A comparative analysis of design rationale representations

(1)

(2)

(3)

(4)

(5)

T55.4

.W2 no.

CSWEY

A

Compararive Analysis

of

Design

Rationale

Representations

Jintae

Lee

Kum-Yew

Lai

March

1992

WP

#

84-92

INTERNATIONAL

CENTER

FOR

RESEARCH ON

MANAGEMENT

OF

TECHNOLOGY

Massachusetts Institute ofTechnology Sloan Schoolof

Management

(6)

(7)

The

International

Center

for

Research

on

the

Management

of

Technology

A

Compararive Analysis

of

Design

Rationale

Representations

Jintae

Lee

Kum-Yew

Lai

March

1992

WP

#

84-92

Sloan

WP#

3295

&

3405

CCS TR#

121 A

revised

and

condensed

version

of this

report appears

in

the

special issue of

Human-Computer

Interaction

on

design

rationale, v.6(3-4),

pp.251-280.

©

1992 Massachusetts

Institute of

Technology

Sloan School

of

Management

Massachusetts

Institute of

Technology

38 Memorial

Drive, E56-390

(8)

d/l.l.T.

(9)

A

revised

and

condensedversion ofthisreportappearsinthe special issue of

Human-Computer

Interaction

on

designrationale,v.6(3-4),pp.251-280

A

Comparative Analysis

of

Design

Rationale

Representations

JintaeLee

and

Kum-Yew

Lai

Centerfor Coordination Science

and

MIT

Artificial Intelligence Laboratory

ABSTRACT

A

fewrepresentationshave been usedforcapturingdesignrationale. Itisimportantto

know

in

what ways

theyareadequateor limitedso that

we know how

to

improve

them. Inthispaper,

we

developa

framework

forevaluating designrationalerepresentations based

on

a set of genericdesigntasks.

We

build the

framework

byprogressively differentiating theelementsof

designrationalethat,

when made

explicit,support anincreasing

number

ofthedesigntasks.

With

thisframework,

we

evaluatetheexpressivenessof the existing representations.

We

also

presentalanguage,

DRL,

that

we

believeisthemost expressiveof the existing representations

without beingtoo

complex

for

human

users.

We

alsodiscuss the limitations of

DRL

as

open

problemsfor further research.

1.

INTRODUCTION

As

the articles inthisissuepointout,anexplicit representation ofdesignrationalecan bring

many

benefits. Sucha representationcanlead to a betterunderstandingofthe issuesinvolved

[McCall1987;

Yakemovic

&

Conklin1990], of thedesign space

[MacLean

etal.1989],

and

of the principlesunderlying

human-computer

interaction [Carroll

&

Campbell

1989]. Itcanalso

providea basis for learning,justification,

and

computational support[Fischeretal.1989;Lee

1990a].

The

extent to

which

we

canactuallyreapthese benefits,however,

would depend

largely

on

thelanguage

we

useforrepresenting designrationale. If,forexample, design

rationale

were

representedinfreetext,the benefits

we

obtain fromit

would

notbedifferent

from

what

we

alreadygetfromthenotes

on

paperthat

we

takeindesign meetings. Also, the

Authors' present address: JintaeLee

and

Kum-Yew

Lai,CenterforCoordinationScience,

MassachusettsInstitute ofTechnology, E40-140, 1

Amherst

Street,

Cambridge,

MA

02139.

(10)

INTRODUCTION

1

2.

WHAT

DO WE

WANT

TO

DO

WITH

DESIGN RATIONALE?

3

3.

WHAT'S

IN

DESIGN

RATIONALE?

5

4.

EXISTING

REPRESENTATIONS

15

4.1. IBIS (Issue Based Information System) 16

4.2. Toulmin's

Model

of

Argumentation

21

4.3.

QOC

(Question,Option,

and

Criteria) 24

4.4. Other Representations 26

5.

DRL

(DECISION

REPRESENTATION

LANGUAGE)

29

5.1. Introduction 29

5.2. Description of

DRL

30

5.3. Evaluationof

DRL

as aDesign Rationale

Language

35

5.4. Relation to

Other

Studies 40

6.

CONCLUSIONS

42

kindsofcomputational supportthat

we

can provide

depends on

what

a representation

makes

explicit

and

how

formaltherepresentationis.

A

few systemshave beenbuilt

and

actuallyused

tocapturedesignrationale orarguments

[Kunz

&

Rlttel1970;McCall1987;Conklin

&

Begeman

1988; Fischer etal.1989;Lee1990a, 1990b;McCall1990],

and most

of

them

usedrepresentations

based

on

the earlierstudiesofdesignactivity

[Kunz

&

Rittel1970]orofargumentation[Toulmin

1958].

However,

thereis

no

systematicattempttojustifythechoiceoftheserepresentationsor

discuss the rationale forusingthem.

This paper is motivated by thefollowing questions:

How

adequate

are the existing representations?

Do

theyallow ustoeasilyrepresent

what

we

want

torepresent? Ingeneral,

how

do

we

evaluatealanguageforrepresentingdesignrationale? Thispaperisan attemptto

answer

thesequestionsby identifying theelementsofdesignrationale thatcould be

made

explicit

and by

exploringtheconsequencesof

making them

explicit. Laying outtheseelements providesa

framework

forplacing the existing

meanings

ofdesignrationaleinperspective, as

well asprovidinga

framework

forevaluatingarepresentationlanguagefordesignrationale,as

(11)

Comparative

AnalysisofDesignRationaleRepresentations 3

We

proceedinthefollowingway. First,

we

identify the tasks that

we

might

want

tosupport usingadesignrationale representation.

Throughout

thepaper,

we

willusethese tasks as a referenceagainst

which

we

would

evaluatetherepresentations that

we

discuss. InSection3,

we

characterizedesignrationaleby presentingprogressively richermodels.

We

startwitha

simple

model

ofdesignrationale,

where

anartifactisassociatedwitha

body

ofreasonsforthe

choiceof thatartifact.

We

thenelaboratethissimple

model by

incrementallydifferentiating

and

making

explicit

what

isimplicitinthe

body

of reasons.

As

we

do

so,

we

discuss

what

each

resulting

model

allows ustodo. These

models

ofdesignrationaleprovidea

framework

in

which

todefinethescopeof arepresentation

and

its

adequacy

withinitsscope.^ Usingthis

framework,

we

evaluatethe existingrepresentationsinSection4. In Section5,

we

presenta

language, called

DRL, which

we

believe is

more

expressive than

most

of theexisting representations

and

overcomes

many

oftheirlimitations.

As

we

describe

DRL,

we

alsodiscuss

itscurrent limitations,

which

we

present as

open

problemsforfuture research.

2.

WHAT DO WE WANT

TO

DO

WITH DESIGN RATIONALE?

To

evaluatea representation,

we

needto

know

what

tasksitisdesignedtosupport.

The

tasks

thatadesignrationalerepresentationcanorshould support can be describedin

many

ways

at different levels of abstractions.^ For example,

[Mostow

1985] lists thefollowing tasks:

documentation, understanding,debugging,verification,analysis,explanation,modification,

and

automation. [Fischeret al.1991] pointsoutthat

documenting

designrationalecan support maintenance

and

redesignofanartifact,reuseofthedesignknowledge,

and

criticalreflection

during design process.

[MacLean

etal. 1991]list

two

majorbenefits

from

designrationale

representation: aidtoreasoning

and

aid tocommunication.

The

tasksofachievingthese

benefitsareelaboratedfurther intermsofsubtasks suchasenablingthedesignerstoenvisage

theavailabledesignalternatives in a

more

structuredway, withtheargumentsfor

and

against

them.

