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INTERACTIVE SYSTEM DESIGN & EVALUATION Usability Requirements…

- - - - Be specific about user tasks and system components
        
        Be specific about exactly what needs to be good or right
        
        Think carefully about priorities and trade-offs
        
        Formulate requirements that will actually help us develop a usable system
      - TYPES OF USABILITY REQUIREMENTS
        
        Learnability - time and effort required to reach specified levels of performance
        
        Throughput - speed of eecution and errors made by experienced users carrying out tasks
        
        Flexibility - the extent to which system can accommodate changes to tasks and environments beyond those first specified
        
        Attitude - the extent of the positive attitude the use of the system generates in user
      - DEFINITION (ISO9241/11)
        
        Usability - degree to which specified users can achieve specified goals in a particular environment with effectiveness, efficiency and satisfaction
        
        effectiveness - measure of accuracy and completeness of the goals achieved
        
        efficiency - measure of resources (e.g. time, money, effort) used to achieve the goals
        
        satisfaction - measure of the physical comfort and subjective acceptability of the product to its users
        
        MORE SPECIFICALLY
        
        Examples:
        
        "User will not perform the wrong action through a physical slip or mistaking similar object"
        
        "Feedback enables smooth real-time control"
        
        "Information flow rate fast enough to engage attention"
        
        "The display facilitates direct perception of comparisons"
        
        EVEN MORE SPECIFICALLY
        
        In exactly which situations?
        
        What do the users require?
        
        How far can we afford to compromise to improve other aspects of the interaction?
- - - - PERFORMANCE MEASURES
        
        A SAMPLING
        
        Time to locate a book at the Barnes & Noble website
        
        Time to fill in customer information and place order
        
        Number of times the Back Button is used (indicating that user cannot find desired information)
        
        Number of clicks to find the time of a TV show
        
        Percentage of tasks completed correctly
        
        Number of calls to support line
      - PREFERENCE MEASURES
        
        How people experienced using the system
        
        Often Obtained using Likert Scale (slide 21)
        https://wpforms.com/beginners-guide-what-is-a-likert-scale-and-how-to-use-it/
      - Useful to set usability objectives so that we can test whether we have achieved them
        
        This involves specifying usability objectives in a measurable form
        
        Performance measures (such as number of tasks completed, number of errors, etc.)
        
        Perference measures (such as first impression, overall satisfaction) using rating scales
        
        Usability becomes another system specification, rather than a loose statement of intent
  - - - LIBRARY INFORMATION SYSTEM
        Efficiency of first time use measured by retrieval time of specified information
        ''90% of student users without any prior training should be able to retrieve the shelf number of a book, given the author and the title within 60 seconds"
      - COMPONENTS
        1. Statement of usability attribute.
        - Efficiency of first time use ...retrieval time of specified information
        2. Statement of how it will be measured.
        - retrieve the shelf number of a book
        3. Statement of the subset of users to which the specification applies
        - student users
        4. Statement of criteria that will represent attainment of the specification
        - 90%
        - within 60 seconds
        5. Statement of pre-condition of measurement
        - without any prior training
    - - Using a prototype, a number of people can be asked to complete the task the requirement applies to
        
        We can measure speed and errors
      - In our library information system example:
        
        Whether a person can find the information or not
        
        The length of time it takes
        
        Wrong button clicks
    - - For the library information system...
        
        If in a sample of 20 students, 18 or more complete the test task successfully within the specified time (i.e. within 60 second) thent the specification has been achieved
    - - COMPONENTS
        
        Attribute: "US16.1: Speed of obtaining 3 closest earlier cheaper flights on same route"
        
        Measurement: "Time from verbal request to appearance of results to the later of (1) end of spoken statement of day of week and departure time of closest earlier flight (2) appearance on screen of descriptions of three closest earlier cheaper flights"
        
        Criteria
        
        Worst Case: "25 seconds"
        
        Planned Case: "12 seconds"
        
        Best Case: "8 seconds
        [assuming single hotkey combination, minimal visual search and reading time, and timings for verbal statements]"
        
        Now level: "30-40 seconds
        [reading paper timetables, making no checks on seat availability]"
        
        Related requirements
        
        "US16.2: Error rate in reporting closest earlier cheaper flights"
        
        Users: "Full time booking staff, basic training or experience with computers running MS Windows"
        
        Pre-conditions: "4 hour training course, plus 4 hours practice including 10 previous attempts to perform task"
    - - COMPONENTS
        
        Attribute: "US12.1 Clarity of standard error indication"
        
        Measurement: "Percentage of correct answers to the usability test debriefing question on standard errors"
        
        Criteria
        
        Worst Case: "60% correct"
        
        Planned Case: "85% correct"
        
        Best Case: "95% correct"
        
        Now Level: "N/A"
        
        Users: "Users who have taken some sort of elementary statistics course"
        
        Pre-conditions: "First-time users of the system"
    - - COMPONENTS
        
        Attribute: "US23.2 Impression of homepage visual clarity"
        
        Measurement: Average rating given on the clear -confusing semantic differential scale question"
        
        Criteria:
        
        Worst Case: "+1.5"
        
        Planned Case: "+2.2"
        
        Best Case: "+3.0"
        
        Now Level: "+1.6 for previous version VLE"
        
        Users: "First year undergraduate students, who have not used previous version of system"
        
        Pre-conditions: "Have used the system between 6 and 10 weeks"
  - - - Partition total time = task time + error time
        
        Error time represents potential saving if time spent in error can be removed by designing out the cause of the error
        
        Determine those errors which contribute the most error time component
        
        Allocating resources to designing out individual errors gives a known saving in task completion time
    - - Agreeing on a definition of usability
        
        Setting this definition in tems of metrics and usability goals
        
        Putting usability on par with other design goals
        
        Providing a method for prioritizing usability problems
    - - Assumption that usability can be operationalized
        
        That all the important aspects of 'goodness' can be expressed in measurable terms
        
        That your metrics are valid - that is, they are good measures of whatever you care about
        
        Danger of prioritizing factors that are easy to measure rather than really important
        
        Requirement that practitioner is familiar with laboratory methods
        
        Unnaturalness of testing environment
        
        Cost of conducting usability tests with enough subjects to get reliable statistical results
    - - Work context
        
        lab test 6 pages
        
        normal work - reports of over 30 pages
        
        Time context
        
        lab test - time when task will be completed is prescribed
        
        real world - individual has some control over scheduling
        
        Motivational context
        
        clear differences between lab and real world
        
        Social context
        
        lab test - no support
        
        real world - social network of support - 'ask my friends'
    - - Usability engineering (as opposed to user testing - looking for qualitative evidence of problems to fix) is only cost-effective for a few projects
        BUT
        
        Writing exact usability requirements forces you to be very specific about
        
        Which usability factors you care most about
        
        In which situations
        
        What your priorities are
        
        How good is good enough
        
        Devising usability metrics is a good discipline, even if you should be a bit sceptical about how valuable they are