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  • 8/11/2019 Bayes Theorem for Choquet Capacities

    1/13

    Carnegie Mellon University

    Research Showcase

    Department of Statistics Dietrich College of Humanities and Social Sciences

    9-1-1990

    Bayes' Teorem for Choquet CapacitiesLarry A. WassermanCarnegie Mellon University

    Joseph B. KadaneCarnegie Mellon University, [email protected]

    Follow this and additional works at: hp://repository.cmu.edu/statistics

    Tis Article is brought to you for free and open access by the Dietrich College of Humanities and Social Sciences at Research Showcase. It has been

    accepted for inclusion in Department of Statistics by an authorized administrator of Research Showcase. For more information, please contact research-

    [email protected] .

    Recommended CitationWasserman, Larry A. and Kadane, Joseph B., "Bayes' Teorem for Choquet Capacities" (1990).Department of Statistics. Paper 16.hp://repository.cmu.edu/statistics/16

    http://repository.cmu.edu/?utm_source=repository.cmu.edu%2Fstatistics%2F16&utm_medium=PDF&utm_campaign=PDFCoverPageshttp://repository.cmu.edu/statistics?utm_source=repository.cmu.edu%2Fstatistics%2F16&utm_medium=PDF&utm_campaign=PDFCoverPageshttp://repository.cmu.edu/hss?utm_source=repository.cmu.edu%2Fstatistics%2F16&utm_medium=PDF&utm_campaign=PDFCoverPageshttp://repository.cmu.edu/statistics?utm_source=repository.cmu.edu%2Fstatistics%2F16&utm_medium=PDF&utm_campaign=PDFCoverPagesmailto:[email protected]:[email protected]://repository.cmu.edu/statistics/16?utm_source=repository.cmu.edu%2Fstatistics%2F16&utm_medium=PDF&utm_campaign=PDFCoverPagesmailto:[email protected]:[email protected]://repository.cmu.edu/statistics/16?utm_source=repository.cmu.edu%2Fstatistics%2F16&utm_medium=PDF&utm_campaign=PDFCoverPageshttp://repository.cmu.edu/statistics?utm_source=repository.cmu.edu%2Fstatistics%2F16&utm_medium=PDF&utm_campaign=PDFCoverPageshttp://repository.cmu.edu/hss?utm_source=repository.cmu.edu%2Fstatistics%2F16&utm_medium=PDF&utm_campaign=PDFCoverPageshttp://repository.cmu.edu/statistics?utm_source=repository.cmu.edu%2Fstatistics%2F16&utm_medium=PDF&utm_campaign=PDFCoverPageshttp://repository.cmu.edu/?utm_source=repository.cmu.edu%2Fstatistics%2F16&utm_medium=PDF&utm_campaign=PDFCoverPages
  • 8/11/2019 Bayes Theorem for Choquet Capacities

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    The

    nnals

    of Statistics

    1990 Vol . 18, No.3

    1328-1339

    BAYES THEOREM FOR CHOQUET CAPACITIES

    By LARRY WASSERMAN AND JOSEPH B.

    KADANE

    2

    Carnegie Mellon University

    We give an upper b ou nd for the posterior probability of a measurable

    set

    when the prior lies in a class

    of

    probability measures The bound

    is a ratio nal fun ctio n

    of

    two Choquet integrals.

    If

    g; is weakly compact and

    is closed with respect to majorization, then the bound is sharp if and only if

    the

    upper

    prior probability is 2-alternating. The result is u se d to compute

    -bounds for sev eral sets of p riors u sed in robust Bayesian inference.

    The

    result may be regarded as a characterization

    of

    2-alternating Choquet

    capacities.

    1.

    Introduction. Sets of probability measures arise

    naturally

    in classical

    robustness, Bayesian

    robustness and group

    decision making. So me sets of

    probability measures give rise to

    upper

    probabilities

    that

    are 2-alternating

    Choquet

    capacities. These

    upper

    probabilities

    are

    pervasive

    in

    the

    robustness

    literature;

    see

    Huber

    1973),

    Huber and Strassen

    1973),

    Buja

    1984, 1985,

    1986), Bednarski 1982)

    and

    Rieder 1977), for example.

