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Cognitive Considerations
From Don Norman’s book, The Psychology (Design) of Everyday Things– Affordances, Constraints, and Mappings– Mental Models– Action Cycle and Gulf of Execution
Slide adapted from Saul Greenberg
Getting serious about design
World War II– invention of machines (airplanes, submarines...) that taxed people’s
sensorimotor abilities to control them– even after high degree of training, frequent errors (often fatal) occurred
Example airplane errors:– if booster pump fails, turn on fuel valve within 3 seconds
test shows it took at least five seconds to actually do it!
– Altimeter gauges difficult to read caused crashes when pilots believe they are at a certain altitude
Result– human factors became critically important
Slide adapted from Saul Greenberg
Other computer psychopathologies
from InfoWorld, Dec ’86– “London—
An inexperienced computer operator pressed the wrong key on a terminal in early December, causing chaos at the London Stock Exchange. The error at [the stockbrokers office] led to systems staff working through the night in an attempt to cure the problem”
Slide adapted from Saul Greenberg
Other computer psychopathologies
from Science magazine– In 1988, the Soviet Union’s Phobos 1 satellite was lost on
its way to Mars, when it went into a tumble from which it never recovered.
“not long after the launch, a ground controller omitted a single letter in a series of digital commands sent to the spacecraft. And by malignant bad luck, that omission caused the code to be mistranslated in such a way as to trigger the [ROM] test sequence [that was intended to be used only during checkout of the spacecraft on the ground]”
Affordances
Affordance: The perceived and actual properties of an object that determine how it could possibly be used.
– Knobs are for turning– Buttons are for pushing
Some affordances are obvious, some learned– Glass can be seen through– Glass breaks easily
Sometimes visual plus physical feedback– Floppy disk example
Rectangular – can’t insert sideways Tabs on the disk prevent the drive from letting it be fully
inserted backwards
Based on slide by Saul Greenberg
Affordances
Perceived can differ from real affordance– Chair: real affordance
Affords sitting Affords standing for changing a lightbulb Affords jamming a door open
– Chair: false affordance Can be moved, but not if bolted down
Remember the Strauss Mouse video
Based on slide by Saul Greenberg
Affordances in Computer Screen-Based Interfaces
Designer only has control over perceived affordances– Display screen, pointing device, selection buttons,
keyboard– These afford touching, pointing, looking, clicking
on every pixel of the display.
Based on slide by Saul Greenberg
Affordances in Computer Screen-Based Interfaces
Most of this affordance is of no value– Example: if the display is not touch-sensitive, even
though the screen affords touching, touching has no effect.
– Example: does a graphical object on the screen afford clicking? yes, but the real question is does the user perceive this
affordance; does the user recognize that clicking on the icon is a meaningful, useful action?
Mappings
Mapping: – Relationships between two things
Between controls and their results
– Related to metaphor discussion– Related to affordances
Mappings
For devices, appliances– Natural mappings use constraints and correspondences in the
physical world Controls on a stove Controls on a car
– Radio volume Knob goes left to right to control volume Should also go in and out for front to rear speakers
For computer UI design– Mapping between controls and their actions on the computer
Controls on a digital watch Controls on a word processor program
Mapping controls to physical outcomes
backright
frontleft
backleft
frontright
24 possibilities, requires: -visible labels -memory
arbitrary full mapping
back front front back
2 possibilities per side =4 total possibilities
paired
Based on slide by Saul Greenberg
Transfer Effects
People transfer their expectations from familiar objects to similar new ones– positive transfer: previous experience applies to
new situation– negative transfer: previous experience conflicts with
new situation
Based on slide by Saul Greenberg
Cultural Associations
Groups of people learn idioms– red = danger, green = go
But these differ in different places– Light switches
America: down is off Britain: down is on
– Faucets America: counter-clockwise is on Britain: counter-clockwise is off
Based on slide by Saul Greenberg
Mental Models
People have mental models of how things work:– how does your car start?– how does an ATM machine work?– how does your computer boot?
Allows people to make predictions about how things will work
Based on slide by Saul Greenberg
Mental Models
Mental models built from– affordances– constraints– mappings– positive transfer – cultural associations/standards– instructions– interactions
Mental models are often wrong!
Slide adapted from Saul Greenberg
Our mental models of how bicycles workcan “simulate” this to know it won’t work
People are always trying to explain things
Mental models often extracted from fragmentary evidence
People find ways to explain things– Computer terminal breaks when accessing the
library catalog– Sure your driving on the correct road
Norman’s Action Cycle
Human action has two aspects– execution and evaluation
Execution: doing something Evaluation: comparison of what happened to
what was desired
Action Cycle
Goals
EvaluationEvaluation of interpretations
Interpreting the perception
Perceiving the state of the world
ExecutionIntention to act
Sequence of actions
Execution of seq uence of actions
The World
start here
Norman’s Action Cycle
Execution has three stages:– Start with a goal– Translate into an intention– Translate into a sequence of actions
Now execute the actions Evaluation has three stages:
– Perceive world– Interpret what was perceived– Compare with respect to original intentions
Gulf of Evaluation
The amount of effort a person must exert to interpret – the physical state of the system– how well the expectations and intentions have been
met
We want a small gulf!
