R. D. L UCE
Measurement representations of ordered, relational structures with archimedean ordered translations
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MEASUREMENT REPRESENTATIONS OF
ORDERED,
RELATIONAL STRUCTURES WITH ARCHIMEDEAN ORDERED TRANSLATIONSR.D.
LUCE *
During
thepast
12 years, research into theconditions
under which mea- surementis feasible -
thatis,
the characterization of thosequalitative, ordered, relational systems
thatadmit
numericalrepresentations -
has pro-gressed
fromaxiomatizations
ofhighly specific systems (see,
forexample, Krantz, Luce, Suppes
&Tversky, 1971,
andRoberts, 1979)
to a far more com-plete understanding
of the entire range ofpossibilities.
At least this istrue for structures that are
highly symmetric - technically, homogeneous.
The notes that
follow,
which areonly
a minormodification
of the handout used at thetalk,
summarize thekey concepts, definitions, major results,
andselected references. The
style
is terse with little comment and noproofs.
For more a
discursive discussion,
see Luce and Narens(1987)
and Narens and Luce(1986);
for more technical surveys seeLuce, Krantz, Suppes,
andTversky
(in press)
and Narens(1985).
There
is,
of course, aclosely
relatedmathematical literature
on order- edalgebraic systems,
inparticular
ordered groups andsemigroups.
For anearly
summary see Fuchs(1963),
and for a more recent one,covering
the nume-rous
developments
after 1960through
the late1970s,
see Glass(1981).
To thebest of my
knowledge,
the resultsreported
here do notduplicate exactly
thosein the
mathematical literature.
To some extentthis
may be due to themotiva-
tion andguidance
of our workissues by representing empirical information
numerically - by
measurement structures.* Harvard
University, Cambridge,
MA02138,
USA.This work was
supported by National
Science Foundationgrant
IST-8602765 to HarvardUniversity. Final preparation
wascompleted
while the author was a Fellow at the Center for AdvancedStudy
in the BehavioralSciences
with financialsupport provided by National
Science Foundationgrant BNS-8700864
andby
the Alfred P. SloanFoundation.
ORDERED RELATIONAL STRUCTURES
Ordered Relation
Structure :A
is
anon-empty
set(of empirical entities
ornumbers),
k
is
a total(or, sometimes, weak) order,
J
is
anon-empty
set(usually integers)
called an indexset,
and foreach j
inJ, S.
Jis
arelation
offinite
order on A .Ordered
Humerical
Structure :where R
is
a subset of the real numbers Re .IsotMTphism : Suppose a,
and a.’ arerelational
structureswith (i)
the sameindex
setJ ,
(ii)
foreach j in J ,
order(S.) =
J order(S.,) .
Jis
a one-to-onemapping
from A onto A’ such that for alla,b
EA ,
and for
all j
E J and alla1 ,a2, ... ,a n (j)
n j EA ,
Numcrical Representation :
Anisomporphism
of ainto (onto)
anumerical
structure
R .
AUTOMORPHISMS OF ORDERED RELATIONAL STRUCTURES
Aut080rphism : An isomorphism
of a structure a, ontoitself. Let J
denote the set ofautomorphisms, which
is a group under functioncomposition.
Translation : Any automorphism
thateither
has no fixedpoint
oris
theidentity,
i .Dilation : Any automorphism with
a fixedpoint.
Asymptotic
Order :~’
onJ defined by :
fora,6
EJ , iff
forsome a E A and all b E A such that b > a ,
It is easy to see that
z’
istransitive
andantisymmetric;
howeverit may not be connected.
Often,
connectednesswill derive
from theassumptions
made.
Relative
tofunction composition, denoted . ,
,it is monotonic :
for allArchimedean
OrderedSubgroup
ofAutomorphisms : T~’,’)
where(i) T is a subgroup of J ,
(ii) ~’
isconnected,
and so a totalorder , (iii) ·
denotesfunction composition,
and(iv)
for eacha,j3
EJ
such thata ~’ i ,
thereis
someinteger
n such thatan ~’S .
Stevens
(1946,1951) suggested
aclassification
of measurementsystems
according
to thebehavior
of theautomorphism
group of anumerical
represen-tation ; it is summarized
in Table 1.Although widely
cited and usedin
thebehavioral
and socialscience literature, this classification
was neververy
thoroughly investigated until
the work to bedescribed.
HOMOGENEITY, UNIQUENESS,
AND SCALE TYPEThe
following concept
ofM-point homogeneity,
which has beenextensively
stu-died
in theliterature
on orderedpermutation
groups(see Glass, 1981,
for asummary of
results),
wasindependently introduced by
Narens(1981a,b)
in arri-ving
at a formaldefinition
of scaletype.