    The purpose of

    this

    paper

    is

    to

    prove a version

    of

    Bayes

    theorem

    for

    sets of prior

    probabilities

    that

    have the 2-alternating property and to see h ow these sets of probabilities may

    be

    exploited in Bayesian robustness.

    This result

    was p roved

    in Wasserman

    1 988, 1990) for infinitely

    alternating

    capacities also

    known as

    belief func

    tions; Shafer, 1976). Since infinitely

    alternating

    capacities

    are

    also

    2-alternat

    ing, the present

    result

    generalizes

    that theorem.

    However, the proof in the

    infinitely

    alternating

    case

    uses

    an argument

    that

    depends

    on

    properties

    that

    infinitely

    alternating

    capacities possess

    that

    are not

    shared

    by 2-alternating

    capacities

    in

    general. Also,

    in

    this paper,

    the

    conditions given are both neces

    sary and

    sufficient. A

    proof of

    sufficiency

    when the parameter

    space is finite is

    given in Walley 1981).

    Section 2

    of

    this

    paper states and

    proves the

    main

    result. In Section 3, we

    apply the resul t to derive explicit bounds for the

    posterior

    probability

    of

    a

    measurable

    set

    using

    various classes

    of

    priors. Section 4

    contains

    a discussion.

    2. Main resul t. Let 0

    be a Polish space, that is, the topology for

    0

    is

    complete, separable

    and

    metrizable,

    and

    le t

    be the Borel u-algebra for

    Let

    PI be the set

    of

    all bounded, nonnegative, real-valued,

    measurable

    func

    tions

    defined

    on Let { o }

    be a

    set of

    probability

    measures on

    a

    sample space Y

    with

    u-algebra

    N.

    Assume that

    each

    o

    has

    a d en si ty

    { YI

    Received October 1988; revised May 1989.

    Research supported by a post-doctoral fellowship from the

    Natural

    Sciences

    and

    Engineering

    Research Council

    of

    Canada.

    2Research supported by NSF grant DMS-87-05646.

    S

    1980 subject classifications

    Primary

    62F15; secondary 62F35.

    Key words

    and

    phrases 2-alternating Choquet capacities, robust Bayesian inference, posterior

    bounds.

    1328

  • 8/11/2019 Bayes Theorem for Choquet Capacities

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    BAYES THEOREM AND CAPACITIES 1329

    with

    respect

    to

    some u-finite dominating

    measure

    and let

    L

    =

    f YI

    be the

    likelihood function for

    having observed

    Y

    E

    Y.

    We

    assume

    that

    LEge

    for

    each Y

    E

    Y.

    Let 9 be a nonempty

    set

    of prior probability measures on

    and define

    the

    upper and lower

    prior

    probability functions by

    P

    A

    =

    sup

    P A and E

    A

    = in f P A .

    P P

    Since and E are r el at ed b y the equation peA = 1 - E AC ,

    it

    suffices to

    concentrate on p. In what follows,

    there

    is no loss in generality by

    assuming

    that 9 is convex.

    For

    each X

    E

    gc, define

    the upper

    expectation

    of

    X by

    E X

    =

    supp

    E

    g;P X ,

    where

    P X

    =

    jX P d .

    The

    lower expectation

    of

    X, denoted by

    ~ X ,

    is defined

    in an

    analogous way. Let g; be

    the

    class of

    posteriors arrived at by applying Bayes theorem to each

    prior P

    in 9 for

    which peL > o. To avoid triviality, we

    assume there

    is

    at

    least one

    such

    P.

    Let P

    y

    be the upper probability generated by g;

    The

    posterior corresponding

    to

    P is de note d b y P

    y

    We say that 9 is 2-alternating if for each

    A,

    B E ~ ,

    peA

    u B ~ peA +

    PCB -

    p A n B .

