Cognitive Modeling 1

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Cognitive Modeling 1

• Bringing a user model into the realm of
  predictive evaluation

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Todays Agenda

• What exactly is cognitive modeling?
• Why do we care about it?
• What are some models?
• What are their relative strengths and
  shortcomings?
• How do we use these models in predictive
  evaluation?

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Cognitive Modeling Explained
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How do we view the user?

• Three prevalent models (not mutually exclusive!)
• A sensory processor (literally,
  stimulus-response)
• An interpreter or predictor (dont you ever
  think?)
• An actor in an environment (a slice of the big
  picture)

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System PhilosophiesShape User Models

• Interaction with philosophy
• Software system as tool, interface
  usability-engineered membrane
• Human-as-processor / -interpreter
• Interaction through philosophy
• Software as communication medium, interface plays
  a role in social context
• Human-as-interpreter / -actor

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User Models Shape HCI

• If a user is a sensory processor
• Your model is informed by experimental
  psychology, quantitative sensory results
• You strive to obey human limits
• If a user is an interpreter/predictor
• Your model is informed by cognitive psychology,
  possibly a touch of AI
• You strive to fit a system into the users
  knowledge base

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User Models Shape HCI, cont.

• If a user is an actor in an environment
• Your model is informed by ecological
  psychology, ideas from anthropology (e.g.
  ethnographic field studies)
• You strive to fit a system into a task and a
  social context
• Roles imply frameworks for design and evaluation

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So why bother?

• Idea If we can build a model of how a user
  works, then we can predict how s/he will interact
  with the interface
• Cognitive model ? predictive evaluation
• No mock-ups or prototypes
• Consider, as we go What do you actually need,
  and what do gaps you fill up/bridge with
  assumptions?

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Model Components

• User qualities
• Understanding
• Knowledge
• Intentions
• Processing
• Levels of detail
• Plans (high-level)
• Motor actions(low-level)

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Your Turn1. Model Human Processor

• Seminal cognitive model
• Microprocessor-human analogue using results from
  experimental psychology
• Card, Moran, Newell (1980s)
• The MHP influence can be seen in some underlying
  HCI principles today
• There are other ways of thinking
• Actors in context vs. users of tools

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Class DiscussionModel Human Processor

• What are the three major subsystems and their
  functions?
• What does it mean to say that certain
  subprocessors have variable rates?
• What is the recognize-act cycle? Is it like
  the fetch-decode-execute of a CPU?
• How do the authors define rationality?
• What are some of the other assumptions underlying
  the MHP model?
• Do you think this is a good model?

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Discussion PointsModel Human Processor

• Three subsystems
• Perceptual, cognitive, motor
• Each has own memories and processors
• Notion of a flow of symbolically coded sensory
  information
• Flow from perceptual to cognitive system
• Cognitive system applies LTM to decide on an
  action, actuates motor system

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Discussion PointsModel Human Processor

• Variable rates
• Cognitive cycle time decreases with increased
  task effort, practice, but increases with
  uncertainty
• Perceptual cycle time decreases with more intense
  stimuli
• Power law of practice (exponential decay of total
  task time with task rehearsal)
• Fitts law (hand tracking to targets in 1D)

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Discussion PointsModel Human Processor

• Procedural model fundamental basis is the
  recognize-act cycle
• WM initiates actions linked to LTM actions
  result in modified WM
• Predicts performance, not actions
• Assumes rationality
• Goals Task Operators Inputs Knowledge
  Process-Limits BEHAVIOR
• Human problem-solving finite state machine

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2. GOMS

• Probably the most widely known and used technique
  in the human as information processor vein
• Heavy MHP influence
• Same authors
• Rationality, goal orientation assumed
• Idea Assign times to each subtask in a linear
  task decomposition

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Class Discussion GOMS

• Define each of the letters in the acronym GOMS
• What is the difference between an operator and a
  method?
• How do you derive task times, and what good are
  they, really?
• What are the assumptions of GOMS?
• When is GOMS appropriate?