The results uncovered appear to bedistinct
from thosereviewed by
Glass.A subset of
B automorphisms is :
M-Point
Homogeneous : Given
any twostrictly increasing
sequences of Melements,
thereis
anautomorphism in B
that takes one into the other.Degree
ofHomogeneity :
Thelargest
M forwhich M-point homogeneity
holds.Homogeneous : Degree
ofhomogeneity >
1 .N-Point Uniqueness :
Whenever twoautomorphisms
ofB
agree at Ndistinct
points, they
areidentical.
Finite Uniqueness : Degree
ofuniqueness
.Scale
Type (M,N) :
M = the
degree
ofhomogeneity,
andN = the
degree
ofuniqueness
of group ofautomorphisms.
Homogeneity
means, among otherthings,
that no element isdistinguish-
able from others
by properties
formulatedin
terms of theprimitives. Thus,
any
system is
excluded that has asingular element,
e.g., a zero or a bound.CLASSICAL MODELS OF MEASUREMENT BASED ON AN OPERATION
Consider an ordered
relational
structure in whicheither
one of the relations is abinary operation
or there is anoperation
thatalong
with the order isequivalent
to therelational
structure(as is
true for some difference andconjoint structures).
The
operation
is assumed to bepositive
andArchimedean.
In the
associative
case,Holder’s
theorem(1901) is
used to show :(i)
theexistence
of arepresentation
ontoRe + >,+> ,
(ii)
therepresentation
isunique
up tosimilarities
x -~ rx , r >0 , i.e.,
the structure and itsrepresentation
are of scaletype (1,1).
Behavorial
andsocial scientists speak
of such arepresentation
thatis
unique
up to thesimilarities
as aratio scale representation.
In the
non-associative
case, a modification ofHblder’s proof (Cohen
&
Narens,1979;
Luce etal.,
in press; Narens &Luce, 1976)
is used to show(i)
theexistence
of a(non-additive)
numericalrepresentation,
suchthat
(ii)
theautomorphisms
form asubgroup
of thesimilarity
group, and so the scaletype
is(0,1)
or(1,1) .
A series of results
(Krantz
etal.,1971;Luce
etal.,
in press; Luce1978;
Luce &
Narens, 1985; Narens, 1976;
Narens &Luce, 1976)
show how such opera-tions
enter into the structure ofphysical dimensions. Consider :
Conjoint
Structure :C = Axp~> ~
where for eacha, b E A
and p , q EP ,
the
following
three conditions aresatisfied :
1. ueak
Ordering : k
is anon-trivial
weakordering.
2.
Independence (or monotonocity) :
Observe that
independence permits
one to defineinduced
weak orders on A andP , which
are denotedZA
Aand k , respectively .
p
3. A
are total orders.Suppose
further :4. 0 A is
apositive operation
on A and that a =A, A,OA>
hasa ratio scale
representation,
thepositive reals;
5. a
distributes
inC = AxP,~)
in the sense that for alla,b,c,d
E A and p,q EP ,
Then there exists a
function ~
on P such that representsC .
If there is also an
operation
0 p p
on P with a ratio scalerepresentation
then for some real p ,
represents C .
This is
thetypical product
of powers exhibitedby physical units.
Itis potentially important
to othersciences
to know thedegree
towhich
theconditions leading
to theproduct
of powersrepresentation
can begeneralized.
So we turn to that.
QUESTION :
HOW DOES ALL OF THIS GENERALIZE WHEN NO OPERATION IS DEFINED ? Inparticular :
1. Is there some natural
concept
ofArchimedeanness
in the absence ofan
operation ?
2. When does an ordered relational structure have a
real,
ratio scalerepresentation ?
3. How can the idea of an
operation distributing
in aconjoint
struc-ture be
generalized
so one continues to arrive at theproduct
of powers re-presentation
ofphysical
dimensions ?These
questions
are ofspecial
interest in the socialsciences
sincethey place mathematical limits
on thepossibility
ofadding
newdimensions
to those
currently
known.ANSWERS IN THE HOMOGENEOUS CASE
Answers are not known
in general,
butassuming homogeneity
the situa-tion
is clear andmoderately simple.
Real Unit Structure :
R =
R,_>,R.>.
J, where
Ris
a subset ofRe+
and.
there is some subset T of
Re
such that1. T is a group under
multiplication,
2. T maps R into
R , i.e.,
for each RE R andt E T ,
then tr
F R ,
3. T
restricted
to R is the set oftranslation
ofR .
