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Human-Computer Interaction

Human-Computer Interaction. Usability Specifications. Lecture Overview. Definition and purpose Usability factors Measuring instruments Usability specification process Setting usability levels Advantages and disadvantages. ISO 9241 Usability Definition (1990).

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Human-Computer Interaction

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  1. Human-Computer Interaction Usability Specifications

  2. Lecture Overview • Definition and purpose • Usability factors • Measuring instruments • Usability specification process • Setting usability levels • Advantages and disadvantages

  3. ISO 9241 Usability Definition (1990) ‘‘The effectiveness, efficiency and satisfaction with which specified users can achieve specified goals in a particular environment.”

  4. Functional and Usability Specifications • Functional specifications are central to ensuring system functionality • Usability specifications are central to ensuring system usability

  5. Purpose of Setting Usability Specifications • Establish when an interface is ‘good enough’ • When to stop iteration

  6. Common Usability Factors • Speed of operation • Completion rate • Error free rate • Satisfaction rating • Learnability • Retainability • Advanced feature usage Main software usability measures identified by IBM

  7. Time Dimension for Usability Factors • Initial performance • Long-term performance • First impression • Long-term user satisfaction

  8. Measuring Instrument • Method for providing values for a particular usability factor • Always quantitative (i.e. numerically measured) • Objective • Observable user performance • Often associated with a benchmark test - involves typical (normally simple and frequent) task a user will perform • Subjective • User opinion • Usually associated with a user questionnaire • Objective and subjective measures are equally important

  9. Usability Specification Process • Defining usability through metrics • Setting and agreeing planned levels of metrics • Analysing impact of design solutions • Incorporating user-derived feedback • Iterating until planned levels are achieved

  10. Usability Specification Process • Defining usability through metrics • Setting and agreeing planned levels of metrics • Analysing impact of design solutions • Incorporating user-derived feedback • Iterating until planned levels are achieved Usability levels become part, first of the requirements document, then of the system specification

  11. Sample Rows from a Usability Specification Table for DTP Package

  12. Electronic Mail: Example Benchmark Tasks • Set up parameters for two terminals (ADM3/VT100) • Set up auto-dial and auto-login • Initiate dial-up and login • Start on-line usage • Send and receive a file • Set slave printer on/off • Set up present unattended dial up/login/file transfer • Read received file • Leave the system Note: Each task would be specified in precise detail to ensure all participants attempt exactly the same task

  13. Setting Levels for Usability • Restrict to major / critical issues • Method: guesses agreed by a team • e.g. user interaction designer, user interface software developer, marketing person, project manager • Generates team commitment to usability targets • Initial performance and first impression are almost always appropriate

  14. Current and Worst Acceptable Levels • Current Level • Present level of value to be measured • Possible even from manual system or initial prototype • May be related to a competitive system • Worst Acceptable Level • Worst acceptable, NOT worst possible • Eventually, if any observed value on any attribute does not meet this level, the system is formally unacceptable • Level should equal or better current level (except for initial performance - slower and error prone - make an educated guess)

  15. Planned Target and Best Possible Levels • Planned Target Level • The ‘what you would like’ level • Attributes not yet at this level focus development effort • Set higher than current level (if known) • Match or exceed competitor’s product • Best Possible Level • Realistic state-of-the-art upper limit • Indicates room for future improvement • Assumes expert user, best design, best available technology

  16. Advantages of Usability Specifications • Part of the management and control mechanisms for the iterative refinement process • Define a quantitative end to the (potentially endless) iterative refinement process • Allows clear assessment of usability during iterative prototyping cycles • Identifies data to be collected, avoids gathering unusable or irrelevant data • Objectively identifies interface weaknesses which need further design effort

  17. Disadvantages of Usability Specifications • Measures of specific user actions in specific situations • No reliable technique for setting usability specifications • Different tasks and user groups may need different usability specifications

  18. Lecture Review • Definition and purpose • Usability factors • Measuring instruments • Usability specification process • Setting usability levels • Advantages and disadvantages

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