11. Quality Of Service
• 10.1. Introduction:
• Quality of service can be measured in terms of telephone call quality, lost connections, customer satisfaction, connection time, cost and other factors.
• Quality of service stems from a basic human value : Time is precious.
• Many people become frustrated, annoyed, and angry due to lengthy or unexpected system response times and slow display rates.
• Most users prefer to work more quickly than the computer allows.
• Second basic human value : Harmful mistakes should be avoided.
• If users work too quickly, they may learn less, read with lower comprehension, commit more data entry errors, and make more incorrect decisions.
• Third quality of service : reduce user frustration.
• With long delays, users may become frustrated enough to make mistakes or give up working.
10. Quality Of Service
10.2. Models of Response-time impacts :
Response time is defined as the number of seconds it takes from the movement users initiate an action until the computer begins to present results on the display, printer, loudspeaker or mobile device.
The user think time is the number of seconds during which users think before initiating the next action.
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Fig 10.2 Simple stages of action model of system response time and user think time
10.2. Models of Response-time impacts :
• In this simple stages of action model, users (fig 10.2)
(i) Initiate
(ii) Wait for the computer to respond
(iii) Watch while the results appear
(iv) Think for a while and initiate again
In a more realistic model, users plan while interpreting results , while typing/clicking, and while the computer is generating results or
Models of response-time impacts
•
Designers of response times and display rates in HCI must
consider:
– complex interaction of technical feasibility – cost
– task complexity – user expectations
– speed of task performance – error rates
– error handling procedures
•
Overall majority of users prefer rapid interactions
– Lengthy response times (15 seconds) are detrimental(harmful) to productivity
– Rapid response times (1 second or less) are preferable, but can increase errors for complex tasks
Models of response-time impacts
•
Display Rate
– Alphanumeric displays: The speed in characters per second at which characters appear for the user to read
– World Wide Web Applications: Display rate may be limited by network transmission speed or server performance
•
Reading textual information from a screen is more difficult
than reading from a book
– Users relax when the screen fills instantly- beyond a speed where someone may feel compelled to keep up
•
Cognitive human performance would be useful for:
– making predictions – designing systems
Tune World Wide Web applications to
improve performance
10-6
Designers can optimize web pages to reduce byte counts and numbers of files or provide previews of materials available in digital libraries or archives
Models of response-time impacts
Limitations of short-term and working memory
• Any cognitive model must emerge from an understanding of human problem-solving abilities
• In George Miller’s classic paper, “Magic number seven - plus or minus two”,
Miller identified,
– The average person can rapidly recognize seven chunks of information at a time
– This information can be held for 15 to 30 seconds in short-term memory
– Size of the chunks depends on the person' s familiarity with the material
• Short-term memory and working memory are used in conjunction for processing information and problem solving
– Short-term memory processes perceptual(aware of things through the senses)input
– Working memory generates and implements solutions
• People learn to cope with complex problems by developing higher-level concepts using several lower-level concepts brought together into a single chunk
• Short term and working memory are highly volatile
– Disruptions(disturbance/trouble) cause loss of memory
Models of response-time impacts
Source of errors
• Solutions to problems must be recorded to memory or implemented
– Chance of error increases when solutions are recorded
• When using an interactive computer system users may formulate plans and have to wait for execution time of each step
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 materials
• Progress indicators shorten perceived elapsed time and heighten satisfaction:
– graphical indicators
– blinking messages
– numeric seconds left for completion
Models of response-time impacts
Conditions for optimum problem solving (cont.)
• Rapid task performance, low error rates, and high satisfaction can come from:
– Users have adequate knowledge of the objects and actions necessary for the problem-solving task
– The solution plan can be carried out without delays
– Distractions are eliminated
– User anxiety is low
– There is feedback about progress toward solution
– Errors can be avoided or handled easily
• Other conjectures in choosing the optimum interaction speed
– Novices may exhibit better performance with slower response time
– Novices prefer to work at slower speeds
– With little penalty for an error, users prefer to work more quickly
– When the task is familiar and easily comprehended, users prefer more rapid action
•
Researchers are extending models of productivity to
accommodate the realities of work and home
environments.