Dedekind Complete : Every
bounded subset of elements has a least upper bound.Order Dense :
A, ~>
is such that ifa,b
E A and a >b ,
then thereexists
c E A such that a > c > b .THEOREM
(Luce,
,1986) Suppose R = R,>,R.>.E J
-- JJis
a realunit
structurewith
Tits
group oftranslations.
(i)
ThenR
can bedensely imbedded
inDedekind complete unit
struc-ture, R* .
(ii)
If R is order dense inRe+ ,
then eachautomorphism
ofR
extends to an
autoporphism
ofR* .
(iii)
If T ishomogeneous
and Ris
order densein Re+ ,
thenR*
of Part
(i) is
onRe+
and T*is homogeneous.
THEOREM
(Luce,
’1986) Suppose a = A, is
arelational structure,
JJT is its
set oftranslations,
and’ is
theasymptotic ordering
of theautomorphisms.
Then thefollowing
areequivalent :
(i)
a, isisomorphic
to a realunit
structure with ahomogeneous
group oftranslations.
(ii) T, -’,.>
is ahomogeneous,
Archimedean ordered group.COROLLARY.
If,
inaddition, A,~>
is orderdense,
then theautomorphism
group of the real unit structure
is
asubgroup
of the power group(see
Table1) restricted
toits domain.
This theorem answers
question
2 - when does a relational structure havea ratio scale
representation ? -
andsuggests
thatcondition (ii)
is an answer toquestion
1 - is there a naturalconcept
of Archimedeanness in the absence of anoperator ?
The latterpoint is discussed
morefully
in Luceand
Narens(in press). Moreover, it provides
aroutine
way toproceed : given
an axiom
system, investigate
whether thetranslations
are :(i) homogeneous,
(ii)
closed underfunction composition,
and so a group,(iii) Archimedean
ordered.The
following is
a sufficientcondition
fortranslations
that form agroup to be
Archimedean
ordered.THEOREM
(Luce, 1987) Suppose a
is a relational structure that is Dedekindcomplete
and order dense. Ifits
setT
oftranslations is 1-point unique
(equivalently,
agroup),
then T is Archimedean.This
means that for orderdense, Dedekind complete
cases theissue is
to show that the
translations
form a group. Thatis
thedifficult part
inproving
thefollowing sufficient condition :
THEOREM
(Alper, 1985, 1987, Narens,
1981a,b).Suppose
thatR =
-J J-
is
anumerical relational
structure that ishomogeneous
andfinitely unique.
Then the
following
are true.(i) R is
of scaletype ( 1 ,1 ) , ( 1,2) ,
or(2,2).
(ii) R
is of scaletype (1,1)
iffR is isomorphic
to a real struc-ture whose
automorphisms
are thesimilarity
group.(iii) R
is of scaletype (2,2)
iffR,’ is isomorphic
to a real struc-ture whose
automorphisms
are the power group(see
Table1).
(iv) R is
of scaletype (1,2) iff R is isomorphic
to a real struc-ture whose
automorphisms
are a propersubgroup
of the power group thatproperly includes
thesimilarity
group.One should not assume
this
resultgeneralizes.
Inunpublished work,
Cameron
(1987)
has shown that structures on therationals
of scaletype
(M,N)
can be found for anyintegers
M and N forwhich
1 M N ~ . .To my
knowledge,
the case M = Nis
not understood.THEOREM
(Luce
&Narens, 1985).
If ais
ahomogeneous concatenation
struc-ture,
then : ,(i) Either
a,is weakly positive
oridempotent [ (V) a0a - a]
orweakly negative [(V) a0a.a]
(ii)
If a is alsofinitely unique,
then itis
of scaletype (1,1), (1,2)
or(2,2).
THEOREM
(Cohen
&Narens, 1979;
Luce &Narens, 1985) a
=Re, > 0,> is
ahomogeneous
andfinitely unique
concatenation structureiff
a.is isomorphic
to a real
unit
structureR = Re+,’> , > ,
inwhich
case a can be charac-terized
as follows : thereis
afunction
f :Re+ (onto) Re+
such that :(i)
fis strictly increasing,
(ii) f/i is strictly decreasing,
where iis
theidentity,
(iii)
for all x,y ERe+ ,
COROLLARY : Consider
... ,...
Then
R is
of scaletype :
(1,1)
iffEq. (1) is satisfied only
for p = 1 .(1,2) iff Eq. (1) is satisfied
for p =kn ,
where k > 0 is fixed and nis
anyinteger.
(2,2) iff Eq. (1)
holds for all p > 0 .Applications
ofhomogeneity ideas, using
bothconcatenation
andconjoint
structures,
aregiven by Falmagne (1985)
topsychophysical problems
andby
Luce and Narens
(1985)
to thestudy
ofpreferences
amonguncertain
alterna-tives.
