User Interface Design
Lecture 9 Response Time and Display Rate
Fitt’s Law
ଶ ଶ
Fitt’s Law
Fitt’s Law
Fitt’s Law
Fitt’s Law
Response Time
• Time is valuable
– lengthy or unexpected delay can produce
• frustration
• annoyance
• eventual anger
– speedy and quickly done work can result in users:
• learning less
• reading with lower comprehension
• making more ill-considered decisions
• committing more data-entry errors
Response Time
• response time – the # of seconds it takes from the moment users initiate an activity until the computer represents results on the display
• user thinking time – the # of seconds the user thinks before entering the next action
User initiates activity
Computer responds
Response Time
• more realistic model
• precise measurements of user thinking time are difficult to obtain
User starts typing
Computer begins response
Response time
User think time User
initiates activity
Computer completes response
User planning time
Response Time
• What need to be considered?
– complex interaction of technical feasibility – task complexity
– user expectations
– speed of task performance – error rates
– error handling procedures
Response Time
• Basic advice
– 0.1 sec – react instantaneously no feedback is needed
– 1.0 sec – notice delay but no interruption
– 10 sec – limit for keeping the user’s attention
• cost versus rapid response time
• loss versus increased errors
Theoretical Foundations
• Cognitive model of human
performance would be useful for
– making predictions – designing systems
– formulating management policies
Theoretical Foundations
Limitations of short-term and working memory
• cognitive model emerge from understanding of human
– problem-solving abilities
– information-processing capabilities
Theoretical Foundations
Limitations of short-term and working memory
• Magic number 7 ± 2
– average person can rapidly recognize seven chunks of information at a time
– This information could be held for 15 to 30 seconds in short-term memory
– size of chunks depends on the person’s familiarity with the material
Theoretical Foundations
Limitations of short-term and working memory
• Short-term memory and working
memory are used in conjunction for processing information and problem solving
– short-term memory processes perceptual input – working memory generates and implements
solutions
Theoretical Foundations (cont)
Limitations of short-term and working memory
• People learn to cope w/ complex
problems by developing higher-level concepts
• short-term and working memory are highly volatile
– disruptions cause loss of memory
Theoretical Foundations
Sources of errors
• Solutions to problems must be
recorded to memory or implemented
– if the solution can be immediately
implemented, the pace of work is quick
– if the solution must be recorded in long-term memory, on paper, or on a complex device,
chances of error increase and the pace of work is slow
Theoretical Foundations
Sources of errors (cont)
• Users may be distracted and may lose the contents of their short-
term or working memory when new information arrives
– i.e., multiplying two four-digit numbers in your head• the intermediate results cannot be maintained
in working memory and must be transferred to
Theoretical Foundations
Sources of errors
• When using an interactive computer system users may formulate plans
and have to wait for execution time for each step
• unexpected result and long delays
– user may forget part of the plan
Theoretical Foundations
Source of errors
• Unskilled and skilled typists worked more slowly and made more errors with longer response time
• When try to work too quickly – no sufficient time to formulate a
solution plan correctly
• for a given user and task, there is a
Theoretical Foundations
Conditions for optimum problem solving
• Longer response time causes
uneasiness in the user because the penalty for error increases
• Shorter response time may cause the user to fail to comprehend the presented material
– incorrect solution plans
Theoretical Foundations
Conditions for optimum problem solving
• Progress indicator
– graphical
– blinking message
– numeric second left for completion – percentage of working
Theoretical Foundations
Conditions for optimum problem solving
• Rapid task performance, low error rate, and high satisfaction can come from:
– user knowledge of the objects and actions – solution plan w/o delays
– no distractions – low user anxiety – progress feedback
– easy error handling, and error prevention
Theoretical Foundations
Conditions for optimum problem solving
• Other conjectures in choosing the optimum interaction speed
– Novices—better performance w/ slower response time – Novices prefer to work at slower speed
– Prefer to work quickly if little penalty for error – Prefer more rapid action with familiar task
– If user have experienced rapid performance previously, they will expect it in the future
