortening muscle
force dynamic
action force
(motion force)
static
action force
(setting force)
driving force
braking force
manipulation
force
(unguided
movement)
actuating force
(guided
movement)
maintenance
force
tensile strength
reaction force
(body support)
vertical,
horizontal,
sagittal-,
frontal,
ductional
and central
force
arm,
hand,
finger,
leg,
knee, foot and
full body forces
extension muscle
force
static
muscle
activity
static (isometric) muscle force
passive
dynamic
effect of
body
masses
dynamic
inertia
force
e.g.
deceleration force
acceleration force
centrifugal force
static effect
of body
masses
static inertia force
= weight force
10 - 29 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Determination of static applied forces according to
DIN 33411-4 by the example of the force direction –B
For a forward-directed arm force, horizontal and parallel to the body‘s
plane of symmetry, with a side angle of β = 0°, a height angle = 10° and
a relative reach of a/amax = 90% a maximal static forces of 170 N can be
generated.
Isodynes describe contours of equal maximum exerted force depending on the position of the body and the effective length of the arm.
10 - 30 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Information generation by motor functions –
agedependency of the maximum force
Source: Lange 2005
Maximal strength
Considering the decreasing strength in old age, and the differences in both men and women
Age in years
Muscle
str
ength
in %
fro
m
the m
axim
al str
ength
Maximal strength of men = 100%
10 - 31 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Information generation by motor functions –
mobility of the hand
Source: BGI 523 2007
10 - 32 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Support of sensorimotor information generation –
manual controls – types of grip
Source: DIN EN 894-3:2010-01
1 Finger
2 Two fingers
3 Thumb opposed
4 Thumb at right
angle
5 Thumb
6 Three fingers
7 Evenly spaced
8 Thumb opposed
9 Fingers
10 Hand
Unidirectional force
Fast actuating
Sensing the setting
Multidirectional force
Precise setting
Continuous actuating
Holding against the
resistance
Load transmission
10 - 33 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Support of sensorimotor information generation – manual
controls – operation & functional characteristics
Method of operation Examples Suitability
Finger Keyboard
Button
Switch
Pusher
Small control forces
High control speed
Hand Lever
Handwheel
Handle
Crank
Medium to large control
forces
Medium and large travel
ranges
Foot Footswitch
Pedal
large control forces
Source: BGI 523 2007, Götz 2007
Examples for rotary knobs
10 - 34 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Support of sensorimotor information generation –
manual controls – controls I
Isotonic control (distance and angle measurement)
Provides one or two continuous control variables and returns automatically to the starting position
Manipulated parameters y proportional to distance x or angle α with respect to the initial position
Rear driving force is about the same size at each deflection distance / angle
Continuous measurement of the deflection (e.g. side stick)
Used as speed system by vehicle and plane guidance → a ~ y
10 - 35 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Support of sensorimotor information generation –
manual controls – controls II
Isometric control (force measurement)
Negligible deflection, measurement of applied action forces
Manipulated control parameters proportional to applied force
E.g. 6-D manual control element to control an industrial articulated robot
Operating as speed system?
Source: Baumann 1998, Kuka
10 - 36 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Type of movement
Axis of movement: x, y or z
Direction of movement + or -
Continuity of movement: discrete
steps or continuous
Support of sensorimotor information generation – manual
controls – movement characteristics and criteria
Source: DIN EN 894-3:2010-01
linear
Along an
axis
Control force
rotary
Around an
axis
Actuating
torque
Performance
criteria
Communication-
related criteria
Safety-related
criteria
Applied force or
torque
Visual control Inadvertent
operate
Positioning
accuracy
Tactile control Friction
Speed of
adjustment
Use with gloves
Ease of cleaning
Task related criteria
10 - 37 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Support of sensorimotor information generation –
manual controls – movement compatibility
10 - 38 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Interaction of displays and manual controls
Source: Schlick et al. 2010
Conformity with expectations
Compatibility with the arrangement of control unit and display in different planes
The arrangement in the center has the highest clarity on between control part movement
(black) and the reaction of the display (gray)
Less favorable is the assignment on the left side
In the illustration on the right with knob and linear scales in offset planes uncertainties
can already arise in the assignment of cause and effect
10 - 39 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Interaction of displays and manual controls –
application
Source: Schlick et al. 2010
10 - 40 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Application –
rotatory manual controls in motor vehicles
Study: Review of the application comfort depending on the applied control
forces and switch types
Investigation of seven rotary
manual controls in terms of their
suitability and their ease of use
under different setting conditions
60 test subjects in the age
between 20 and 75
Continuous variation of the
adjustable rotational resistance
moment in a region between min.