    We say that 9 generates a Choquetcapacity if

    p F n ~ p F

    for each sequence of closed sets F

    n

    F. It can be shown that 9 generates a

    Choquet capacity if and only if {P; P ~ p} is weakly compact. Here, P ~

    means

    that peA

    ~

    peA

    for every

    A

    E

    ~ . See Choquet

    1953)

    and Huber

    and Strassen 1973) for details on capacities. We say

    that

    9 is closed with

    respect to majorization, or is

    m-closed, if

    P

    ~

    implies that P

    E

    It is

    common practice

    in

    the literature

    to assume

    that 9 is m-closed. We will show

    Section 3, Ex ample

    1

    that repla cing 9 with its m-closure has nontrivial

    consequences.

    For each X E

    g

    the

    upper

    Choquet integral of X is defined by

    C X

    =

    [OP X

    t

    dt,

    o

    where X

    t

    { E X > t}. Similarly, the lower Choquet integral Q X is

    defined by

    It follows easily that

    Q X

    ~

    E X

    ~ E X ~ C X .

    If

    9 consists

    of

    a single

    probability measure

    P,

    then

    C X

    = Q X = E X = E X = P X ; if this

    probability

    measure

    is our prior probability, then the posterior probability of a

    subset

    A,

    after

    observing

    y

    and

    applying Bayes theorem, may be expressed

    as

    P LA / P L

    A

    P LAc , where L

    A

    = L I

    A

    and I

    A

    is

    the

    indicator

    function for A. Note that C and Q may b e e xte nde d t o the

    set

    of all bounded,

    mea su ra bl e func tions b y a ddi ng and subtracting a sufficiently large positive

    constant.

  • 8/11/2019 Bayes Theorem for Choquet Capacities

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    1330

    L. A WASSERMAN AND J B KADANE

    THEOREM Let

    g;

    a nonempty set

    of

    prior probabilities on

    an d

    let

    g;

    the corresponding class ofposterior probabilities. Then for each A E

    when the ratios are well defined. If

    g;

    generates a Choquet capacity

    and

    i f

    g;

    is m-closed then the following three statements are equivalent:

    i

    P is 2-alternating.

    ii The first inequality is

    an

    equality for each A E

    and

    each LEge

    iii

    The second inequality is

    an

    equality for each A E

    an d

    each LEge

    Before proving

    the theorem

    we shall

    state a few

    lemmas The first

    lemma is

    from

    Huber and

    Strassen

    1973 .

    LEMMA 1 If

    g;

    is m-closed

    an d

    generates a 2-alternating Choquet capac

    ity then for each upper semicontinuous

    X

    E ge there exists

    aPE g;

    such that

    P X) =

    E X)

    and

    P X

    t

    =

    P X

    t

    ),

    for each real number t.

    The next

    two

    lemmas are from Bednarski 1982 .

    LEMMA

    2.

    If

    0,

    P L

    A

    PiA = P L

    A

    P L

  • 8/11/2019 Bayes Theorem for Choquet Capacities

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    BAYES THEOREM AND CAPACITIES

    Now, Py A is

    of

    the form

    1331

    1

    h P

    =

    1

    g P / f P

    where

    rep

    = P L

    A

    and g P

    = P LAc . h P is maximized where g p jr p

    is minimized.

    But

    which proves

    the

    first inequality.

    The

    second inequality follows from

    that

    fact

    that E X ~ C X

    and

    ~ X

    ~ Q X .

    Now suppose

    g

    is m-closed

    and

    generates

    a

    Choquet

    capacity. We begin by

    showing

    that

    i) implies ii). Assume

    that

    P is 2-alternating. We need

    to

    show

    that the first inequality is an equality for each A E

    and

    for each L E fJ If

    E L

    A

    = 0

    this

    follows easily. Now suppose

    E L

    A

    >

    0

    Let b

    =

    SUPOEACL O .

