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The unit-temperature likelihood update is ordinary Bayes in a finite model

Example

Let Θ={0,1} have prior masses π(0)=1/3 and π(1)=2/3. Let the observation space be X={0,1} and define sampling laws by P0(Y=1)=1/4, P0(Y=0)=3/4, P1(Y=1)=3/4, and P1(Y=0)=1/4. These finite spaces with power-set sigma-algebras are standard Borel, and the displayed laws define a statistical experiment. Take ℓ(θ,y)=−log⁡Pθ(Y=y) for every (θ,y)∈Θ×X. For observed y=1 at w=1, the unnormalized masses are 1/12 and 1/2, so the update is (1/7,6/7). The same values follow by conditioning the joint prior-sampling law on Y=1.

Facts & Assumptions

Given: The parameter and observation spaces {0,1} with power-set sigma-algebras, prior (1/3,2/3), and the four displayed sampling probabilities.

[F1]

A countable discrete measurable space is standard Borel: a nonempty at-most-countable set with its power-set sigma-algebra is standard Borel.

[F2]

Probability measures and probability spaces: a probability measure has total mass one.

[F3]

Statistical experiments: a statistical experiment consists of a sample measurable space, a parameter set, and a probability law on the sample space for each parameter.

[F4]

Generalized Bayes loss update and temperature: at w=1 the update weights are proportional to e−ℓ(θ,y) and are normalized by their integral against the prior.

[F5]

A finite-normalizer loss update is a probability measure: a finite-positive-normalizer update is a probability measure.

[F6]

The natural logarithm as the inverse of the exponential function: log⁡ is the inverse of exp⁡ on (0,∞).

[F7]

Conditional probability P(A∣B) for P(B)>0: in a finite probability space, Pr⁡(A∣B)=Pr⁡(A∩B)/Pr⁡(B) when Pr⁡(B)>0.

Verification

1.1F1F2F3givenalgebra

Both parameter and sample spaces are nonempty finite sets, so [F1] makes them standard Borel. The prior masses are nonnegative and sum to one; each pair of sampling probabilities is nonnegative and sums to one. By [F2] these are probability measures, and [F3] therefore gives the stated finite statistical experiment.

2.1F6F4step 1.1algebra

All four likelihoods are positive, so [F6] makes ℓ real-valued on Θ×X. Each parameter section is measurable because the parameter sigma-algebra is the power set, satisfying the loss hypotheses of [F4] for every y∈X. At y=1, the inverse relation in [F6] gives e−ℓ(θ,1)=elog⁡Pθ(Y=1)=Pθ(Y=1). Hence the unnormalized weights are π(0)P0(Y=1)=112,π(1)P1(Y=1)=12, and their normalizer is Z=1/12+1/2=7/12, finite and positive.

3.1F4F5step 2.1algebra

By [F4]–[F5], the update is a probability measure with q(0)=1/127/12=17,q(1)=1/27/12=67..

4.1F7step 1.1step 2.1step 3.1algebra∎

Define the joint masses by Pr⁡(θ,y)=π(θ)Pθ(Y=y). The four masses are 1/4,1/12,1/6,1/2 at (0,0),(0,1),(1,0),(1,1), respectively; they sum to one, and Pr⁡(Y=1)=7/12>0. Applying [F7] to each parameter singleton gives Pr⁡(θ=0∣Y=1)=1/127/12=17,Pr⁡(θ=1∣Y=1)=1/27/12=67. Thus the loss update equals the ordinary finite-model posterior at y=1.

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