Another

way

ofcharacterizing the tasksistolistthequestionsthat

we

oftenneedto

answer

to

accomplishthegeneraltasksmentioned aboveinthedesignprocess.

To

theextent that

we

want

our designrationalerepresentationtohelp

answer

these questions,answeringthese

Inthispaper,

we

use the terms,model andrepresentation,in the followingway. Modelisan algebraicconcept,andrepresentationalinguisticone. Sincethesamestructurecan be describedin

many

ways,amodelcanhaveseveralrepresentations. Therefore,

when we

wanttodiscussa

structureindependentof a particular

way

of describingit,

we

usetheterm model.

On

theotherhand,

we

usetheterm representationto refer toaparticularnotationfordescribing thestructure.

There are

many

gooddiscussionsofthe benefitsortasks forwhichadesignratioanlecan be used.

(12)

4

questions

become

thetasks that the representationshould support.

The

followingisasetof

representative questionsthat

we

gatheredfrom our experiments with designrationale[Lee

1991a],from walking through examples [Lewisetal.1991],

and

fromcreating scenarios [Carroll

&

Rosson1990].

•

What

isthe status of the currentdesign?

•

What

did

we

discusslast

week and what do

we

needto

do

today?

•

What

are the alternativedesigns

and

what

are theirpros

and

cons?

•

What

are the

two

mostfavorable alternatives so far?

•

Sun

Microsystemsjust released their

X/NeWs

server.

How

would

the releasechange our evaluations?

•

What

if

we

do

not considerportability?

•

Why

isportabilityimportant

anyway?

•

What

are the issues that

depend on

thisissue?

•

What

are theunresolvedissues?

What

are

we

currentlydoing about

them?

• What'stheconsequencesofdoing

away

withthispart?

•

How

did other people deal withthisproblem?

What

can

we

learn

from

thepastdesign

cases?

The

abovelistof thequestionsis

by no

means

completeasthereare

many

possiblepathsthat

we

did not

walk

through

and

many

scenarios that

we

did notconstruct.

We

alsoleftoutthose

questionswhich,though important,

do

not

seem

tobethe jobofdesignrationaleto

answer

(e.g.

How

can

we

compute

thetotalcostofthisdesign?) Nevertheless,thequestionsinthelist

provideauseful

framework

forassessing theexpressivenessofthe differentrepresentations.

When we

discuss the limited or increasedexpressivenessof agivenrepresentation,

we

referto

those questionsthatcanorcannot be

answered

asaresult. Ifourtaskincludes answeringa

question

which

isnot representedinthelist,then

we

canalways evaluatethe representations

by

asking

whether

they

would

supportansweringthequestion

and

ifnot,

what

additional

objects, attributes,or relations

would

havetobe

made

explicit.

We

want

to

emphasize

further that

we

areassessingonlytheexpressivenessofthe existing representations ofdesignrationale. Thatitisdesirableto

answer

thequestionsinthelistdoes not

mean

that

any

representation fordesignrationaleshouldsupporttheansweringofthese questions. Eachrepresentation

must weigh

the costs

and

benefitsinvolvedin tradeoffs

among

threegeneral dimensions: expressiveness,

human

usability,

and

computational tractability (Figure1). Thesetradeoffsshouldinturn be motivated

by

the tasks thatareintendedtobe

(13)

Comparative

AnalysisofDesign Rationale Representations 5

accomplished usingthe representation. In short,

we

arenotdictating

what any

existing

representationshouldorshould nothave.

However,

we

do hope

thattheanalysispresentedin thispaper

would make

the architects of a representationfordesignrationale

more

consciousof

what

theirlanguage canorcannotexpress

and

why.

Domain

Figure1. Elementsincomputer supportedactivities.

To

besure,

we

cannotseparateour concern withexpressiveness entirelyfromotherconcerns such

as

human

usability orcomputationaltractability. For example,ifalanguageis

meant

tobe

usedby peopletocapture designrationales,butifitistoo

complex

forpeopletomanage, then

thereisnot

much

pointinevaluatingitsexpressiveness.

Whether any

ofthe representations that

we

discussfallsinto thatcategoryisan empiricalquestion. Allthelanguages discussed herehave beenactuallyused

by

people,butthatis

no

guaranteethattheywillallsucceedat

their "industrial strength"use [Conklin

&

Yakemovic

1991]. Nevertheless,

we

believeit

would

bedifficulttoevaluatetradeoffs

among

thethreedimensions withoutcalibratingindividual dimensions suchasexpressiveness(cf.

[Brachman

&

Levesque

1984]

on

tradeoffs

between

expressiveness

and

computationaltractabilityforgeneral

knowledge

representation).

3.

WHAT'S

IN

DESIGN

RATIONALE?

What

is design rationale? Design rationalefor an artifacthas at least threedifferent

meanings

initscurrentusage: a historicalrecordofthereasonsfor thechoiceofanartifact

[Yakemovic

&

Conklin1990], asetofpsychologicalclaims

embodied

by anartifact[Carroll

&

Rosson1990],

and

a description of thedesign space

[MacLean

etal.1989].'^

Therepresentationusedin[Yakemovic

&

Conklinl990] describeslogical aswellas historicalaspects ofdesignrationale,as

we

discussfurtherlater. For example,thesupportrelationhas both thelogical

aspectandthehistoricalaspect because,

We

associate[Yakemovic

&

Conklin 1990],aswellas[Lee

1990a;Potts

&

Bruns1988] withthe historicalrecord becauseoneof theirgoalistocaptureand documenttheactualprocessofdesign.

(14)

6

Designrationale often

means

the historicalrecordofthe analysis that ledtothechoiceofthe

particular artifactor the featurein question.

To

illustrate, letus takeas an

example

a particularfeature of theMacintosh operating system,

namely

theplacementofallthe

window

commands

intheglobal

menubar

atthe top of thescreen."*

By

a

window

command,

we

mean

a

command

specifictoa

window,

e.g.

SAVE

isa

window

command

thatsavesthecontents of the

window.

A

designrationalefor thisfeature inthesenseof historicalrecord

would

be something

like: "Theissue of

where

toputthe

window commands

was

raised

by Mark on

January20.

Kevin proposedtheidea of incorporating

them

intothe global

and

pointedoutthatitsaves screenspace(e.g.,as

opposed

toputtingthe

commands

on

each

window,

as in the Starenvironment). Julieobjectedbecauseitrequires along

mouse

travelfrom

thecurrently active

window

inexecutinga

command.

But,

we

decidedtohavethe global

anyway

because people generally agreedthat thattheadvantage, together with others

suchas

more

efficientimplementation,outweighthe objection."

We

canprovide

more

structure

tothishistoricalrecord, as

we

discussinthe rest of thepaper.

Such

structureisusually designedto

make

explicitthe logical structure(e.g.,an

argument

supportsaproposal)

and/or

the historical structure(e.g.,aproposalreplacesanotherproposal).

Another

meaning

ofdesignrationaleisthesetofpsychological claims

embodied by

anartifact

inthesenseof [Carroll

&

Kellogg1989], i.e."theclaimsthat

would have

tobetrueifthe artifactistobesuccessful"or"theclaimsaboutthepsychological consequencesforthe user" [Carroll

&

Rosson

1990]. These claimsare differentfromthe historical record; theclaimsneed notbe presentinthe historical record;eveniftheywere, they

would

havetobeextractedfrom

therecord

and

formulatedina testableform. Forexample,thedesignrationale inthissense

would

besomethinglike:"Theglobal

makes

the

environment

easier tousebecauseit

reducesscreenclutter."or

"Dimming

the irrelevantitemsintheglobal

makes

iteasier to

learnaboutthe

commands."

