i213: User Interface Design

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i213 User Interface Design Development

• Marti Hearst
• Tues, April 17, 2007

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Today

• Evaluation based on Cognitive Modeling
• Keystroke-Level Model
• low-level description of what users must do to
  perform a task.
• GOMS
• structured, multi-level description of what users
  must do to perform a task
• Fitts Law
• Used to predict time needed to select a target

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Keystroke-level Model

• Another discount usability method
• Main idea
• Walk through the interface, counting how many
  operations it would take an expert user to
  perform
• Look for ways to optimize
• Look for potential sources of error
• KLM is very low-level (tiny operations)

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Keystroke-Level Model

• How to make a KLM
• List specific actions user does to perform task
• Keystrokes and button presses
• Mouse movements
• Hand movements between keyboard mouse
• System response time (if it makes user wait)
• Add Mental operators
• Assign execution times to steps
• Add up execution times
• Only provides execution time and operator sequence

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KLM Example

• Replace all instances of a 4-letter word.
• (example from Hochstein)

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What is GOMS?

• A family of user interface modeling techniques
• Goals, Operators, Methods, and Selection rules
• Higher-level than KLM
• Input detailed description of UI and task(s)
• Output various qualitative and quantitative
  measures

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Applications of GOMS analysis

• Comparing UI designs
• Profiling
• Building a help system
• GOMS modelling makes user tasks and goals
  explicit
• Can suggest questions users will ask and the
  answers

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What can GOMS model?

• Task must be goal-directed
• Some activities are more goal-directed than
  others
• Even creative activities contain goal-directed
  tasks
• Task must be a routine cognitive skill
• Can include serial and parallel tasks

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GOMS Output

• Functionality coverage and consistency
• Does UI contain needed functions?
• Are similar tasks performed similarly? (NGOMSL
  only)
• Operator sequence
• In what order are individual operations done?
• Abstraction of operations may vary among models

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GOMS Output (contd)

• Execution time
• By expert
• Error recovery
• Procedure learning time (NGOMSL only)
• Useful for relative comparison only
• Does not include time for learning domain
  knowledge

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How to do (CMN-)GOMS Analysis

• Generate task description
• Pick high-level user Goal
• Write Method for accomplishing Goal - may invoke
  subgoals
• Write Methods for subgoals
• This is recursive
• Stops when Operators are reached
• Evaluate description of task
• Apply results to UI
• Iterate

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Operators vs. Methods

• Operator the most primitive action
• Method requires several Operators or subgoal
  invocations to accomplish
• Level of detail determined by
• KLM level - keypress, mouse press
• Higher level - select-Close-from-File-menu
• Different parts of model can be at different
  levels of detail

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GOMS Example 1 PDA Text Entry

• goal enter-text-PDA
• move-pen-to-text-start
• goal enter-word-PDA
• ...repeat until no more words
• write-letter ...repeat until no more letters
• select goal correct-misrecognized-word ...if
  incorrect
• expansion of correct-misrecognized-word goal
• move-pen-to-incorrect-letter
• write-letter

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GOMS Example

• Move text in a word processor
• (example from Hochstein)

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GOMS Example 2

• Move text in a word processor
• (example from Hochstein)

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GOMS Example 2

• Move text in a word processor
• (example from Hochstein)

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Members of GOMS Family

• Keystroke-Level Model (KLM)
• Card, Moran, Newell (1983)
• CMN-GOMS
• Card, Moran, Newell GOMS
• Natural GOMS Language (NGOMSL)
• -Kieras (1988)
• Critical Path Method or Cognitive, Perceptual,
  and Motor GOMS (CPM-GOMS)
• John (1990)

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Other GOMS techniques

• NGOMSL
• Regularized level of detail
• Formal syntax, so computer interpretable
• Gives learning times
• CPM-GOMS
• Closer to level of Model Human Processor
• Much more time consuming to generate
• Can model parallel activities

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Real-world Applications of GOMS

• KLM
• Mouse-based text editor
• Mechanical CAD system
• NGOMSL
• TV control system
• Nuclear power plant operators associate
• CPM-GOMS
• Telephone operator workstation

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Advantages of GOMS

• Gives several qualitative and quantitative
  measures
• Model explains why the results are what they are
• Less work (?) than usability study
• Easy (?) to modify when interface is revised
• Research ongoing for tools to aid modeling process

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Disadvantages of GOMS

• Not as easy as heuristic analysis, guidelines, or
  cognitive walkthrough
• Only works for goal-directed tasks
• Assumes tasks are performed by expert users
• Evaluator must pick users tasks/goals
• Does not address several important UI issues,
  such as
• readability of text
• memorability of icons, commands
• Does not address social or organizational impact

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GOMS Summary

• Provides info about many important UI properties
• But does not tell you most of what you want to
  know about a UI
• Substantial effort to do initial model, but still
  (potentially) easier than user testing
• Changing later is much less work than initial
  generation

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Fitts Law
Models movement time for selection tasks

• The movement time for a well-rehearsed selection
  task
• increases as the distance to the target
  increases
• decreases as the size of the target
• increases

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Fitts Law
Time (in msec) a b log2(D/S1)
where a, b constants (empirically derived)
D distance S size ID is Index of
Difficulty log2(D/S1)
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Fitts Law
Time a b log2(D/S1)
Target 1
Target 2
Same ID ? Same Difficulty
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Fitts Law
Time a b log2(D/S1)
Target 1
Target 2
Smaller ID ? Easier
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Fitts Law
Time a b log2(D/S1)
Target 1
Target 2
Larger ID ? Harder
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Determining Constants for Fitts Law

• To determine a and b
• design a set of tasks with varying values for D
  and S (conditions)
• For each task condition
• multiple trials conducted and the time to execute
  each is recorded and stored electronically for
  statistical analysis
• Accuracy is also recorded
• either through the x-y coordinates of selection
  or
• through the error rate the percentage of trials
  selected with the cursor outside the target

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A Quiz Designed to Give You Fitts

• http//www.asktog.com/columns/022DesignedToGiveFit
  ts.html
• Microsoft Toolbars offer the user the option of
  displaying a label below each tool. Name at least
  one reason why labeled tools can be accessed
  faster. (Assume, for this, that the user knows
  the tool.)

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A Quiz Designed to Give You Fitts

• The label becomes part of the target. The target
  is therefore bigger. Bigger targets, all else
  being equal, can always be acccessed faster, by
  Fitt's Law.
• When labels are not used, the tool icons crowd
  together.

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A Quiz Designed to Give You Fitts

• You have a palette of tools in a graphics
  application that consists of a matrix of
  16x16-pixel icons laid out as a 2x8 array that
  lies along the left-hand edge of the screen.
  Without moving the array from the left-hand side
  of the screen or changing the size of the icons,
  what steps can you take to decrease the time
  necessary to access the average tool?

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A Quiz Designed to Give You Fitts

• Change the array to 1X16, so all the tools lie
  along the edge of the screen.
• Ensure that the user can click on the very first
  row of pixels along the edge of the screen to
  select a tool. There should be no buffer zone.

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Summary

• We can use Cognitive Modeling to make predictions
  about interface usability
• Complementary to Usability Studies
• In practice
• GOMS not used often
• Fitts law often used for determining best case
  for new kinds of input methods