Human factors in mobile systems Lin Zhong ELEC424, Fall 2010

Outline Psychology theories for mobile HCI Human limits Human factors and energy efficiency 2

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 3

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) 4

Cognitive process Hick-Hyman Law – N distinct and equally possible choices Applicable only to simple cognitive tasks – Selection: menu, buttons, list 5

General form Hick-Hyman Law – p i : the probability that the ith choice is selected – p i can be estimated based on history 6

Motor process Stylus operation Fitts ’ Law – A: distance to move – W: target dimension along the moving direction – Parameters adopted from (MacKenzie and Buxton, 1992) 7

Power Law of practice Speed on n th trial – S n = S 1 n a, 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 8

Human capacity limitations Human capacity Perceptual Cognitive Motor …… 9

Perceptual limits D Ω Visual and auditory output E min ≈ Ω·D 2 · (Joule) About (Joule) for most handheld usage Point source Minimal energy requirement for 1-bit change with irreversible computing (Joule) (Landauer, 1961) 10

Insights for power reduction D Ω Point source P∝P∝ Ω·D 2 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 11

Weight of electronic systems Warwick, ounce ≈ Weight decreased from 397 to 176 grams from 1996 to

13 137g

14 540g

15 680g

Human thermal comfort Starner & Maguire, 1999 and Kroemer et al,

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 17

Motor limit: text entry speed Speakingmini hardware keyboardVirtual keyboard with stylusHandwriting Speed (words per minute) Raw speedCorrected speed 18

The slow-user problem Energy efficiency – = (User productivity)/Average power consumption Fast computer vs. slow human user Using Calculator on Sharp Zaurus PDA 99% time and 95% energy spent waiting during interaction Reducing idle power most important 19

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 factorsLow-power design 20

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 21

Comparison: Text entry Handwriting recognition is inferior to alternatives Speech recognition can be the most energy-efficient Display off for speech recognition 22

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 # of taps Maximal # of words per command Ideal 95% accurate 95% accurate/No LCD 95% accurate/No LCD/Lighting 23

OLED display power management User productivity may decrease 2.5 times power reduction HP Labs, MobileHCI

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 25

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 28