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Task Analysis (continued)

Task Analysis (continued). Task analysis. Observations can be done at different levels of detail fine level (primitives, e.g. therbligs, keystrokes,GOMS - the “micro” level) intermediate level (flow charts, plans, or steps for sequences of actions)

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Task Analysis (continued)

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  1. Task Analysis(continued)

  2. Task analysis Observations can be done at different levels of detail • fine level (primitives, e.g. therbligs, keystrokes,GOMS - the “micro” level) • intermediate level (flow charts, plans, or steps for sequences of actions) • high level (cognitive goals; social impact - the “macro” level)

  3. General principles of human information processing • Fitt's Law • Reaction time (the Model Human Processor) • Power Law of Practice • Principle of uncertainty • GOMS - an approach to task analysis

  4. The Model Human Processor • Perceptual system (sensors) • Cognitive system (processors) • Motor system (effectors) (Card, Moran, & Newell, 1983)

  5. Important parameters • Memory capacity • Decay • Representation • Processing cycle time

  6. Sample times Eye-movement = 230 [70~700] ms Typical time = 230 ms “Fastman” = 70 ms “Slowman” = 700 ms Perceptual processor: 100 [50~200] Cognitive processor: 70 [25~170] Motor processor: 70 [30~100]

  7. Model of simple RT problem: Task: Press button when symbol appears.

  8. Model of simple RT problem: Task: Press button when symbol appears. 1. Perceptual processor captures it in the visual image store & represents it in working memory. 100 [50~200]

  9. Model of simple RT problem: Task: Press button when symbol appears. 2. Cognitive processor recognizes the presence of a symbol. 70 [25~170]

  10. Model of simple RT problem: Task: Press button when symbol appears. 3. Motor processor pushes the button 70 [30~100]

  11. Model of simple RT problem: Task: Press button when symbol appears. 1. The perceptual processor captures it in the visual image store and represents it in working memory. 100 [50~200] 2. The cognitive processor recognizes the presence of a symbol. 70 [25~170] 3. The motor processor pushes the button 70 [30~100] Total time?

  12. Each of these action primitives takes some small amount of time (in msec.). The Model Human Processor provides a range of parameters you can use to predict precisely how long something will take, or to compare the time needed for alternative actions

  13. More complex RT example Task: you see one symbol, then another. Push yes if they match, no if they don’t. Same first step as in simple RT problem: 1. The perceptual processor captures symbol #1 in the visual image store and represents it in working memory 100 [50~200]

  14. Complex RT example, cont. 2. Ditto for symbol #2 100 [50~200] 3. If symbol #1 is still in the visual store, the cognitive processor can compare the two symbols 70 [25~170] 4. If they match, the cognitive processor decides to hit “yes” 70 [25~170] 5. The motor processor hits “yes” 70 [30~100] How long from step #2 until the end?

  15. Something to think about: • If you’re driving down the highway at 60 mph, how quickly can you react to an emergency? Mean RT in simplest situation is 240 msec. You travel 5280 * 60 = 316,800 ft./hr. 1 hour = 60 * 60 = 3600 sec. You travel 88 ft./sec. or > 21 ft. in 240 msec. before you can even react (let alone stop)!

  16. What about Fastman & Slowman? • If you’re driving down the highway at 60 mph, how quickly can you react to an emergency? Mean RT in simplest situation is 240 sec. You travel 5280 * 60 = 316,800 ft./hr. 1 hour = 60 * 60 = 3600 sec. You travel 88 ft./sec. or > 21 ft. in 240 sec. [between ~11 and~41 ft.] before you can even react (let alone stop)!

  17. General principles of human information processing • Reaction time • Power Law of Practice • Fitt's Law • GOMS - an approach to task analysis • Principle of uncertainty

  18. Power Law of Practice When something is done again and again, performance follows a power law (You keep improving with practice, but as you become an expert, you improve less and less.)

  19. Power Law of Practice

  20. Note: The power law of practice describes quantitative changes in skilled behavior (both cognitive and motor), but not qualitative changes (changes in strategies).

  21. GOMS(Card, Moran, & Newell) • Goal - what the user wants to achieve • Operator - elementary perceptual, motor, or cognitive act • Method - a series of operators that forms a procedure for doing something • Selection rule - how the user decides between methods (if...then...). Skill is particularly important here.

  22. GOMS(continued) Examples: • Goal - editing a paper (high level) cutting and pasting text (low level) • Operator - typing a keystroke • Method - set of operators for cutting • Selection rule - how the user chooses a method

  23. Advantages of GOMS • very general purpose • allows for individual differences • much predictive power about timing • good at predicting "ideal" performance

  24. Disdvantages of GOMS • not so good at predicting errors • takes a long time to conduct analysis • whole may not be the sum of the parts • ignores the nature of internal symbolic representations - focus is very low-level

  25. Skill acquisition and transfer • Transfer (positive transfer) • Interference (negative transfer)

  26. Hick’s principle of uncertainty • Predicts how long a response will take in a given situation, based on how likely (or uncertain) the different possibilities are

  27. Hick’s principle of uncertainty • A secretary has a telephone console with 10 buttons for answering calls on 10 lines. When a light behind a button comes on, his job is to push the button and answer the phone. • Which of these situations is going to be faster to react to? A: where each line gets an equal number of calls B: where two lines are used heavily, getting 50% and 40% of the calls, with the other 10% divided evenly among the other eight lines.

  28. Hick’s principle of uncertainty T = I * log2(n+1) T = time I = a constant n = number of possible responses, assuming all are equally probable +1 is due to uncertainty whether to respond

  29. Hick’s principle of uncertainty A: where each line gets equal number of calls 3.46 units B: where two lines are used heavily, getting 50% and 40% of the calls, with the other 10% divided evenly among the other eight lines. 2.14 units So the RT for B is 62% of the RT for A. (2.14/3.46)

  30. Miscellaneous points to review • Experiment design • Breadth-first vs. depth first in menus and in spoken dialog design • Writing • VoiceXML info

  31. Improve your writing • Everyone can write better (and you are no exception!) • Advice from Clark • Addendum: Brennan • Writing for the Internet (Nielsen) • How users read on the Web

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