Chapter 11

Quality of Service

Outline

I. Introduction

– Time is precious

• Lengthy or unexpected system response time 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

II. Models of response-time impacts

• Response time

– The number of seconds it takes from the moment users initiate an activity until the computer presents results on the display

• User think time

– The number of seconds the user thinks before entering the next action

• 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 to productivity

– Rapid response times (1 second or less) are preferable, but can increase errors for complex tasks

• Display Rate

• Reading textual information from a screen is a challenging cognitive and perceptual task

• Cognitive human performance would be useful for:

– making predictions

– designing systems

– formulating management policies

Limitations of short-term and working memory

• Any cognitive model must emerge from an understanding of human problem-solving abilities

• Magic number seven - plus or minus two

• Short term and working memory are highly volatile

– Disruptions cause loss of memory

– Delays require that memory be refreshed

Source of errors

• Long (1976) found unskilled and skilled typists worked more slowly and made more errors with longer response times

– For a given user and task, there is a preferred response time

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

• 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

– If users have experienced rapid performance previously, they will expect in future situations

III. Expectations and attitudes

• Related design issues may clarify the question of acceptable response time

– E.g. how long before hearing a dial-tone

• Two-second limit (Miller, 1968) appropriate for many tasks

Response-time choke

Response time across web sites varies

Three things influence response-time:

• Previous experiences

• The individual's tolerance for delays

• Task complexity

IV. User productivity

• Repetitive tasks

• Problem solving tasks

V. Variability

• People are willing to pay substantial amounts of money to reduce the variability in their life e.g. insurance

• Modest variations in response time (plus or minus 50% of the mean) appear to be tolerable

• It may be useful to slow down unexpected fast responses