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Human factors in mobile systems
Lin ZhongELEC424, Fall 2010
2
Outline
• Psychology theories for mobile HCI
• Human limits
• Human factors and energy efficiency
3
Model Human Processor
Cognitive process
Perceptual process
Motor process
Model Human Processor: Card, Moran & Newell’83
Three processes involved in the user reaction to a computer
Perceptual process• Fixations and saccades– Fixation: information absorbed in
the fovea (60ms)– Saccades: quick movements
between fixations (30ms)– Each GUI object requires one
fixation and one saccade• Rauding rate– Raud: read with understanding– 30 letters/second (Carver, 1990)
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5
Cognitive process
• Hick-Hyman Law– N distinct and equally possible choices
• Applicable only to simple cognitive tasks– Selection: menu, buttons, list
(s) 1Nlog7
1delay Cognitive 2
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General form
• Hick-Hyman Law
– pi : the probability that the ith choice is selected
– pi can be estimated based on history
)1
(1 log7
1delay Cognitive
i1 pp
N
ii
7
Motor process
• Stylus operation
• Fitts’ Law– A: distance to move– W: target dimension along the moving direction
– Parameters adopted from (MacKenzie and Buxton, 1992)
(s) )1(log166.023.0delayMotor 2 W
A
8
0 5 10 15 20 25 30 35 40 45 50
0
5
10
15
20
25
30
35
40
45
50
Power Law of practice
• Speed on nth trial – Sn = S1 na, where a ≈0.4 – Applies to perceptual & motor processes– Does not apply to cognitive process or quality
Learning curve of text entry using Twiddler, Lyons, 2004
Power Law predictionMeasurement
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Human capacity limitations
Human capacity
• Perceptual• Cognitive• Motor• ……
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Perceptual limits
D
Ω
Visual and auditory output
Emin ≈ Ω·D2·10-13 (Joule)
About 10-14 (Joule) for most handheld usagePoint source
Minimal energy requirement for 1-bit changewith irreversible computing
10-21 (Joule) (Landauer, 1961)
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Insights for power reduction
D
ΩPoint source
P∝ Ω·D2
η(λ)·V(λ)
η(λ): conversion efficiency from electrical power
V(λ): relative human sensitivity factor
Reflective layer to control Ω
λ: wavelength of light/sound
Smaller D with head-mounted display and earphone
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Weight of electronic systems
Warwick, 1995
1 ounce ≈ 28.35
Weight decreased from 397 to 176 grams from 1996 to 2010
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137g
14
540g
15
680g
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Human thermal comfort
Starner & Maguire, 1999 and Kroemer et al, 1994
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A hot case: 3-Watt Nokia 3120
Phone case temperature will be 40 deg C higher.
Every One Watt increases surface temperature by about 13 deg C
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Motor limit: text entry speed
150
2313 15
25 2212 7
0
40
80
120
160
200
Speaking mini hardware keyboard Virtual keyboard with stylus Handwriting
Spee
d (w
ords
per
min
ute)
Raw speed Corrected speed
0
0.2
0.4
0.6
0.8
1
0 1 2 3 4 5
Time (s)
Po
we
r (W
att
)The slow-user problem
• Energy efficiency – = (User productivity)/Average power consumption
• Fast computer vs. slow human userUsing Calculator on Sharp Zaurus PDA
99% time and 95% energy spent waiting during interaction
Reducing idle power most important
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Human factors & energy efficiency
• Energy efficiency– Energy consumption per task– # of tasks completed in the battery lifetime– User productivity/Avg. power consumption– or (User productivity) * (Power efficiency)
Human factors Low-power design
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Human factors & energy efficiency
Energy efficiency = User productivity
Avg. power consumption
It is all about tradeoffs between user productivity and power consumption
• Increase productivity without much power increase
• Reduce power consumption without much productivity decrease
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Comparison: Text entry
Handwriting recognition is inferior to alternatives
Speech recognition can be the most energy-efficient
Display off for speech recognition
0.1
1
10
100
0 20 40 60 80 100 120 140 160
Speech recog. input rate (cwpm)
r inpu
t
HW MKB VKB Letter Recog.HW MKB-Lighting VKB-Lighting Letter Recog.-Lighting
Comparison: Command & control• Speech vs. GUI operation
Assume each stylus tap takes 750ms
Single-word voice command is more energy-efficient than GUI operation with 2 taps
0
1
2
3
4
5
6
7
8
9
1 2 3 4 5
# of taps
Max
imal
# o
f wor
ds p
er c
omm
and
Ideal
95% accurate
95% accurate/No LCD
95% accurate/No LCD/Lighting
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OLED display power management
• User productivity may decrease
2.5 times power reduction
HP Labs, MobileHCI 2004
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Predictive system shutdown
• About four eye fixations & saccades– 60*4 + 30*4 =360ms
• Four different choices– 286 ms
• Suppose A= 1/4 screen height– 615 ms
It takes more than 1 second for the user to respond
0
0.2
0.4
0.6
0.8
1
0 1 2 3 4 5
Time (s)
Po
we
r (W
att
)
0
0.2
0.4
0.6
0.8
1
0 1 2 3 4 5
Time (s)
Po
we
r (W
att
)
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Examples of energy-inefficient interfaces
Kyocera KX2325 LG VX 6100 Microsoft Voice Command 1.01
Buttons are protrusive. Often triggered accidentally in the pocket to activate the back lighting
The flip display uses the same back light as the main display
Display is on while not useful