The

third

meaning

ofdesignrationaleis

one

used

by [MacLean

etal.1989],

namely

how

agiven

artifactislocated inthespaceofpossibledesignalternatives:

what

are theotherpossible alternatives,

how

are these alternatives related,

what

are the tradeoffs

among

them?

Inour

Macintosh

example, thedesignrationale in thissense

would

be

some

descriptionof the

logicallypossible alternatives for placing

window

commands,

how

theyare related,

and what

the tradeoffsare. Itisoftendifficult toprovidesuchadescriptioninasystematic way, but an

* Since theunit ofanartifactisoftenambiguous(e.g.,Macintosh,MacintoshOS,Macintosh

window

system, Macintoshmenubar,or the positionofthemenubar),

we

willusethetermartifactin the

(15)

Comparative

example

isfoundin[Cardetai.1990;

McKinlay

etal.1990],

which

providesavocabularyof the

primitives

and

a set ofcomposition operatorsfordescribingthedesign spaceofpossibleinput

devices. This

meaning

ofdesignrationale

seems

different

from

the first

meaning

in its

emphasis

on

designrationalenotbeingarecordbuta construction,

and

fromthesecondinits

emphasis not

on

a particularartifactbut

on

the relation

among

possible alternatives.

Models

of

Design

Rationale

We

now

developa series ofprogressivelyrichermodelsofdesignrationale,

which

providea

framework

in

which

we

canplace the three different

meanings

ofdesignrationale.

The

first

two meanings

arediscussed immediately below.

The

third

meaning

ofdesignrationale,asa

possibilityspace,isdiscussedshortlyafter.

Designrationaleinthe

most

generalsenseisan explanationof

why

anartifactisdesignedthe

way

itis. Soinourfirst

model

ofdesignrationale,anartifactisassociatedwitha

body

of

reasonsas

shown

in Fig. 2a.

There are differentkindsofreasons that

we

can givefor

an

artifact.

The

reasonscanbe

historical orlogical,roughlycorrespondingtothe

meanings

ofdesignrationale, respectively, asa historicalrecord

and

as asetofclaims

embodied by

anartifact.^

The

recordoftheprocess

thatledtothechoiceofanartifacttellsus

one

kindofreason

why

that artifact

was

chosen.

The

free text

example

aboutthe

window commands

aboveisan example. If

we

wanted

a logical

justification,

we

would

haveto extractitfromthe record;butatleastsucharecordtellsusthe

historicalcircumstances

and

sequencethat ledtothe design,

and

providesa basisfrom

which

to inferthe logical reasons.

On

theother hand,

we

can representthe logicalreasonsdirectly,i.e.

thereasonsjustifyingthechoiceof

an

artifact

no

matter

how

or in

what

order they

were

articulated.

The

setofclaims

embodied by

anartifactisan

example

becausetheseclaims

would

justifythedesignoftheartifact. These claimsare logical also in thesensethat the

contextin

which

theyaretruehavetobe

made

explicit. For example,aclaim should not say "Theglobal

means

"The

We

believethatthe distinctionbetweenhistoricalandlogicalreasons breaksif

we

pushittoofar

becauseapurelyhistoricalrecord persedoes notreallygiveusareason. Itwouldgive usareason onlytotheextentthat

we

canextractsomelogicalstructure outofit. Nevertheless,

we

believethat

the distinctionis useful for the purposeofevaluating representationsbecausefor a given representation

we

wouldliketo

know

whatitmakesexplicitand what

we

haveto inferfromit.

(16)

(a)

MODEL

I:Anartifactisassoaated withabody ofalltheargumentsrelevant to thedesignof the artifact

(b)

MODEL

2:Alternativesandtheirrelationsaremadeexplicitandthearguments aboutindividualahematives can bedifferentiated.

Ct^'^

_''^'k^lr^

'2.^*%a

CTi)

"^^3

Figure 2(a)-(c). Progressively

more

differentiated

models

ofdesignrationale

(17)

Comparative

AnalysisofDesign Rationale Representations

^'ne,^

(d)

MODEL

4:Criteriausedforevaluationsandtheirrelationsaremadeexplicitandthearguments aboutthem can be

further differentiated in theargumentspace.

(e)

MODEL

5;Individualissuesaremadeexplicit,eachofwhichcontains the alternatives, evaluations,andcriteriausedin

discussing the issue.Apart of theargument spaceincludes themeta-arguments aboutthe issuesandtheir relations.

Figure 2(d)-(e).Progressively

more

differentiated

models

ofdesignrationale

(continuedfrom previouspage).

global

Macintosh

because it leads to smaller implementation. This is important in a

system

like the

Macintosh

which

has a small

memory."

Extracting these logicalreasonsisnotan easytask;onceidentified,however, they

(18)

10

The

internalstructure of thesereasons can be

made

explicittodifferentdegrees.At one extreme, theycan be completelyundifferentiated.

An

example

isthe naturallanguagedescription that

we

gaveearlier as

an

example

of a historical record. If

we

were

to

make

the historical

relations

more

explicit,

we

candifferentiatefurtherby

making

explicitsuchroles

and

relations

suchas:Initiator,Second-Motioner,Initiates,

and

Replaces.

An

example

thatisnothistorical

istherepresentationused by[Carroll

&

Rosson1991] fordescribing theclaims"embodied"inan

artifact. Inthisrepresentation, theclaimsthemselvesarerepresentedinnaturallanguage, but

theclaimsare

grouped

bythequestions they

answer

to:

what

canIdo,

how

doesthatwork,

and

how

do

I

do

this?

We

canalsoimaginearepresentation

where

the logicalsupportrelationscan be

made

more

explicit by providing such constructsasLogically-Implies,Supports,Denies,

Qualifies,

and

Presupposes.

We

willusetheterm.

Argument

Space,toreferto

what

we

have

called a

body

of reasons,becausethereasonsarecapturedeither as a historicalrecordof the

various

arguments

relevant for thedesignoftheartifactorlogical

arguments

underlyingthe design.

Thereis

much

we

can

do

with ourfirst

model

ofdesignrationale.

A

representationbased

on

this

model

can help us

answer

thequestion,

"What

did

we

discusslast

week and what do

we

needto

do

today?" Sucha representationcanalsohelp us

answer

the questions:

How

didotherpeople

dealwiththisproblem?

Can

we

learnfromthe pastdesigncases? Carroll

and

Rosson[Carroll

&

Rosson

1991]providea

good

example.

They

report in detail

how

theirrepresentationof

designrationale,mentioned above, suggested

many

issues forredesigninganartifact(the

View

MatcherinSmalltalk).

They

discuss

how

these issuescan be couchedas adesignhypothesis,

which

can betested

and

compiledtoform,inthelongrun,"acontextualizedscienceoutof

practice"of

human

computer

interaction.

Our

firstmodel,however, doesnothelpvery

much

withtheother questions,though

we

should

qualifythisstatementimmediately. Sayingthatitdoes not help

much

isnottosaythat

we

cannot

answer

these questions.

Of

course,iftheuser

works

hard enough,

and

aslongasthe representationbased

on

the

model

has

enough

informationcaptured,evenintheformof natural

languagefreetext,

we

can

answer

these questions. Sothe realissueis

how much

the

model

itselfhelp us

answer

thesequestionseitherby helpingus see the structure better orby enabling

ustodefinecomputationalservices thathelp us

answer

thequestions.

We

willsee

how

more

differentiated

models

allow usto

answer

thesequestions

more

easily,although theyincrease the cost in

some

other

ways

[Conklin

&

Yakemovic

1991].