    Define a function a

    on

    e

    by a O =

    L O

    b on

    A

    and

    a O = -

    L O

    b on

    AC.

    For

    notational convenience, define

    E L

    A

    B=

    E LA ~ L A c .

    We will show

    that

    for ea ch

    e

    >

    0,

    there

    exists

    P

    ~

    P

    such

    that

    P L

    A

    B e<

    0,

    there

    exists, by

    Lemma

    4, a nonnegative, bounded,

    upper

    semicontinuous function h such

    that

    h

    ~

    a

    and E a -

    8

    ~ E h ~

    E a .

    Set

    k

    =

    E L

    A

    ~ L A c .

    Note that

    k >

    0 Choose 8

    t =

    P h > t ,

    for e ac h

    t.

    It

    is easily verified that

    P L

    A

    ~ E L

    A

    -

    8 and P LAc

    ~ ~ L A c

    8. Thus

    P L

    A

    E L

    A

    8

    P

    y

    A =

    P L

    A

    P LAc E L

    A

    -

    8

    E LAc

    8

    8 8

    = B

    -

    E L

    A

    ~ L A C

    =

    B

    -

    k

    >

    B

    -

    e.

    Since

    this

    holds for e ac h

    e >

    0

    supP

    E gPy A = B so

    that

    ii) holds.

    Further

    i) implies iii) because

    of Lemma

    4.

    To prove that ii) implies i), first

    assume

    that e is finite. Suppose

    that

    Py A = E LA j E L

    A

    ~ L A c

    for every

    subset A

    and

    every

    L E fJ

    g is

    closed,

    with

    respect

    to

    the

    Euclidean topology, so

    there

    exists, for e ac h

    A

    a

    PEg

    such that Py A = Py A . Therefore, .

    E L

    A

    P L

    A

    E L

    A

    ~ L A c

    P L

    A

    P LAc ,

  • 8/11/2019 Bayes Theorem for Choquet Capacities

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    1332

    L.

    A

    WASSERMAN AND

    J. B

    KADANE

    from which it follows that

    1

    This

    holds for each

    A an d

    each

    L E [ C. Le t A l C A

    2

    C C An

    be an

    increasing sequence

    of

    subsets.

    For

    i =

    1 2 . . .

    n -

    1

    define

    L

    i

    on to

    be

    equal to 1 on

    Ai an d Ai+l

    an d

    0 otherwise.

    From

    1) we deduce

    that there

    exists

    Pi E

    9 such that Pi A

    i

    )

    =

    P A

    i

    ) an d

    Pi A

    i

    +

    l

    )

    =

    P A

    i

    +

    l

    )

    for i

    =

    1 2 . . .

    n

    - 1

    Define

    A

    o

    =

    0

    and

    let

    gi

    =

    A i

    -

    A i - I Let ~

    be

    th e

    field

    generated by {gl

    ,gn}

    Define an additive se t function

    S

    on

    ~

    by

    S gl) = P l A

    l

    ),

    S g2) =P

    2

    A

    2

    ) -P l A

    l

    ),

    S gn-l) =

    P

    n

    -

    l

    (

    An-I)

    - P

    n

    -

    2

    ( A

    n

    -

    2

    ),

    S gn)

    =

    Pn-l A

    n

    ) - Pn-l A

    n

    -

    l

    )

    and S A ~

    = 1 -

    SCAn)

    Then,

    S A

    i

    )

    =

    P A

    i

    )

    for

    i

    = 1, 2,

    . . .

    ,

    n.

    Also,

    S gl) ~

    o

    and S gi)

    = Pi A

    i

    ) - Pi-l A

    i

    -

    l

    )

    =

    P A

    i

    ) - Pi-l A

    i

    -

    l

    )

    =

    Pi-l A

    i

    )

    Pi-l A

    i

    -

    l

    ) ~ 0

    so that

    S

    is a probability

    m easure on ~

    which maximizes th e

    probability

    of

    the

    Ai S

    and

    since-

    S

    is dominated by

    on ~

    S

    may be

    extended

    by

    t he Hahn-Ba na ch

    theorem

    to

    2.

