SIMS 213: User Interface Design &

Development

Marti Hearst

Tues, Jan 30, 2001

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?

Visual affordances of a scrollbar

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

Slide adapted from Saul Greenberg

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

Slide adapted from Saul Greenberg

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

EvaluationExecution

The World

start here

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

Digital Watch Redesigned for Affordances (Rachna Dhamija)

Digital Watch Redesigned for Affordances (Ping Yee)

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

Phone list

List metaphor Rolodex metaphor

Slide adapted from Saul Greenberg

Games

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”)

Slide adapted from Saul Greenberg

Conventional Applications: A Mix

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