Display Rate and Variability
• Display rate
– alphanumeric displays: the speed in characters per second at which characters appear for the user to read
– WWW applications: display rate may be limited by network transmission speed or server performance
Display Rate and Variability
• Reading text from screen or printer is more difficult than reading from a book
• Allow users to control the display rate to synchronize with their reading speed
• If only scan to pick out relevant material, faster display rates will speed up
performance
• Performance and error rate determine
Expectations and Attitudes
• Acceptable response time
– 0.1 sec for pressing a keyboard, turning the wheel of a car, changing channels on TV
– 2 sec to hear dial tone on the phone, see a picture on TV
– 30 sec for a red traffic light – 2 days for a letter to arrive
– 1 or more months for flower to grow
Expectations and Attitudes
• People establish expectations based on their past experiences
• Users become concerned about the response time if the change is much greater
• Response-time choke
– allows a system to be slowed down when the load is light
Expectations and Attitudes
Factors influencing response-time expectation
• Previous experiences
• Individual’s tolerance for delays
• Task complexity
Expectations and Attitudes
• C. M. Williams (1973) made the experiment
– 24 subjects working for 4 hr/day for 5 days on a 15-cps printing terminal
– 2-, 4-, or 8-second response times on four types of data-entry tasks
– each subject worked on four tasks at the same response time
– press “attention” key to get immediate
Expectations and Attitudes
Standard response
time
Total Number
of Trials
Average Delay (seconds)
Standard Deviation (seconds)
Percent of Attention
PressedKey
2 seconds 11,634 1.98 0.53 1.42
4 seconds 9,754 3.50 2.08 17.44
8 seconds 10,103 2.27 5.63 82.92
Results of the experiment
Expectations and Attitudes
How people prefer response time?
1. People will work faster as they gain experience, it may be useful to
allow customize pace of interaction 2. If (1) cannot be done, people will
force response time to well under 1 second
User Productivity
• Shorter response time higher productivity
• Concurrent processing and shortcut while waiting for long response time
• Generally, with shorter response time
– reduce performance time
User Productivity
• Exceptions
– nature of the task
– difficulty in repairing an error – feedback from the system
– possibility of using different methods to solve the given problem
– expectations of the users
User Productivity
Repetitive control task
• Nature of the task has a strong influence on whether changes in
response time alter user productivity
• shorter system response time
– operator picks up the pace
– decisions on command may be < optimal
– penalty for poor choice may be small easy to try another command
User Productivity
Problem-solving tasks
• Users will adapt their work style to the response time
• Users will change their work habits as the response time change
User Productivity
Programming tasks
• types of task
– text editing – compilation
– program testing – debugging
• knowledge programmer build a plan
and then seek to carry out as rapidly as possible
User Productivity
Summary
• Users pick up the pace of the system to work more quickly w/ shorter response time
• There is an optimal pace for each user-task situation
• Higher throughput of work need attention to minimize the cost & delay of error recovery
• The optimal response time may be longer than the minimum possible time
Variability
• Modest variations in response time (±50% of mean) – tolerable and have little impact on performance
• With extreme changes, users appear to be adaptable
• It may be useful to slow down
unexpected fast response time to avoid surprising the user
Variability
• A serious effort should be made to avoid extremely slow response time
• We can determine the optimal response time for a specific
application and user by measuring
– the productivity (correctly completed tasks) – cost of errors
Response Time Guidelines
• User prefer shorter response times
• Longer response times are disruptive
• Users change usage profile with response time
• Shorter response times leas to short user think times
• A faster pace may increase
productivity, but error rates also
Response Time Guidelines
• Error-recovery ease and time
influence the optimal response time
• Response time should be appropriate to the task
– Typing, cursor motion, mouse selection: 50 to 150 milliseconds
– Simple frequent tasks: < 1 second – Common tasks: 2 to 4 seconds
– Complex tasks: 8 to 12 seconds
Response Time Guidelines
• User should be advised of long delays
• Modest variability in response time is acceptable
• Unexpected delays may be disruptive
• Empirical tests can help to set suitable response times