    By

    Lemma

    2,

    satisfies

    th e

    2-alternating

    condition. Therefore 9 is 2-alternating.

    Now we show

    that

    iii) implies i)

    and

    we continue

    with th e assumption that

    is finite. Let th e second inequality be an equality for each A E

    ~ and

    each

    L E

    [ C. Then

    E X ) = C X ) for each bounded function X.

    For

    suppose E X )

    a}. We

    claim that

    E X

    A

    )

    < C X

    A

    .

    To see this ,

    note

    that if

    a = Q t. en

    E X ~

    =

    E X )

    < C X)_= C X

    A

    . If a > 0 then-py th e

    ~ i n i t i o n

    of C,

    C X

    A

    ) = C X ) - aE(AC) >

    E X

    A

    ) + Ll where

    Ll

    = E X ) -

    E X

    A

    )

    aE(AC).

    Now, for each

    P E 9

    P X

    =

    P X

    A

    )

    +

    P XAc)

    =

    P X

    A

    )

    +

    aP AC)

    ~

    P X

    A

    )

    +

    aE AC),

    so

    that

    E X )

    ~ E X

    A

    )

    +

    aE(AC).

    Hence,

    Ll

    ~ 0 which implies that

    E X

    A

    )

    b > c > d >nd b > c d. Now let P be a

    probability

    measure

    with values ~ ~ 0, 0 on e and let Q have values

    0, o ~ Finally, let 9 be

    the

    convex hull of P and Q It is easily verified

    that

    9 is m-closed

    and that P

    is

    not

    2-alternating. Let

    = {I}.

    Then

    _ a E L

    PiA

    =

    a

    b

    and

    Q X =

    1

    -

    e P X

    e inf X O .

    JEE>

    Applying

    the

    theorem, we have

    P

    A

    = 1 - e P i

    A

    ea/P L

    y

    l - e +ea jP L

    where

    a

    = sUPo E L O . This result was stated

    in

    Huber 1973 .

  • 8/11/2019 Bayes Theorem for Choquet Capacities

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    BAYES THEOREM AND CAPACITIES 1335

    EXAMPLE

    4. Tot al varia ti on neighbourhoods. As

    in

    Example 3, fix

    and e

    and let < be compact. Define by

    9= {Q;Ll P,Q)

    e},

    where Ll P, Q) = SUPAE.98IP A - Q A)I. This is the e t ot al -varia ti on class

    generated by

    P.

    Let

    x

    V

    y

    denote the maximum

    of x

    and

    y

    and

    x

    \

    y

    the

    minimum of

    x and

    y.

    Then peA)

    = P A)

    + e) \

    1 A 0 and

    E A)

    =

    P A) - e) V

    A < Again, is m-closed and generates a 2-alternating

    Choquet capacity. For every X E f C define to X)

    =

    sup{t; P X

    t

    ) 1 - e} and

    so X)

    =

    inf{t; P X

    t

    )

    e}. Also, let

    Then

    C X) =P X) esupX l - e t

    o

    X)

    -J X,O, to X))

    and

    Q X) =

    P X)

    + e

    inf X -

    eso X)

    - J X, so X), sup

    X).

    Let to = to L

    A

    ) and So = so LAc). Then,

    _ P L

    A

    )

    1 -

    e)t

    o

    ea

    - J L

    A

    ,

    0,

    to)

    y {

    = ) ) ) ,

    P L + I - e

    t o+ea - J

    LA,O,t

    o

    - e s o - J

    LAc,so,b

    where a = SUPOEE> LA O and b = SUPOEE> LAc O .

    Bednarski 1981) and B uj a 1986) considered an upper probability that

    contains e-contamination and

    total

    variation neighbourhoods as special cases.