0,015 Nm and complete blockade
of the operating element
10 - 41 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Application – Touchscreen with tactile feedback on the
driver's work station
Study: Survey on the use of touch-sensitive displays with tactile feedback
on the driver's work station
Study on the users acceptance
and distraction behavior while
using touch-sensitive display with
tactile feedback on the driver's
work station
72 test subjects in the age
between 20 and 75
Surveying different tactile
feedback concerning their
suitability for the transmission of
information to the driver
10 - 42 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Which laws of psychophysics exist for assimilating information?
Which sensory modalities can be distinguished?
Which types of displays exist and to what extent are these suitable for different applications?
What does movement compatibility and obviousness mean?
What are the important aspects of human vision in terms of the perception of displays?
To what extent do the visual acuity and maximum force change with age?
Quick Knowledge Check
10 - 43 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Baumann, K. & Lanz, H.: Mensch-Maschine-Schnittstellen elektronischer Geräte. Springer Berlin, 1998
BGI 523: Mensch und Arbeitsplatz 2007
DIN EN 894 Part 2 + 3: Safety of machinery ― Ergonomics requirements for the design of displays and manual controls –
Part 2: Displays. 1997 ,Part 3: manual controls . 2009
DIN 33411 Part 1 + 4: Physical strengths of man; Part 1: Concepts, interrelations, defining parameters.
1982, Teil 4: Maximum static action forces . 1987
M. O. Ernst, H. H. Bülthoff: Merging the Senses into a Robust Percept. Science 8 (4), 2004, 162-169.
Fechner, Gustav Theodor: Elemente der Psychophysik. Breitkopf und Härtel, 1860
Goldstein, E.: Sensation and Perception. Wadsworth, Pacific Grove (USA), 2002
Götz, M.: Die Gestaltung von Bedienelementen unter dem Aspekt ihrer kommunikativen Funktion, TU München, 2007
Herczeg, M.: Mensch-Computer-Kommunikation Teil 1 + 2. http://www.medieninformatik.uni-
luebeck.de/Portal/studierzimmer/lernmodule/index.html, 2003. Stand: 04.04.12
Lange W.; Windel, A.: Kleine Ergonomische Datensammlung. Köln: TÜV-Verlag GmbH 2005
Schlick, C.; Bruder, R.; Luczak, H.: Arbeitswissenschaft, Springer Berlin 2010
Stevens, S. S.: Psychophysics: Introduction to Its Perceptual, Neural, and Social Prospects. New York, NY: John Wiley and
Sons. 1975.
Literature
10 - 44 © Chair and Institute of Industrial Engineering and Ergonomics, RWTH Aachen University
Wandke, H: Seminar Psychologie und Technik. http://www3.psychologie.hu-berlin.de/ingpsy/alte%20Verzeichnisse%20-
%20Arb1/Lehrveranst/seminar/psych_technik/alte_am_automaten/Ver%C3%A4nderungen%20in%20Alter%20sch%C3%B6
n.htm , 2000. Stand: 09.05.2012.
Wiener, E. L., Nagel, D. C. (1989): Human Factors in Aviation.
Wickens, C. D., Lee, J. D., Liu, Y. & Gordon Becker, S. E. (2004). An introduction to human factors engineering (2nd ed.).
Upper Saddle River, NJ: Pearson Education, Inc. p. 201.
Literature