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Human factors in mobile systems

Human factors in mobile systems. Lin Zhong ELEC424, Fall 2010. Outline. Psychology theories for mobile HCI Human limits Human factors and energy efficiency. Model Human Processor. Three processes involved in the user reaction to a computer. Perceptual process. Cognitive process.

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Human factors in mobile systems

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  1. Human factors in mobile systems Lin Zhong ELEC424, Fall 2010

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

  3. Model Human Processor Three processes involved in the user reaction to a computer Perceptual process Cognitive process Motor process Model Human Processor: Card, Moran & Newell’83

  4. 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)

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

  6. General form • Hick-Hyman Law • pi : the probability that the ith choice is selected • pi can be estimated based on history

  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)

  8. 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 Measurement Power Law prediction Learning curve of text entry using Twiddler, Lyons, 2004

  9. Human capacity limitations • Perceptual • Cognitive • Motor • …… Human capacity

  10. Perceptual limits Visual and auditory output Emin ≈ Ω·D2·10-13(Joule) About 10-14 (Joule) for most handheld usage D Point source Ω • Minimal energy requirement for 1-bit change • with irreversible computing • 10-21(Joule) • (Landauer, 1961)

  11. Reflective layer to control Ω Insights for power reduction Ω·D2 P∝ η(λ)·V(λ) D λ: wavelength of light/sound Point source η(λ): conversion efficiency from electrical power Ω V(λ): relative human sensitivity factor Smaller D with head-mounted display and earphone

  12. Weight of electronic systems 1 ounce ≈ 28.35 Weight decreased from 397 to 176 grams from 1996 to 2010 Warwick, 1995

  13. 137g

  14. 540g

  15. 680g

  16. Human thermal comfort Starner & Maguire, 1999 and Kroemer et al, 1994

  17. A hot case: 3-Watt Nokia 3120 Every One Watt increases surface temperature by about 13 deg C Phone case temperature will be 40 deg C higher.

  18. 200 Raw speed Corrected speed 150 160 120 Speed (words per minute) 80 40 25 23 22 15 13 12 7 0 Speaking mini hardware keyboard Virtual keyboard with stylus Handwriting Motor limit: text entry speed

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

  20. 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

  21. User productivity Energy efficiency = Avg. power consumption Human factors & energy efficiency • Increase productivity without much power increase • Reduce power consumption without much productivity decrease It is all about tradeoffs between user productivity and power consumption

  22. 100 HW MKB VKB Letter Recog. HW MKB-Lighting VKB-Lighting Letter Recog.-Lighting 10 input r 1 Speech recog. input rate (cwpm) 0.1 0 20 40 60 80 100 120 140 160 Comparison: Text entry Display off for speech recognition Handwriting recognition is inferior to alternatives Speech recognition can be the most energy-efficient

  23. 9 8 Ideal 95% accurate 7 95% accurate/No LCD 6 95% accurate/No LCD/Lighting 5 Maximal # of words per command 4 3 2 1 0 1 2 3 4 5 # of taps 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

  24. OLED display power management • User productivity may decrease 2.5 times power reduction HP Labs, MobileHCI 2004

  25. 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

  26. Examples of energy-inefficient interfaces LG VX 6100 Kyocera KX2325 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

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