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Chapter 10

Chapter 10. Response Time and Display Rate February 3, 2004. 10.1 Introduction. Response time – seconds from user ativity to computer response User think time – seconds during which user thinks before action Display rate – characters per second of display

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Chapter 10

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  1. Chapter 10 Response Time and Display Rate February 3, 2004

  2. 10.1 Introduction • Response time – seconds from user ativity to computer response • User think time – seconds during which user thinks before action • Display rate – characters per second of display • Must balance fast response with accuracy

  3. 10.2 Memory • 7±2 chunks • Working memory is used to generate and implement solutions • More efficient if you can cluster into larger chunks • Complex problems often dealt with by developing higher-level concepts that group several lower level concepts into a single chunk • Easy to disrupt short term memory (highly volatile)

  4. Error • Long delays can be distacting and lead to wasted effort • Short delays can lead to ill thought out plans • There might be a preferred response time to maxmize performace

  5. Optimum Problem Solving • Driving analogy • Graphical indicators make happy users • Users have knowledge • The solution plan has little delay • Distractions are low • Anxiety is low • Feedback about progress • Errors can be avoided and handeled easily

  6. 10.3 Expectations and Attitudes • How long will users wait for the computer to respond before they become annoyed??? • For many tasks 2 seconds, sometimes within 0.1 second- turning the wheel of a car- pressing a key on the keyboard, piano or telephone – 30 seconds, red traffic light to turn green, two days for a letter to arrive or a month for flowers to grow

  7. If the task is completed much more quickly or much more slower than expected, users become concerned or frustrated. Even though people can detect 8% changes in 2-4 second response time users do not become concerned until the change is much greater. • Individual’s tolerance for delays also impacts the response-time expectations- personality, costs, age, mood, time of the day , pressure to complete the work, ….

  8. Less than 100 milliseconds wait – user controlled three-dimensional animations, flight simulations, graphic design, …. • Three primary factors influences user’s expectations 1. Previous experiences 2. Individual personality differences 3. Task differences

  9. Three conjectures emerge • Individual differences are large and users are adaptive. work faster as they gain experience. • For repetitive tasks, users prefer and will work more rapidly with short response times. • For complex tasks, users can adopt to work with slow responses time.

  10. 10.4 User Productivity • Shorter system response time usually lead to higher productivity. But sometimes working too quickly may lead to errors that reduce productivity. • Just like driving – there is no general rules , just as a new route evaluation must be done for each trip

  11. Repetitive Tasks • The nature of the task has a strong influence on whether changes in response time alter user productivity. • Operators may learn to use the system more quickly with short system response times because they can explore alternatives more easily.

  12. Problem Solving Tasks • Users will adapt their work style to the response time. • Study shows that the time to solution was invariant with respect to response time!

  13. Summary • User pick up the pace of the system to work more quickly with shorter response time and that they consistently prefer a faster pace. • The optimal response time may be longer than the minimum possible response time.

  14. 10.5 Variability • Predictability = peace of mind • Insurance industry based on the idea that most people are willing to sacrifice a reduction of immediate pleasures in exchange for peace of mind • Extreme variation in response time should be prevented or addressed by the system (i.e. through some progress indicator) • As variability increases, performance may decrease • Dissatisfaction is most noticeable if delays are unusually long (i.e. 2x anticipated response time)

  15. 10.6 Practitioner’s Summary • Computer system response time closely linked with user productivity, error rates, satisfaction, working style, etc (refer to Box 10.1, pg. 367) • In general, user satisfaction is inversely proportional to response time • Optimal response time can be established by measuring Δ productivity associated with cost of errors • More informative the feedback the better

  16. 10.7 Researcher’s Agenda • Study error rates as a function of response time for a range of tasks and users? • How else are user work styles affected, other than through response time? • Effectiveness of diversionary tasks vs. progress reports when technical feasibility prevents short response? • Strive for specification of upper/lower limits for response time for respective commands

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