(19)

Comparative

Our

second

model

(Fig.2b) differsfromthefirstby

making

multiplealternatives

and

their

relations explicit. Design involves formulatingseveral alternatives,

comparing

them,

and

merging

them,as

many

of thequestionsinourlistindicate.Inourfirstmodel, onlya single

alternativeis

made

explicit at agiventime,

and

themultiplealternativesarepresentonly

implicitlyinthe

argument

space.

Our

second

model

make

these alternativesexplicit,including

theonesthathave beenrejected.

Once

the alternatives

become

explicit,

we

cantalkabout

their attributes(e.g.currentstatussuchas "rejected" or'waiting for

more

information'),

make

the relations

among

thealternatives explicit(e.g. specialization, historical precedence), define computational operations

on

them

(e.g.

comparing

alternatives, displaying the

alternatives that specializethisalternative),oreven argue about

whether

analternativeis

worth

considering.

The

alternatives,otherthantheonefinallychosen,are interestingbecause

many

of the issues

and

the

knowledge

usedinevaluating

them

are useful in other contexts, for

example,

when

situational constraintschange.

We

usetheterm. AlternativeSpace,torefer to

thissetofmultiplealternatives

and

theirrelations.

Theserelations

among

the alternativescanalsobehistorical orlogical. Historical relations

may

be not onlythe linearsequencethat

we

usually describe as versions,butalso

more complex

relationssuchas layers

and

contexts

[Bobrow

&

Goldstein1980].

The

logical relations

may

include Specializes,Generalizes,Elaborates, or Simplifies.

Or

alternatives can be related

throughadesignspace [McKinlayet al.1990].

To

the extent that

we

want

a representationto

representthese different alternatives

and

their relations,

we

saythat the alternativespaceis

withinthescope ofthe representation. glBlS, forexample, seemstoincludethe alternative

space withinitsscope because oneofitsgoalsis"tocapturealternative resolutions (including

those

which

arelaterrejected),[and] trade-off analysis

among

these alternatives"[Conklin

&

Begeman

1988].

The

constructsinglBlSforrepresentingthe alternative spaceconsistof:

Position,with

which

we

can describe multiplealternatives,

and

the specialization relation

among

the Positions.

By now,

we

have analternativespace connectedtothe

argument

space, asshovwiin Fig.2b. For

eachofthe alternatives, there arearguments describingthereasonsforitscurrentevaluation,

justas inourfirst

model

there arearguments describingtheevaluationstatus of that single alternative,i.e.,thatit

was

chosen.

Some

ofthe

arguments

can be shared;forexample, an

argument

can support analternativewhile

denying

another; soitisbetter tothinkofthe

arguments aboutthe different alternativesforminga single large

argument

space, as

shown

in

(20)

Once

the alternativespaceisrepresented,

we

canimagine

how we

can

make

asystemthathelp

us

answer

some

ofthequestionsinourlist.

To answer "What

and

what

are their pros

and

cons?,"

we

canassociate an

argument

space with each ofthe

alternativesthroughthe linkssuchasSupportsor Objects-To, asingIBIS.

To answer

"Why

do

we

even considerthis alternative,

and

how

isitrelated tothe

one

that

we

discussedlast

week?"

we

needto

some

historical relations,suchas Rqjlaces,

among

the alternatives.

With

therepresentation of the alternative space,

we

canimagine

how

we

can

make

asystem help us

answer

some

ofthequestionsposedinSection2.

To answer "What

and

what

are theirpros

and

cons?,"

we

canassociatean

argument

spacewith eachofthe

alternativesthroughthe linkssuchasSupports orObjects-To, asingIBIS.

To answer

"Why

do

we

even considerthisalternative,

and

how

isitrelatedto the

one

that

we

discussedlast

week?"

we

needtouse

some

historical relations(e.g.Replaces)or structural relations(e.g.

Is-A-Part-Of),

among

the alternatives.

However,

once

we make

explicitmultiplealternatives,

we

needtoarticulate

more

carefully

what

the

argument

spaceisabout(Fig. 2b). Inourfirst

model

when

we

had

a singleartifact,

namely

thechosenone, the

argument

space contained reasonsforthechoiceof thatartifact.

Similarly,the

arguments

fortheotheralternativesareabout

why

they

were

not chosen,or,to generalize,

why

they

have

theirparticularevaluationstatus, e.g."Still inConsideration", "Waiting for

More

Information","Rejected".

These

evaluation status could be

nominal

categories(suchas theabove examples),ordinal categories(suchas"Very

Good,"

"Good,"

and

"Poor") oracontinuous

measure

(suchastheprobability that the alternative willachievea

givensetof goals).

Therefore,

we

introducetheevaluationspace(Fig. 2c),

where

theevaluationstatus are

made

explicit

and

related. Usually,

we

do

not

and

need notspecify

any

elaborate relation

among

the

evaluationmeasures

we

use. Often,the implicitordinalrelation

among

thesevalues(e.g.

"Very Good," "Good," "Poor," "Very Poor")issufficient

when

we

leaveitforthe

human

userto

assignthesevaluestothe alternatives.

However,

if

we

want

todefine

any

computational

service that

manages

these values, for

example

thatautomatically propagates

and merges

them

toproduceahigherlevel

summary,

then

we

needtobe verycarefulabout

what

these

valuesmean.

We

needtospecifythe units ofmeasurement,acalculus forcombining them,

and

a

model

specifying

what

theymean.

Even

inthe case

where

these actions areleftto

human,

for

example,ifthe

human

userisexpectedto

combine

thesevaluesto

produce

ahigherlevel

summary

measure, then

we

need

to set

down

what

these values

mean

so that their

(21)

Comparative

Making

theevaluation spaceexplicitallows ustodifferentiate

two components

of the

argument

space:(1)

arguments

about

why

an

alternativehas its currentevaluationstatus,

and

(2)

arguments aboutthe alternativesthemselves,e.g.,

why we

shouldorshould noteven consider

an

object as

an

alternativeor

whether

this alternativeis reallyaspecialcaseofanother

alternative. That is,as

shown

in Fig. 2c, these different kinds of

arguments

can be

differentiated in the

argument

space.

With

therepresentation of theevaluationspace,

we

can

now

answer

questionssuchas:

"What

are the

two most

favorable alternatives sofar?"

and

"Sun

Microsystems just released their

X/NeWs

server.

How

would

the release

change

our

evaluations?"

We

canalso explain

how

an evaluation

was

made

by pointingtothearguments

inthe

argument

space behind the artifact in question,

and

explaining

how

this particular

evaluationmeasureisderivedor

computed

from

them

or relatedtoothermeasures.

Our

modelssofar

do

not

make

explicit thecriteriausedinproducinganevaluation.

However,

thecriteriausedfor theevaluation

and

theirrelations are usually quiteimportanttorepresent

explicitly. For example,itisimportant to

know

that the

argument

"We

do

not

need

to

duplicate

which

isusedas a criterion for evaluation.

By making

this criterionexplicit,

we

can

group

alltheargumentsthatappealto thiscriterion

and weigh them

against

one

another. Ifthe criterionchangesor

becomes

lessimportant, then

we

can

do

appropriatethingsto allthe

arguments

thatpresupposethisgoal(forexample,

making

theseargumentslessimportant).

Knowing

how

thiscriterionisrelatedtoothers(e.g.

"Reducing ScreenClutter"isa

way

ofachieving "EasytoUse"), alsoallows ustoassignproper importancetothiscriterionorchangeitsimportance,

when

the relatedcriteriachanges.

We

usethetermcriteriaspacetorefertothesecriteria

and

their relations.

As

Fig.2d shows, once

we

have

the criteriaspace explicit,

we

can further differentiate the

argument

space

by

groupingthosearguments

which

areaboutthecriteria

and

their relations.