    It may be shown that this general ized class is an m-closed, 2 alternating

    Choquet capacity as well.

    EXAMPLE

    5

    Density bounded classes. Let Land U be two u-finite mea

    sures on

    such

    that

    L

    U;

    that

    is,

    L A)

    U A)

    for e ach

    A

    E

    Lavine

    1987) considers classes of probability

    measures

    of the form = {P; L P

    U}.

    He

    calls

    this

    a

    density bounded class.

    We shal l

    assume

    that

    L

  • 8/11/2019 Bayes Theorem for Choquet Capacities

    10/13

    1336

    L. A WASSERMAN AND J. B. KADANE

    L X)

    -

    SO X) d

    U

    J X,

    0,

    so X)).

    Here, U X)

    = jX O)U dO) and L X) =

    jX O)L dO). Therefore,

    - U L

    A

    ) tod

    L

    -

    J L

    A

    ,

    0,

    to)

    P

    y

    A

    =

    U LA)

    to d

    L

    - J LA

    0,

    to)

    L LAc)

    -

    So d

    u

    J

    L c 0,

    so)

    where

    to

    to L

    A

    ) and So

    so LAc).

    EXAMPLE 6 Probabilities with

    partition

    restr ictions. Moreno

    and

    Cano

    1988 consider

    the

    following

    set of

    priors.

    Let P

    be a fixed probability

    measure on yg

    e

    a fixed

    number in

    [0,1]

    and H

    =

    {hI , h

    k

    }

    a measurable

    partition

    of

    8

    Assume

    that

    8 is compact. Let P = PI

    ,Pk)

    be such

    that

    each

    Pi

    nd

    LPi

    =

    1

    Then

    define

    9= {Q;Q

    = 1 -

    e)P

    e8,

    8

    E

    J} .

    Here

    is

    the set

    of all probability measures that satisfy 8 h

    i

    ) = Pi i =

    1 2

    . . .

    k. The interpretation

    is

    the

    same as

    the

    e-contaminated class

    of

    Example 3, except that we are

    putting

    constraints on the possible contamina

    tions by forcing

    them

    to have fixed probabilities on some part it ion. This is a

    simple, reasonable way to restrict the size

    of the

    possible contaminating

    distributions.

    This class contains two

    important

    special cases. By

    taking

    H =

    8

    we get

    back

    the

    general e-contaminated model

    of

    Example 3 By

    taking

    e

    =

    1 we

    get

    the set of

    all probability measures with fixed probabilities on a par ti tion. This

    latter class has been studied by Berliner

    and

    Goel 1986 .

    Define

    A

    =

    {i; hi c

    A}

    and J

    A

    =

    {i; hi

    n A 0}

    Then

    f A = 1 - e)P A) e L Pi

    iE I

    A

    and

    P A) = 1 -

    e)P A) e L Pi.

    iE J

    A

    It

    may

    be shown

    that

    P

    is

    n alternating

    for every

    n and

    hence is 2-alternat

    ing.

    Further 9

    generates an m-closed, Choquet capacity. It follows

    that

    Q X)

    =

    1 -

    e)P X)

    eL KiPi

    and

    where Ki = n

    OEht

    X O) and

    Xi

    = sUPOEh

    t

    X O).

    Therefore,

    - 1 -

    e)P L

    A

    ) eLPiLA.

    PiA

    =

    1 -

    e)P L)

    eLPi[

    L

    Ai

    i

    EXAMPLE 7

    Symmetric, unimodal contaminations. Sivaganesan

    and

    Berger

    1989 consider

    the

    following modification

    of the

    e-contaminated class for 8

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    11/13

    BAYES THEOREM AND CAPACITIES

    equal to the real line. For fixed

    P

    and

    set

    g;

    = {Q = 1 - )

    P

    S , S

    E

    J }

    1337

    where

    J

    is

    the

    se t of all probability measures

    that

    possess densities

    that

    are

    unimodal

    and

    symmetric about a fixed point Jo.