Based on slide by Saul Greenberg
Good Example
Scissors– affordances:
holes for insertion of fingers blades for cutting
– constraints big hole for several fingers, small hole for thumb
– mapping between holes and fingers suggested and constrained by appearance
– positive transfer learnt when young
– conceptual model implications clear of how the operating parts work
Based on slide by Saul Greenberg
Bad Example
Digital Watch– affordances
four push buttons, not clear what they do
– contraints and mapping unknown no visible relation between buttons and the end-result of their
actions– negative transfer
little association with analog watches
– cultural standards somewhat standardized functionality, but highly variable
– conceptual model must be taught; not obvious
Slide adapted from Saul Greenberg
Interface Metaphors Revisited
Definition of Metaphor– application of name or descriptive term to an object to which it is not
literally applicable Purpose
– function as natural models – leverages our knowledge of familiar, concrete objects/experiences to
understand abstract computer and task concepts Problem
– metaphor may portray inaccurate or naive conceptual model of the system
A presentation toolis like
a slide projector
Slide adapted from Saul Greenberg
Interface Metaphors
– Use metaphors that matches user's conceptual task – desktop metaphor for office workers– paintbrush metaphor for artists...
– Given a choice, choose the metaphor close to the way the system works
– Ensure emotional tone is appropriate to usersE.g., file deletion metaphors
– trashcan– black hole– paper shredder– pit bull terrier– nuclear disposal unit...
Slide adapted from Saul Greenberg
Metaphors continued
Caveat– metaphors can be overdone!
Common pitfalls– overly literal
unnecessary fidelity excessive interactions
– overly cute novelty quickly wears off
– overly restrictive cannot move beyond
– mismatched does not match user’s
task and/or thinking
Slide adapted from Saul Greenberg
The Metaphor of Direct Manipulation
Direct Engagement– the feeling of working directly on the task
Direct Manipulation– An interface that behaves as though the interaction was with a real-
world object rather than with an abstract system Central ideas
– visibility of the objects of interest– rapid, reversible, incremental actions– manipulation by pointing and moving– immediate and continuous display of results
Almost always based on a metaphor– mapped onto some facet of the real world task semantics)
Slide adapted from Saul Greenberg
Object-Action vs Action-Object
Select object, then do action– interface emphasizes 'nouns' (visible objects) rather than 'verbs'
(actions) Advantages
– closer to real world– modeless interaction– actions always within context of object
inappropriate ones can be hidden– generic commands
the same type of action can be performed on the object eg drag ‘n drop:
–
my.doc
move
Slide adapted from Saul Greenberg
Direct manipulation Representation directly determines what can manipulated
Slide adapted from Saul Greenberg
Direct Manipulation
Xerox Star: pioneered in early '80s, copied by almost everyone– simulates desktop with icons
in and out baskets file folders and documents calculators printers blank forms for letters and memos
– small number of generic actions applicable system wide move, copy, delete, show properties, again, undo, help
– eg same way to move text, documents, etc property sheets
– pop-up form, alterable by user
– What you see is what you get (WYSIWYG)
Slide adapted from Saul Greenberg
Xerox Star continued
Star's observers:– objects understood in terms of their visual characteristics
affordances, constraints
– actions understood in terms of their effects on the screen causality
– intuitively reasonable actions can be performed at any time conceptual model
A subtle thing happens when everything is visible: the display becomes reality
Slide adapted from Saul Greenberg
Is direct manipulation the way to go?
Some Disadvantages– Ill-suited for abstract operations
spell-checker?
– Tedium manually search large database vs query
– Task domain may not have adequate physical/visual metaphor
– Metaphor may be overly-restrictive
Solution: Most systems combine direct manipulation and abstractions word processor:
– WYSIWYG document (direct manipulation)– buttons, menus, dialog boxes (abstractions, but direct manipulation “in the
small”)
Based on slide by Saul Greenberg
Guidelines for Design
Provide a good conceptual model– allows users to predict consequences of actions– communicated thorugh the image of the system
Make things visible– relations between user’s intentions, required actions, and
results should be sensible consistent meaningful (non-arbitrary)
– make use of visible affordances, mappings, and constraints– remind person of what can be done and how to do it
Slide adapted from Saul Greenberg
Summary
Good Representations– captures essential elements of the event / world– deliberately leaves out / mutes the irrelevant– appropriate for the person, their task, and their interpretation
Metaphors– uses our knowledge of the familiar and concrete to represent abstract concepts– need not be literal– has limitations that must be understood
Direct manipulation– visibility of the objects of interest– rapid, reversible, incremental actions– manipulation by pointing and moving– immediate and continuous display of results
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