Hence,itisimportantthatalanguage

whose

scope includesthecriteriaspace representthe

different attributes of thecriteria

and

the relationship

among

them. For example,itshould allow ustorepresenttheimportanceofthesecriteria

and

the synergistic or tradeoff relations

among

them.

A

setofcriteriacan besub-criteria ofanotherinthesensethat satisfying

them

facilitatesthe satisfaction of thelatter. Thesesub-criteriacan berelated

among

themselvesin

various ways.

They

can be mutually exclusive inthe sensethat satisfying

one makes

it

impossibletosatisfyothers.

They

can be independentofeachotherinthesensethat satisfying

(22)

theirparentcriterion invarious

ways

as well.

They

can be exhaustiveinthesensethat satisfyingallof

them

isequivalenttosatisfyingthe parent.

With

thecriteriaspacerepresented,

we

can

now

see

how

thesystemmight beabletohelp us

answer

thequestionssuchas:

what

if

we

do

not considerportability? or

why

isportability

important

anyway?

The answer might

be"If

we

give

up

thegoalof portability,thenthe

evaluationof thealternative

X

changesto"High" becausealltheseclaimsthatargueagainst

X were

based

on

theimportanceof portability."or"Portabilityisimportant becauseitisa

subgoalofanother importantgoal,

Have

a

wide

distribution." Theseanswers can be derived

from

arepresentationiftherepresentation

makes

explicitthe relation

between

evaluations,

criteria,

and

arguments.

Of

course, representation of thecriteriaisnotsufficient foranswering

these questions. It isnotobvious

how

thesequestionscan be answered evenif

some

parts ofthe

criteriaspacearerepresentedexplicitly.

However,

the explicitrepresentationof thecriteria

spaceseemsanecessaryconditionif

we

aretoanswerthese questions. Atleast,

we

would

have

theinformation necessarytodefinean operationthat willgiveorsuggesttheanswersto these questions.

We

willgive

some

examplesofsuchoperationslaterinSection5.

Sofar,

we

have

identified

and

discussedthe structure of a singledecision underlying

an

artifact,

namely which

ofthe alternativedesignsshould

we

choose?

However,

withthe

representation ofsuchlocalstructures alone,

namely

its

argument

space, alternativespace,

and

criteriaspace,

we

stillcannot ask

some

ofthequestionsin thelistsuchas:

What

are the

unresolved issues

and

what

are

we

currentlydoing about

them?

What

are the issues that

depend

on

thisissue?

To answer

suchquestions,

we

needa

more

global picture of

how

individual issues are related.

A

decision often requires

and

/orinfluences

many

other decisions.

Forexample,adecisioncan beasub-decisionofanotherifthelatterrequires

making

thefirst

decision.

A

decisioncan bea specialization ofanotherifthefirstdecisionisa

more

detailed case of thesecond. Itisimportanttocapture

how

these decisions are related,

and

we

usethe

termIssueSpacetorefer tothem.

A

unitin thisissuespaceis,therefore,a single decision that

hasasitsinternal structure theother spaces, as

shown

in Fig. 2e.

Once

we

have anissue asan

explicitelement,

we

canassociate the attributessuchas "Status"

and

"Actions

Taken"

with

issues

and answer

questionssuchas

"What

are theunresolvedissues

and what

are

we

currently

doing about them?" Representingthe

dependency

relation

among

the issues willallow usto

answer

thequestion

"What

arethe issues that

depend

thisissue?"

Therearestill

some

questionsthat

we

have notyetcovered suchas:

How

didotherpeopledeal

(23)

Comparative

spacessofaridentified,thespacesofarguments,alternatives,evaluations,criteria,

and

issue,

cancontain

enough

informationtoanswerthese questions.InSection5,

we

discusscomputational operationsthathelp us

answer

theseadditionalquestions

by

exploiting the structure of the

fivespaces.

4.

EXISTING

REPRESENTATIONS

Thereareonlyahandful ofrepresentationsthathave been used forrepresenting design

rationale[Toulmin1958;

Kunz

&

Rittel1970;Marshall1987;McCall1987;Conklin

&

Begeman

1988; Potts

&

Bruns 1988;Lee 1990a;

MacLean

etal.1991]. Most,though notall,ofthese representations areheavily influencedbythe IBISstructure forrepresentingissues

[Kunz

&

Rittel1970]or

by

Toulmin's

model

ofargumentation[Toulmin1958].

[MacLean

etal.89;

MacLean

91] alsoproposea representationbased

on

theirexperienceswith constructing design

rationales. Inthenextthreesubsections(4.1-4.3),

we

discuss these three representationsin

detailby definingtheirscope

and

evaluating

them

withinthe

framework

outlined above. In thenext subsection (4.4),

we

discuss a few other studies thatbear

on

designrationale representation. InSection5,

we

presentalanguage,called

DRL, which

we

believe

overcomes

many

ofthe limitations of the existinglanguagesthat

we

discussin thissection.

Some

qualificationsareinorderbefore

we

proceed.

Our

intentionisnotto crihcize the existing

representations,but onlytoevaluate

them

asadesignrationalerepresentationlanguage.

Even

then,our evaluation is

mainly

with respect to theirexpressiveness.

We

would

liketo

emphasize

againthattheadequacyof a representationcan be evaluated only withrespecttoa

set of tasks.

One

representation

may

be

much

more

expressivethananother; as a result

we may

beableto

do more

withit.

However,

if

what

itenablesisnotinthesetof desirable tasks,orif

itenables thoseonly

by

sacrificingother

more

importantconstraints,thentheadditional expressivenessisnotworth

what

itgivesus.

What

we

discuss

below

isonly

what

the existing representationsallowor

do

not allow ustodo;

We

do

notintendto

make

value judgments about whethertherepresentationsshouldorshould notdo.

Some

oftheselanguages

were

designed withdifferent goals,

which

inturndeterminethe tradeoffsadoptedinthe designs. Therefore,

thefollowingdiscussionshould not be construedas a criticism of these representations,but only

asan articulation oftheir scope

and

their

adequacy

asdesign rationalerepresentation languageswithrespecttoexpressiveness.

We

also

want

to

make

clearthat

whenever

we

saythata representationcannot express

some

information, it does not

mean

that people cannot infer that information

from

the

(24)

Comparative

representation.

Take

a naturallanguagerepresentation ofdesignrationale that

we

gaveasan

example

inthebeginningofthepaper. If

we

keepa detailed

enough

recordof

what

happened,

orevenavideo recordingof the

whole

designprocess,

we

canalwaysretrievetheinformation

thathasbeen ever expressedif

we

work

hardenough.

When we

saythat arepresentation cannot express

some

information,

we

mean

that therepresentationdoes not provideconstructs

that

make

theinformationexplicit insucha

way

thathelp peoplesee thestructure betteror

that

makes

it

amenable

tocomputational manipulation.

4.1.

IBIS

(Issue

Based

Information

System)

The

IBISstructure

was

originally developedin

[Kunz

&

Rittel 1970] forthe

purpose

of

representingdesigners'argumentationactivities. Sincethen,

some

variationsofithave been used

by

afew systemsforrepresentingdesignrationale.

One

variationistherepresentation

usedby gIBIS [Conklin

&

Begeman

1988],"ahypertexttool forexploratorypolicy discussion. SincegIBISis

most well-known

[Conklin

&

Yakemovic

1991]

and

can be regardedas a

modem

incarnation of theoriginal IBIS ,

we

use gIBIS as the contextfor discussing theIBIS

representation.^ Othervariationsinclude

PHI

(ProceduralHierarchyof Issues)[McCall1987]

and

the

one

usedby[Potts

&

Bruns1988] for the rationale

module

in theirrepresentation.

