1

Quality of Service

Quality of Service

Dropped connections

Unavailable websites

Network outages & Delays

Goal is to reduce user frustration

Long delays can result in mistakes

2

Quality of Service

Models of Response-Time Impacts

Response time – number of seconds from the user initiating an action (e.g., ENTER key or mouse-click) until the computer begins to present results (e.g., on display, printer, mobile device)

User think time – the time the user thinks before initiating the next action

• Model

– Initiate

– Wait for computer to respond

– Watch while the results appear

– Think for a while, and initiate again

• More accurately, users plan while:

– Interpreting results

– The computer is generating results or retrieving information

3

Quality of Service

Models of Response-Time Impacts

UserStartstyping

UserInitiatesactivity

User planning time

ComputerBegins

Response

Response time

ComputerCompletesResponse

User think time

4

http://www.youtube.com/watch?v=kDsOtdRtG0Q

5

Quality of Service

Short Term and Working Memory

Sensory memory processes perceptual input

Working memory is used to generate and implement solutions

If many facts and decisions are needed to solve a problem, then short term and working memory may become overloaded

Humans learn to cope with complex problems by developing higher-level concepts into single chunk

The Working Memory Model (Baddeley and Hitch, 1974)

6

Quality of Service

Short Term and Working Memory

Novices tend to work with smaller chunks until they can cluster concepts into larger chunks

Experts rapidly decompose a complex task into a sequence of smaller tasks

Disruptions cause loss of information in sensory and working memory

Anxiety reduces the size of available memory

If users must record solutions in long term memory (e.g., paper or a complex device), the chances for error increase and the pace of work slows)

The Working Memory Model (Baddeley and Hitch, 1974)

7

Quality of Service

Short Term and Working Memory

Challenges to process control (e.g., air traffic control)

• Task often require integration of information (in short term and working memory)

from several sources

• Need awareness of the total situation Rapid task performance, low error rates and high satisfaction result from:

• Users have knowledge of the object and actions needed to solve the problem

• The solution plan can be carried-out without delay

• Distractions are eliminated

• Anxiety is low

• The is feedback about progress towards the solution

• Errors can be avoided or, if they occur, can be handled easily

8

Quality of Service

Interaction Speeds

• Factors involved in choosing the optimal interaction speed

– Novices may perform better with slower response times

– Novices prefer to work at speeds slower than those chosen by

knowledgeable frequent users

– When the penalty for error is low, 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 demand it

in future situations

• Design approaches

– Allow users to block interruptions

– Providing quick links to specific sites (e.g., bookmarks)

9

Quality of Service

Expectations and Attitudes

For repetitive tasks, uses prefer and will work more rapidly with short response times

For complex tasks, users adapt to working with slow response times with no loss of productivity

In some cases, long response times result in users developing short-cuts, or parallel processing and completing the task in less time

In some cases, fast response times can actually result in more errors

Each task appears to have an optimal pace – response times that are shorter or longer than this pace lead to more errors

Physiologically, higher systolic blood pressure occur with shorter response times

Frustration occurs when delays are twice the anticipated time

Tradeoff: Fast starts vs. Rapid Usage

• Is it worth a slower start to download a mechanism to improve rapid usage?

10

Quality of Service

Frustrating Experiences

Studies indicate that 46 to 53% of user’s time is wasted

Dropped network connections

Application crashes

Long system response times

Confusing error messages

Based on applications

• Web browsing

• E-mail (spam/advertisements account for 80% off all emails)

• Viruses

– The ILOVEYOU virus caused $5.5 billion of damage

11

Quality of Service

Frustrating Experiences

Spam

Thirty years ago Gary Thuerk sent an email to 393 users of Arpanet

Today, spam makes up 80 to 90% of all emails sent

Approximately 120 billion messages are sent per day

Today spammers target not just email, but also websites, blogs, social networking sites, and cellphones.

$140 billion globally in 2008