    Without

    loss

    of

    generality, we

    take

    0

    0

    = 0. Now, g; generates a 2-alternating upper probability if and only if

    J generates

    a 2-alternating upper probability. Hence, we confine attention to

    the

    class J . As Sivaganesan

    and

    Berger (1989) remark

    when

    computing

    extrema, one need only consider the smaller class consisting of

    uniform

    probability measures

    that

    are centered at the origin. A

    point

    mass at the origin

    must

    be

    included

    as

    well. Let

    S be the

    upper probability generated by

    J .

    We

    claim

    that

    S

    is

    not

    2-alternating

    and

    that

    J

    is

    not

    m-closed.

    To see that S is not 2-alternating,

    set

    A = [- 3, - 1] U [1,2]

    and

    B

    =

    [-2 - 1] U [1,3].

    Then

    i =

    S A

    U

    B) >

    S A) S B) -

    S A n B)

    = ~

    ~ - To see that J is

    not

    m-closed, let

    R

    = ~ o

    ~ u

    where is a point

    mass at

    nd

    u is the uniform [0 1] distribution. Now, if

    A, R A) 1 =

    S A). Otherwise, R A) = R A

    n

    [0,1]) = u A

    n

    [0 1] /2 =

    u A n

    [0, 1]

    u A) S A), where u is uniform [

    1

    1]. Thus R S but R . J so J is

    not m-closed.

    As a result, the Choquet integral fails miserably

    as

    an approximation to

    For

    example,

    let

    X

    n

    be defined by

    0

    X

    n

    = n

    n

    i f n ~ O ~ n

    if 0 > n

    i fO < n .

    Then E X

    n

    = E X

    n

    = 0, but it is easily shown that C X

    n

    = n/2 Hence,

    limn

    -+oo(C(X

    n

    ) - E X

    n

    =

    00 .

    This is an

    extreme

    example

    that

    shows how

    important the 2-alternating and m-closure conditions are. It would

    be

    interest

    ing to investigate how much of the difference between E and

    C

    is accounted

    for

    by

    the failure

    of

    the

    2-alternating

    condition

    and

    how much is because

    of

    the

    lack

    of

    m-closure.

    4. Discussion. The main result of

    this

    paper helps to unify

    the

    process of

    finding bounds for posterior probabilities in robust Bayesian inference. Also,

    this

    theorem may

    be

    thought of as

    a characterization

    of 2-alternating

    Choquet

    capacities.

    Other authors

    have given different characterizations.

    Huber and

    Strassen (1973) showed that the 2-alt

    /

    ernating

    structure is necessary

    and

    sufficient for generalizing the Neyman-Pearson lemma to

    sets

    of probabilities.

    Buja (1984, 1985) generalized

    this

    result from the case of two hypotheses

    to

    a

    finite

    number of

    hypotheses. Similarly, Bednarski (1982) showed that this

    condition is necessary

    and

    sufficient for

    the

    construction

    of

    least in(ormative

    binary experiments in

    the

    sense of Blackwell (1951), Le

    Cam

    (1964, 1969,

    1972)

    and

    Torgersen (1970). These

    authors

    were concerned

    with

    uncertainty

    with respect to sampling distributions, as

    represented

    by sets

    of

    probability

    measures dominated

    by 2-alternating

    Choquet

    capacities. We have focused on

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    1338

    L.

    A

    WASSERMAN AND J. B KADANE

    uncertainty in

    the

    prior distribution. Other properties of 2 alternating capaci

    ties may be found in Walley 1981).

    Acknowledgments.

    We

    are grateful to the

    referees for

    their

    insightful

    comments and suggestions.

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    DEPARTMENT OF STATISTICS

    232

    BAKER

    HALL

    CARNEGIE MELLON UNIVERSITY

    PITTSBURGH PENNSYLVANIA

    15213-3890


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