We

discuss

them

briefly inSection4.4.

The

goal ofgIBISistocapture"thedesignproblems,alternative resolutions, tradeoff analysis

among

these alternatives,

and

the tentative

and

firm

commitments

that

were

made

in the

processof thedecision

making

process". Figure3

shows

the objects

and

relations thatformthe

languageofgIBIS

and

Figure4

shows

an

example

representation. IngIBIS,

one

raisesanIssue

suchas

where

toputthe

window

commands.

Positionsare created to Responds-to the issue(e.g.,

"intheglobal

menubar

atthetop of thescreen.",

and

"Commands

atthetopof eachwindow.")

Arguments can becreatedtoSupportor Object-to a Position. For example,the

argument

"Don't needtoduplicate

commands

foreach

window

"supportsthefirstposition,

and

the

argument

"Requireslong

mouse

travel."objects toit.Also,anIssuecanberelated to other objects as

shown

in the figure.

The

gIBIS

model

extends the original IBIS

model by

introducing: the generalize/specializerelation

among

Arguments

aswellas

among

Positions,

an

additional

We

believethatthe distinctionbetweenhistoricalandlogicalreasonsbreaksif

we

pushittoofar

becauseapurelyhistoricalrecordpersedoes notreallygive usareason. Itwouldgiveusareason onlytotheextentthat

we

canextractsomelogicalstructureoutofit. Nevertheless,

we

believethat

thedistinction is useful for thepurposeof evaluating representationsbecausefor a given representation

we

wouldliketo

know

whatitmakesexplicitand what

we

havetoinferfromit.

(25)

Comparative

C_J_^>

Figure3.

The

glBIS vocabulary.

f Commandsatth^lCommandsatthd

Itopofthewindow.IlsideotthewindowI

C ^"-/"•" responds-to

^ ^ 'I commanasrea

(^N°J—

responds-to

V

jistjacting?

_J

Dimmedcommandsstilldistracting

bytheirmerepresence.

(26)

Other type

which

serveasan"escape"

mechanism

forrepresentingthingsnot representable withthe available constructs,

and

anExternaltypeforexternal objectssuchas

documents

or

sketches. Inthefollowing,ourdiscussion referstothestructure

common

tobothIBIS

and

gIBIS except

when

we

discuss the generalize/specialize relation for Position

and

Argument.

The

scopeof thegIBISrepresentation

depends

on what

anissueis. If

we

takeanissueinavery general sense to

mean

any

question that takes a set of Positions, then the

Issue/Position/Argument structurecan representa fairlylargepart of thedesignrationale spaces.

The

internalstructure ofthese spaces,however,isnot welldifferentiated in gIBIS, as

we

discussbelow.

The

alternativespaceisrepresentedingIBIS

by

Positions

and

the relation

among

them. Since

multiplePositionscan becreated for agivenissue,gIBIS allowstherepresentation ofmultiple

alternatives,thusoffering at least the richness ofour

model

2.

The

onlyrelation,however,

among

thePositions that

we

can representingIBISistheSpecializes/Generalizes relation,

althoughthereareotherrelations thatcan connectaPositiontoobjectsofother types(e.g.

Questions or ls-Suggested-By)7 Thus, gIBIScan

answer

thequestion

"What

are theother

alternativesbeing considered?" but notthequestion

"How

arethese alternatives related?"

unlessthey

happened

toberelated viathe Specializes relation.

The

unit oftheargumentspaceingIBISisArgument.

An

Argument

(e.g."The

commands

not

applicabletothecurrent

window

would

be

shown and

distracting.")can supportor objecttoa

Position(e.g."Intheglobal

menubar

at thetopof the screen"),indicating areason

why

the

positionisorisnota

good

one.Thereis

no

construct,however,forqualifyingan argument. For example,

we

cannotindicate thatthe existingArgument,"theglobal

Argument

"theglobal

become

so bigitself"isvalid.

The

relation

among

the

arguments

isagainlimitedtothe Specializes relation. Thislimitation

implies that

an Argumentcannot

Supportor Object-toanother

Argument

directly. For example,

the

Argument

"Irrelevant

menus

can be

dimmed"

isan

argument

objectingtothe original

Argument,

"The

commands

not applicable to the current

window

would

be

shown

and

distracting."

To

expressthisrelationship

among

ArgumentsingIBIS,

we

havetocreateanIssue

about

whether

the original

Argument

isrightornot,create Positions "yes"

and

"no",

and

then

(27)

Comparative

argue aboutthese Positions.^

A

possible criticism ofthisrepresentationisthatitleadstothe proliferation ofIssues. For example,inordertojustpointoutthe factualinaccuracyof a claim,

we

would have

tocreate three

new

objects: a

new

Issue,a"No"Position,

and

tin

Argument

supportingthe

"No"

Position.

However,

we

might beabletoreducethecomplexitywithan

interface thathidestheintermediatedetails.

A

more

serious limitation ofthisrepresentation

isthatit does not help us

answer

questionssuchas

"Show

me

alltheclaimsthatrespondto thisclaim."

We

might

tryto

answer

thesequestions

by

followingtheIs-Suggested-Bylink thatconnectsthe originalclaim

and

the issues thatcontaintherespondingclaims.

However,

theIs-Suggested-By link istoogeneral for that

purpose

because itdoes not allow usto distinguish the issues that containrespondingarguments fromthose that

do

not.

IngIBIS

we

cannotargue aboutrelationalclaims. For example,thereisno

way

ofsayingthat

we

agreewith

A

and

B,but notthat

A

Supports

B

becausea linkingIBISisnotanobject that

you

canargueabout.

To

illustratewith another example,suppose

we

want

toargue about

whether

portabilityshould bea criterion forevaluatingtheposiHonsforagivenissue.

The

only

way

we

can

do

soinIBISisbycreating a

new

Issue,say"Should

we

considerportabilityas

acriteria?",

and

suggestpossibleanswersintheformof positions,say"Yes,

we

should" or"No,

we

shouldnot". Then,

we

canargueaboutthese positions.

We

canalso relatethis

new

issueto

the originalissue

by

anIs-Suggested-By relation.

However,

thisrepresentationagaindoes not

make

explicitthe relationbetweenthe

new

issue

and

thecriteriabeing questioned. If

we

make

a relationanexplicit object,then

we

canarguedirectlyaboutthe relation, Is-a-Criteria-for,

between

the object portability

and

the original issue.

Beingabletoargue aboutrelationalclaimsisimportantforotherreasonsas well. People often

want

tosaythatthey

may

agreewith

A

and

B but not withtheclaimthat

A

supportsB. For example,

one

may

agreethat the global

menubar

isa

bad

idea

and

thatseeingirrelevant

commands

isdistractingbut notthatthesecond claim supportsthefirst. Thatis,

one

may

believe thatthe global

menubar

isa

bad

ideabut not becauseitdistracts theuser

by showing

irrelevant

commands;

he

may

point outthatthe global

menubar

does not

have

to

show

the

irrelevant

commands.

The

user

may

want

to

make

the distinction clearbecause they

do

not

want

tobe construedasdenyingeither of theclaimsalone.

The

distinctioniseven

more

crucial

when

therepresentationistobeusedtoprovide computationalservices. Mixingdenial of

node

Anotheralternativeistoshowthefirstargumentassupportingthepositionthatthe secondobjects to,instead of objectingtothesecondargumentdirectlybecausean argumentcannotrespondto

anotherargumentingIBIS. This representation does notmakeexplicitthe relationbetweenthetwo arguments. Probably forthatreason, gIBISdoesnotencouragethisrepresentation (Conklin, personalcommunication)

(28)

20

withdenial of linkjustisnotrightsemantically. If

you

define acomputationalservicebasedon

thistypeof carelesssemantics,suchasevaluation

management

thatpropagates

and

merges

evaluations,

what you

get

would

not

make

much

sense.

