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 Must balance fast response with accuracy

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)

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

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

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

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, ….

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

Three conjectures emerge Individual differences are large and users are adaptive. work faster as they gain experience. 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 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. For complex tasks, users can adopt to work with slow responses time.

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

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.

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!

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.

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)

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

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