DISTRIBUTION IN A CONJOINT STRUCTURE
Suppose C = A x P, ~ ) is
aconjoint
structure(defined above) :
SimdLlarn-Tuples :
Then-tuples (a1,...,an)
and(b1, ... ,bn)
from A aresuch that for some p,q E P and for each
i - 1,...,n ,
Distributive Relation in C :
Sis
a relation of order n on A such thatif (a ,...,an)
1 n and(b1,...,bn)
In aresimilar
and one isin S ,
then so isthe other.
Note : the
induced
order kA is always distributive
inC .
Distributive
Structurein C : Any relational
structure on acomponent
ofC
each of whose relationsdistributes
inC .
The
following is
theculmination
of aseries
ofincreasinly
more gene- ral resultsby
Narens and Luce.THEOREM
(Luce, 1987). Suppose a = is
an orderedrelational
JJ
structure
with translations T
andasumptotic ordering ’ .
Then the follow-ing
areequivalent :
( i ) T, ~’ ,
> is ahomogeneous, Archimedean
ordered group.(ii) T
is1-point unique
and thereexists an~Archimedean,
solvableconjoint
structureC with
a relational structure a.’ on thefirst component
such thata.’
isisomorphic
to a, and a’distributes
inC .
COROLLARY. The
conjoint
structure of Part(ii) satisfies
theThomsen condi-
tion :
fora,b,c E A
and p,q,u EP ,
Solvability :
Given any three ofa,b-C.A ,
p,q EP ,
the fourth exists such that(a,p) -(b,q) .
In solvable
conjoint
structures with anoperation,
thegeneral concept
ofdistribution implies
the one introducedearlier.
Archimedean :
If{ai}
and{ai+1}
aresimilar
andbounded,
then the sequen-ce
is finite.
And asimilar condition
on theP-component.
THEOREM
(Luce, 1987). Suppose C
=A x P, g5>
is aconjoint
structure thatis
solvable and
Archimedean. Suppose, further,
that a =A, z ,S.>. A J
jEJis
a relational structure whosetranslations
form anArchimedean
ordered group.(i)
If a.distributes in. C ,
then ais 1-point homogeneous
andC satisfies
the Thomsencondition.
(ii) If, in addition,
a is Dedekindcomplete
and orderdense,
thenunder some
mapping
from A ontoRe+
a has ahomogeneous
real unit
representation
and thereexists
amapping
from Pinto Re+
such thatis
arepresentation
ofC .
(iii) If, further,
there is aDedekind complete
relational structure onP
that,
under some(pl,
from P ontoRe+ ,
has ahomogenèous
unit representation,
then for some real constant p , APrepresentation
ofC .
The latter result is
highly
relevant toissues
inthe
foundations of dimensionalanalysis;
see Luce(1978).
It says :Real
unit
structures thatdistribute
in aconjoint
structure are suchthat
products
of powers of theirrepresentations
form arepresentation
of the
conjoint
structure.This is the
typical pattern
ofphysical dimensions,
e.g., E=(1/2)mv2
and is reflected in the
pattern
ofphysical units.
This result establishes that such a
dimensional triple
isnot,
as wasfor some time
believed, restricted
toextensive
structures; it can be true for any real unit structure. The net effect is to increaseconsiderable
theopportunities
foraugmenting
thesystem
ofphysical units.
For a somewhat
different,
butrelated, approach
tothis problem
of pro- ducts of powers, seeFalmagne
and Narens(1983).
CONCLUSION
We now understand a
good
deal abouthomogeneous
measurementstru.ctures;
butwe
still
do not have anequally satisfactory understanding
ofnon-homogeneous
ones. Some of these are
important.
Inparticular,
we do not understand theinterlock
ofconjoint
structures and bounded structures on one of the factors.One
example is relativistic velocity, which superficially
looks like any otherphysical
dimensionexcept
forbeing bounded; however, unlike
the unboundedphysical attributes, velocity
does notdistribute in
thedistance = velocity
x time
conjoint
structure. Anotherexample is probability, which
has neverbeen
incorporated into
thesystem
ofphysical units although it
hasbeen,
toan
extent, into economic
measures(e.g., subjective expected utility).
Pre-sumably,
sensoryvariables, like
loudness andbrightness,
are bounded. Thesestructures, unlike
many othernon-homogeneous
one, arerichly
endowed withautomorphisms
orpartial automorphisms,
and seem in many wayscomparable
tothe
homogeneous
ones. So results about them may beanticipated.
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TABLE 1*
Stevens’ Classification
of ScaleTypes (Augmented)
* This is Table 20.1 of