The

evaluationspaceused

by

gIBISconsistsof

some

nominalcategoriessuchas"Rejected"

and

"Chosen"

assignedtothe Positions.

We

could usefinercategories,suchas"Waitingfor

More

Information," but

beyond

thatitisnotclear

what more

sophisticatedevaluationmeasures can

be defined

on

thegIBISstructure.

We

couldassignuncertaintymeasurestothearguments,

and

try toderive

some

ordinalranking

among

thepositionsbased

on

supportingor objecting

arguments.

However,

itisdifficult toimagine

what

thecalculus

would

belike.

The

criteriaspaceis

beyond

thescopeof gIBIS. gIBISprovides

no

vocabularyfordescribingthe

criteria

we

useinevaluatingthe alternatives. Thislack of explicitrepresentation ofcriteriain

gIBISisa serious limitation for adesignrationalerepresentationlanguage. Sincecriteriaare

notexplicit,

we

cannotargue about them;

we

cannot representthereasonsforhavingthese

criteria; nor can

we

indicate

any

relationship,suchasmutuallyexclusiveness,

among

the

criteria. Again,

we

couldindirectlyrepresent these relations

by

creating additionalIssues. For

example.Fig.4

shows an

Issue("Should smallimplementation bea goal?") relatedtoour

original Issue viaan Is-Suggested-by link. But, again, this representationdoes not

make

explicitthe relation

between

thisissue

and

the criterion in question. Itwillbedifficultfor

peopletoseethisrelationship;itwillbe even

more

difficultfor amachine.Lackof explicit

representation of thecriteriaentailsother limitations. Forexample,

when

goalschange,there

is

no

easy

way

to

accommodate

the changes. It is

more

difficult to isolate the real

disagreements

among

people becausethecriteriathey useintheirarguments remainimplicit.

As

we

will see in thenextsection,explicit representationofgoals can provide

modular

representation ofarguments,multipleviewpoints,

and

a basis forrelevance matching.

InglBlS,theunit of the issuespaceisanIssue,

and

gIBIS providesseveralconstructs for

describingthe relationsaboutissues.

An

IssuecanGeneralize, Specialize,Replace,Question,

and

Suggestanother Issue. Figure4

shows

that the original Issueaboutthe placementof

window commands

suggests other Issues:

whether

the

command

pop-up

and

whether

we

shouldhavesmallimplementationas a goal.

An

IssuecanalsoQuestiona PositionoranArgument; anIssuecan be Suggested-bya Position orby an

Argument.

Theserelationsarequiteimportantindescribinga

more

global relationship

(29)

Comparative

differentiated, the relationssuchasReplace

and

Specialize

seem

essential. It

would

benice,

however,if

we

can

somehow

show

whetheragivensetof relationsiscompleteoradequate.

4.2.

Toulmin's

Model

of

Argumentation

Stephen Toulmin,a philosopher,proposeda

model

of

argument

in1958. Since then, the

model

hasbeen adoptedfor

many

purposes, includingthecomputationalrepresentation ofarguments

[Bimbaum

et al.1980;

Lowe

1986;Marshall1987; Streitzet al.1989]. Figure 6

shows

an

example

Toulmin

representation ofan argument.^

CLAIM

commands neednot be duplicated for eachwindow

X

usually Placingwindow commandsinthe

globalmenureduces

screenclutter.

No

duplication

savesspace.

unless

5

on-account-of

tlieglobalmenu

containingallpossible

commandsbecomes

so big

Common

Sense Knowledge

Figure5.

An

example

ofan

argument

inToulmin'srepresentation.

A

Claimisthe

main

assertionbeing

made

(e.g.,"Placingthe

window commands

intheglobal

We

supporttheClaim

by

producinga

Datum,

"Commands

need

notbe duplicatedforeach window."^^

We

justify the leap

from

the

Datum

to theClaim

by

producingaWarrant,e.g. "If

you

need notduplicate things, thatsavesspace."

A

Warrantis

usuallyaninference ruleor aprinciple that licenses the inferencefrom

Datum

toClaim.

A

Warrantisnotoften absolute; soif

we

want

toexpress

how

stronglythisWarrantsupportsthe

inference

from

the

Datum

totheClaim,

we

usea Qualifier. Inour example,the Qualifieris

We

will usetheterm Toulmin's representation rather than Toulmin's modelbecause

we

are

interested inthemodeltotheextentithasbeenorcan be usedas arepresentation.

Aspointedoutin

[Newman

&

Marshall90],Toulmin wantsthedatumtobeasingularfact,suchas

somethingthatyoucan pointto,because

we

thinkhestillwantstoretainthesyllogisticform,where onegetstheconclusionbyhavingastheminorpremise,asingularfact.Sohe seemsshllunderthe

boundof syllogism,atleast atthetimeof his writing,despitehis rebellionagainstit.Thedatumofour

exampleis amoregeneral statement. But

we

usethisexamplefirstbecause thedistinctionitself

betweenasingularfactandauniversalstatementhasbeencalled intoquestioninphilosophyand

(30)

22

"usually",

which

qualifies the strength of the inference.

We

indicate the excepting circumstances

where

theWarrantfailsthroughRebuttal(e.g.,"Theobject tobesharedmight havetobelargerthanthe

sum

ofthese objects locally existing.') IfaWarrantiscalled into

question,then

we

appealtoaBacking,i.e.,a

body

of

knowledge

thattellus

why

ingeneral

we

shouldaccept thewarrant. Inour example,

we

appealtoour

commonsense

knowledge. Other

typesofbackingmight bephysical laws, historicalfacts,orlegalprecedents.

The

scopeof the

Toulmin

structure as adesignrationalerepresentationislimitedto

what

we

called the

argument

space.

The

goal of

Toulmin

structureisto

make

explicit

how

aclaimis

supported. Thereis

no

attempttorepresentalternatives,goals,

any

evaluationmeasure,or

how

theyare related.

As

such,

we

evaluateToulmin'srepresentationonlyasa representation

module

forthe

argument

space,

which might

interfacewith representationsfor theother

elementsofdesignrationale.

Even

then,

we

should keepin

mind

thatthe originalgoalof

Toulmin

was

to delineate the logicalsupportstructureofan argument, notnecessarilytoprovide

a representation tobeused dynamicallyforcapturingdesignrationale.

As

a representation for the

argument

spaceofdesignrationale,Toulmin's has

many

limitations.

Forexample,

we

can only

deny

aClaimby supportingitsnegation.

Suppose

you want

to

deny

thattheglobal

command

items(letus

assume

thatthisisbeforepull-down

menus were

invented).

The

only

way

to

do

soisbycreating aClaim"theglobal

and

presents "the global

Datum

forthis

new

Claim.

Whether denying "A

reducesB"

means

the

same

assupporting

"A

does not reduceB",itis

certainlyan

awkward way

ofdenyingaClaim;

and

in

any

case,without

some

construct that indicates that the

two

Claimsare thenegationofeachother,thedenial relationship

would

not beexplicit atall.

Even

withsuchconstruct,such

awkward

representation

would

slow

down

human

comprehensionor

machine

computation.

Some

people

[Bimbaum

etal.1980]have solved

this

problem by

explicitly

naming

the original link

between

Datum and

ClaimasSupports

and

introducinganotherlinkcalledAttacks. So Toulmin's representation should be extendedat

least this

much

to

accommodate

denyingclaims.

Itisalsonoteasyto qualifyaclaimusing Toulmin'srepresentation.

