Lecture 8: Tradeoffs

1
Lecture 8 Tradeoffs
09/10/99
2
Goals

• Understand the importance of tradeoffs in design
• Understand what a tradeoff analysis is and how to
  carry one out

3
Tradeoffs in Systems Design

• Tradeoffs occur in almost every system, e.g.
• Size vs. Cost
• Functionality vs. Usability
• Some Tradeoffs require hard choices
• Some times good design creates a win-win
  situation
• E.g,. Proximity arning was expensive way to
  reduce rear-end collisions
• Third brake ligth was cheaper and better solution

09/10/99
4
Y2K bug results from a tradeoff

• One person's brilliant tradeoff (saving 2 bytes
  of memory per date field)
• Becomes a bug 35 years later if memory costs drop
  a thousandfold and a product meant to be used for
  two years lasts well into the next millenium
• a paragon of silliness and stuff for Newsweek
  cartoons.
• Its easy to take things out of context

5
IP Addresses (Internet Protocol)

• when the size was set during the design of the
  Internet's predecessor
• a network mostly designed to enable computer
  scientists to communicate
• who could have foreseen that twenty-five years
  later we would need a new standard supporting
  five IP addresses for every square meter on the
  planet?

6
False Tradeoffs

• Safety vs. Cost
• Safety vs. productivity
• Rest vs. Productivity
• Functionality vs. Usability?

09/10/99
7
Trade Study

• systematic process for selecting preferred
  concept
• can be performed at any phase of the system life
  cycle

09/10/99
8
Human Factors in Trade Studies

• number of operators required for a system
• operator's skill levels
• amount of additional training required
• effectiveness of the system
• safety
• comfort and satisfaction operators

09/10/99
9
User Differences and Tradeoffs
10
Trade Study Methodology Design

• Define the problem
• Identify constraints
• Generate problem statement
• Review requirements
• Select candidate concepts
• Choose and set-up methodology.
• Develop and quantify criteria
• Determine weights to be applied to criteria

09/10/99
11
Trade Study Methodology Evaluation

• Measure performance with each candidate
• Analyze the results
• Evaluate the candidates
• Perform a sensitivity analysis
• Select preferred candidate
• Document the process and results

12
Example (Figure 8.3 in Chapanis)
13
Sensitivity Analysis

• How robust, how stable is the final solution?
• avoid solutions that do not work well if they are
  modified just slightly
• under normal operating conditions the properties
  of the system may change slightly
• Dont over-tune the design

09/10/99
14
Tradeoffs within Product Lines

• Execution time vs. mental effort
• Learning vs. execution time
• Aesthetic Design vs. function
• Cost vs. usability

09/10/99
15
Execution time vs. mental effort

• Fast to execute designs need information to be
  stored in the user's
• This speed comes at a cost, since they require
  more mental effort on the part of the user

16
Learning vs. execution time

• E.g., a single function key for performing a
  command.
• The single function key can be faster than using
  a mouse.
• Tradeoff occurs when the user must remember which
  key performs which function.
• This tradeoffs also translates into a novice vs.
  expert tradeoff.

17
Design vs. function

• Cleaner design has fewer controls and buttons
• This uncluttered design may appear aesthetically
  pleasing,
• but could tradeoff with functionality.
• Reducing the number of keys while maintaining the
  functionality means more operation modes
• This may reduce usability usability.

18
Cost vs. usability

• Making products more usable can be costly.
• E.g., telephones might be more usable with a
  small screen to provide feedback to user.
• This would increase cost.
• Past phone designs reduced costs but also reduced
  functionality/usability.

19
Tradeoffs in Products
20
Summary

• When carrying out human-machines systems design,
  be prepared to consider tradeoffs.
• look for tradeoffs and problems.
• It is no good improving the usability and "fit"
  of a system that is too expensive or unproductive.

21
Where we are

• Completed the lecture module on systems and
  systems design
• On Thursday well start a five lecture module on
  human error and user-centred design
• Followed by midterm exam on October 14.
• Closed book exam, so youll need to learn the
  material.