•
Eg: e-mail messages, phone calls, request from fellow
workers, etc..
•
Another useful functionality is to
provide users with
feedback about the amount of time spent on various
tasks and a log(duration) of how they handled
interruptions.
10
10. 3. Expectations and Attitudes
10.3. Expectations and Attitudes :
Three primary factors influence users’ expectations and attitudes regarding response time :
1. Previous experiences
2. Individual Personality Differences
3. Task differences
1. Previous experiences :
• People have established expectations based on their past experiences of the time required to complete a given task.
• If a task is completed more quickly than expected, people will be pleased.
• But if a task is completed much more quickly than expected, they may become concerned that something is wrong. Similarly, if a task is completed much more slowly than expected, users become concerned or frustrated.
• Response-time choke : it slow down the system when the load(in network) was
10. 3. Expectations and Attitudes
An important design issue is the issue of rapid start-up. Users are annoyed if they have to wait several minutes for a laptop or a digital camera to be ready for usage, and consequently fast starts are a strong distinguishing feature in consumer
electronics.
2. Individual Personality Differences
These variations are influenced by many factors such as personality, cost, age, mood, cultural context, time of day, noise and perceived pressure to complete work.
3. Task differences
• For repetitive tasks, users prefer and will work more rapidly with short response times.
• For complex tasks, users can adapt to working with slow response times with no loss of productivity , but their dissatisfaction increases as response time lengthen.
• An increasing number of tasks place high demands on rapid system performance.
eg : 3D-animations, flight simulation and graphic design, video conferencing.
10. 4. User Productivity
10. 4. User Productivity:
Productivity - -the ratio of the quantity and quality of units produced to the user per unit of ti
me.
• The nature of the task has the strong influence on whether changes in response time alter user productivity.
• A repetitive control task involves monitoring a display and issuing actions in response to changes in the display.
• In a study of data-entry task, users adopted one of the three strategies, depending on the response time.
• With response times under one second, users worked automatically without checking whether the system was ready for the next data value. It leads to many errors as user typed data values before the system could accept those values.
• With response times above two seconds, users monitored the display carefully to make sure that the prompt appeared before they typed.
10.5. Variability in Response time :
10.5. Variability in Response time :
• The more difficult issue is the effect of modest variations in response time.
• It may not be technically feasible to provide fixed short response time(such as 1 sec) for all actions, several researchers have suggested that the time be fixed for classes of actions.
• Many actions could have a fixed response time of less than 1 second, other actions could take 4 seconds and still other actions could take 12 seconds.
• Experimental results suggest that modest variations in response time do not
severely affect performance. Users are apparently capable of adapting to varying situations.
• The physiological effect of response time is an important issue for stressful,
long-duration tasks such as air-traffic control, but it is also a concern for office workers and sales personnel.
• Eg : higher error rates, higher systolic blood pressure.
• Fig 10.6. Error rates(pg: 419)
10. Quality Of Service
10.6. Frustrating Experiences :
• QOS is usually defined in terms of network performance, but another perspective is to think about the quality of user experiences.
• Frequent complaints include dropped network connections, application crashes,
long system response times, and confusing error messages.
• The major sources of problems were the popular applications for web browsing, e-mail and word processing.
• Recommendations for reducing frustration include interface redesign, software
quality improvement and network reliability increases.
• Others : increased learning, careful use of services and self-control of their attitudes.
10. Quality Of Service
• E-mail has become the source of frustrating spam.
• Antispam legislation is being passed in many nations, but the internet’s international reach and open policies limit the success of legal controls.
• User controlled spam filters also help, but the complexity of user control undermines many users willingness to use these tools.
• Another problem : Malicious viruses that once installed on the machine, can destroy data , disrupt usage, or produce a cancerous spread of the virus to everyone on the user’s
contact list.
• Most network service providers offer virus filters that stop known viruses, but professional programmers must make weekly or even daily revisions to anti-virus software. Eg : McAfee and Symantec.
• Box 10.1. Response-time guidelines (pg: 425)
• Box 10.2. Reducing user frustration