We

canqualify aWarrant using Qualifier

and

Rebuttal,butif

we

want

to qualify aClaim or a

Datum,

thereis

no

correspondingconstruct.

We

couldofcoursebuild into theClaim

any

qualificationso that the

Claimitselfisoftheform, "Global

Comparative

purposeof

making

these

components

distinctistoallowustosee the relations

more

clearly, to

formallymanipulatethese

components and

relations,

and

toincrementallyconstruct pieces of theargument.

Toulmin'srepresentation also suffersfrom buildingintoobjecttypescontext-dependentroles,

suchaswarrantor backing.

The

differencebetween warrant

and

backingisnotinthenatureof

the objectitselfbutonlyinthefactthatbackingissomethingthatsupports warrant.

The same

objectcan beaWarrantor aBackingoreven Claimor

Datum, depending on

the context. So

either

we

assign atypetoanobjectbased

on

the inherent properties of the object

and

indicate

itscontext-dependentrolein

some

otherway,as

we

show

inSection5,oranobjecttype should dynamically

change

depending

onthe context.

Most

oftheToulmin-basedrepresentations

[Marshall1987; Streitzetal.1989;

Newman &

Marshall1990] take thelatterapproach. For example,if

we

want

tosupporta

Datum

object,

we

firstchangeitsrolefrom

Datum

toClaim,

theninstantiatethefive-component schema,

and

fillitin.

However,

thenitisnotclear

why

we

need

aseparatetypecalledBacking. If

we

want

tosupport or

deny

aWarrant,

why

not

changeitsroletoClaim,instantiatethe

now

four-components(thatis,the

Toulmin

structure

without Backing) schema,

and

usea

Datum

tobacktheClaim? Itisnotclear

what

isspecial

about Warrantthat

we

needaseparatetypeforitssupport

when

we

do

notneedaseparate

typetosupportother thingssuchas aDatum.

We

understandthatToulmin's reasonforintroducingBacking as a separatetypeistoprovidea

categoryfor thekindof

knowledge

thatisrarely called into question,suchas the written law,

common

senseknowledge,or physical laws.

We

agreethatforan

argument

toberesolved, there

must

be somethingthattheinvolvedpartiescan agree

and

appealto.

However,

we

believe

thatitismisleadingto representsuch

knowledge

as a distinct

component

ofa rigid structure

suchasToulmin's. For

one

thing,thefive-component

schema

leadspeopletobelieve thatan

argument

hasto

make

explicitallthese

components

tobe complete. Thisbelief often forces

peopletounnecessarily representtheobvious;for

example

many

warrants,

when

explicitly represented, are often asobviousas theoneinour example, "Noduplicationsaves space". Also

often in the process,peopleare ledtoargue about whether somethingisawarrantorbacking,

ratherthan aboutthesubstanceof theargument. Conversely,theclosednatureof the

schema

leadspeopletobelieve thatifan

argument

hasthese fivecomponents,itiscomplete. But,

havingfive

components

isarbitrary,becausetheremighthavetobe multipleData,multiple

Warrants, orchainofWarrants before

we

get tosomethingthat

we

agreeon.

We

believe that

itisbettertodefinetherepresentation inan open-ended

way

thatallowsas

many

objectstobe createdas

needed

withoutnecessarilyspecifyingthe

boundary

ofan

argument and

without

(32)

requiring theobvioustobe representedunlessitneedstobe,e.g.,unlessitiscalled intoquestion

orarguedabout.

Thereare

numerous

otherlimitationswithToulmin'srepresentation, as

[Newman

&

Marshall

1990] pointout fromtheirexperiencesofusing Toulmin'sforrepresentinglegalarguments. For example, they

had

to

make

Rebuttalhaveatleastfouradditional

meanings

torepresentthe

differenttypes of objections,eachcorrespondingto

what

isbeingobjectedto.

We

believe that these differenttypesofobjectionsshouldexplicitlyindicate

what

theyobjectto,

and

that

we

can

do

soina gracefulway,as

we

hope

to

show

inSection5.

4.3.

QOC

(Question,

Option,

and

Criteria)

QOC

isarepresentationproposed by

[MacLean

et al.1989,1991] for "constructing"design

rationale.Designrationalein

QOC

issaidtobe notarecordofthedesignprocess,butir\steadis

aco-productofdesignthathastobeconstructedalongsidethe artifactitself. Thisemphasis

makes

QOC

differentfrom,saygIBIS,

whose

goalat leastincludescapturingthe rationale asit

unfolds. I DATA I In globalmenubar allopofscreen Oneach window Positiveevaluation Negativeevaluation DATA: commandsnot applicable

tocurrentwindowwould

(33)

Comparative

The

majorconstructs of

QOC

are straightforward

and

map

clearlytothe

framework

proposedin thispaper. Figure6

shows

an

example

representedin

QOC.^^ The

unit of the issuespacein

QOC

isaQuestion.

The

unit of the alternativespaceisan Option. Questions

and

Options roughly correspondtoIssues

and

PositionsingIBIS.

However,

unlike gIBIS,

QOC

canrepresent thecriteria space with Criteria.

A

Criteria (e.g.

Reduce

ScreenClutter)issaid to bea

"bridgingcriteria"if itisa

more

specific

one

thatderivesitsjustificationfroma

more

general

one

(e.g.Easytouse).

The

units ofevaluation spaceare links labelledwith "+"

and

"-,"

correspondingtowhether an option doesordoes not achieveagivencriterion.

The

constructsfor

representingthe

argument

spaceareData, Theory,

and

Mini-Theory.

One

supportsthe

evaluation ("+" or"-") ofan option withrespecttoa criterion

by

appealingtoempiricalData

(e.g.

"The

mouse

isaFitt's

Law

Device") ortoan accepted Theory(e.g."Fitt'sLaw").

When

thereis

no

relevantdataat

hand

orexistingtheoryto

draw

on,thedesigners

may

haveto

constructa Mini-Theory,

which

is

an approximate

explanation of partof the

domain.

[MacLean

etal.1991]provideanilluminatingdiscussion of the otherformsof justifications for

design,suchas variousformsofdependencies

and

metaphors, nospecificconstructs arediscussed

forrepresentingthem.

QOC

as

we

understandithasa

number

of limitations as a representationlanguage. First,in the

argumentspace,constructs like Data, Theory,orMini-Theory

do

not

seem

tocapture

many

aspectsofarguments. For example,it isnotclear

how

an argument

suchas "Irrelevant

connmands

canbe

dimmed"

should betreatedgiventhatitisneither a piece of empiricaldata

noratheory.

Nor

is itclearwhether

and

how we

can argueabouttheories, or individualclaims

intheories. Inthe alternative space, thereisa referencetocross-option dependency, but

no

specificconstructs arediscussedforrepresentingit. Inthecriteriaspace. Bridging Criterion

seems

to treatedas a fixedtype, ratherthanarolethata criterionplays withrespect to

another criterion.

However,

building a role into a fixed type results in unnecessary

ir\flexibility. Thatis,beingaBridgingCriterionisnotaproperty inherentinthe objectitself

butisarelationthatthe objecthas toanothercriterion.

As

such,

one

should not

have

to

classify agivenobject asaBridgingCriterionbutinstead indicateitsrolethroughthe relation

ithaswith anothercriterion. Otherwise,

we

have

to unnecessarily

change

object types depending

on

whichobject

we

focus on.

The

QOC

constructs for the evaluation space are"+"

and

"-"links. These evaluationsare said tobe supported

by

appealingtoempiricaldata

and

acceptedtheories. Itisnotclear,however.

1^

_We

could represent onlyapart ofourexamplebecauseitwasnotclear tous

how

torepresent the