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Discussion Points GOMS

• Goals desired endstates
• Subdivide into lower-level operations
• Operators lowest-level task-oriented actions
  (move mouse, read dialog box)
• Methods sequence of operators for accomplishing
  a goal
• Selection rules to choose between multiple
  methods
• GOMS attempts to predict method choice

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Discussion Points GOMS

• Goal photocopying a piece of paper
• GO-TO-COPIER
• if (user is Bob)
• KEY-CARD-ACTIVATE-COPIER
• else
• COIN-ACTIVATE-COPIER
• PLACE-ORIGINAL-ON-GLASS
• MAKE-COPY
• Is this decomposition detailed enough?

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Discussion Points GOMS

• Determine times for each operator, and for the
  task sequence, just add up the times to get total
  time sequence
• Assumes expert users behaving as rational
  problem-solvers why?
• Assumes you know a good sequence of tasks and can
  estimate times decently well
• GOMS power degrades when one of these
  assumptions does not hold

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One More GOMS Question

• Book talks about case study with NYNEX telephone
  system
• Specialized GOMS analysis (equipped for parallel
  tasks) used to determine critical path, task
  timings
• Analysis concluded new system would be slower
  system was abandoned, saving millions of
  dollars
• Anything wrong with that conclusion?

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Variant GOMS-KLM

• GOMS keystroke-level model (KLM)
• Analyze only observable interactions with
  standard input elements (mouse, kbd.)
• GOMS w/ operators K, B, P, H, M, D, R
• Keypress, mouse Button press, Point, Homing (or
  Hand move to input device), Mental prep, Draw
  (line segments) with mouse, Response by system
• Each operator takes a prescribed time

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A Fun GOMS-KLM Problem

• Consider a GOMS-KLM decomposition of selecting
  File / Print from a pulldown menu
• Now consider the same task using only the
  keyboard, with the ALT-F accelerator to open the
  File menu and then the P key to select the Print
  option
• Use texts operator timings for these scenarios
  assume hands start on keyboard

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GOMS-KLM ProblemMouse for menu selection?

• What is the right operator sequence?
• HmousePBleft-clickMPBleft-click
• Complicated rules for placing Ms but boils
  down to chunking (one M before each chunk of a
  task)
• Candidate Ms before each B, K, and P involved in
  specification or selection of a command
  eliminate the Ms that are fully anticipated or
  in a cognitive unit
• Textbook timings (all in seconds)
• H 0.40, P 1.10, B 0.20, M 1.35
• Total predicted time 4.35 s

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GOMS-KLM ProblemKeyboard for Menu Selection?

• Recall mouse operator sequence over two chunks
  (open File menu, select Print option) HPBMPB
• Assuming same two chunks, you have
  MKALTKFMKP
• Book times for K based on typing speed
• Good typist, K 0.12 s, total time 3.06 s
• Poor typist, K 0.28 s, total time 3.54 s
• Non-typist, K 1.20 s, total time 6.30 s
• Possible moral Shortcut keys not necessarily
  faster than using the mouse

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Other models/variantsto know about

• know about know they exist
• NGOMSL similar to GOMS, but expresses goals as
  noun-action pair
• more sophisticated, handles expert-novice better
• Cognitive Complexity Theory uses a hierarchical
  goal decomposition with production rules (if a,
  then b) for a generalized transition network
• better predictive power, size of production rule
  set a good measure of task complexity

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SummaryIssues in Cognitive Modeling

• Terminology
• Whats expert vs. novice?
• Granularity problems (see GOMS)
• Still no user, per se
• No notion of what the real users want
• Time-consuming and lengthy
• One user, one computer
• No social context

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SHW2 Heuristic Evaluation

• Well discuss in detail next time (once Ive had
  a chance to read your papers)
• Problems you encountered?
• Questions you have about heuristic evaluation?

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Next on the Menu

• Some loose ends
• SHW2 post-mortem
• Motor behaviors Fitts law
• Interpretive evaluation
• Ethnography
• Cognitive modeling with context
• Situated action, activity theory, distributed
  cognition
• Design, DOET