MCMC Inferenceof Bayesian Network:

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Inference of Bayesian Network:

MCMC

Jin Young Choi

1

(2)

Outline

 Latent Dirichlet Allocation (LDA) model(Topic Modelling)

 Inference of LDA Model

 Markov Chain Monte Carlo (MCMC)

 Gibbs Sampling

 Collapsed Gibbs Sampling for LDA

 Estimation of Multinomial Parameters via Dirichlet Prior

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LDA Model (Topic Modelling)

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LDA Model

𝐿𝑖𝑘𝑒𝑙𝑖ℎ𝑜𝑜𝑑: 𝑝(𝑤|𝑧, 𝜃, ∅, 𝛼, 𝛽) 𝑃𝑜𝑠𝑡𝑒𝑟𝑖𝑜𝑟𝑖: 𝑝(𝑧, 𝜃, ∅|𝑤, 𝛼, 𝛽)

Dir(K, α): 𝑝(𝜇₁, … , 𝜇_𝐾|𝛼₁, … , 𝛼_𝐾)=^Γ(σ^𝑖=1

𝐾 (𝛼_𝑖))

ς_𝑖=1^𝐾 Γ(𝛼_𝑖) 𝜇₁^𝛼¹⁻¹ … 𝜇_𝐾^𝛼^𝐾⁻¹

Mul(K, 𝜇): 𝑝(𝑥1, … , 𝑥_𝐾|𝜇₁, … , 𝜇_𝐾)=_𝑥 ^𝑛!

1!…𝑥_𝐾!𝜇₁^𝑥¹ … 𝜇_𝐾^𝑥^𝐾

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LDA Model

Dir(K, α): 𝑝(𝜇₁, … , 𝜇_𝐾|𝛼₁, … , 𝛼_𝐾)=^Γ(σ^𝑖=1

𝐾 (𝛼_𝑖))

ς_𝑖=1^𝐾 Γ(𝛼_𝑖) 𝜇₁^𝛼¹⁻¹ … 𝜇_𝐾^𝛼^𝐾⁻¹

Mul(K, 𝜇): 𝑝(𝑥₁, … , 𝑥_𝐾|𝜇₁, … , 𝜇_𝐾)=_𝑥 ^𝑛!

1!…𝑥_𝐾!𝜇₁^𝑥¹ … 𝜇_𝐾^𝑥^𝐾

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Inference of LDA Model

 Maximum A posteriori Probability (MAP) given observation 𝑤, 𝛼, 𝛽

Not Convex

Closed-form solution is not available 𝐿𝑖𝑘𝑒𝑙𝑖ℎ𝑜𝑜𝑑: 𝑝(𝑤|𝑧, 𝜃, ∅, 𝛼, 𝛽) 𝑃𝑜𝑠𝑡𝑒𝑟𝑖𝑜𝑟𝑖: 𝑝(𝑧, 𝜃, ∅|𝑤, 𝛼, 𝛽)

Likelihoods

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Discrete Variables, Dirichlet

 Parameters: 𝛼

₁

, … , 𝛼

_𝐾

> 0 (concentration hyper-parameter)

 Support: 𝜇

₁

, … , 𝜇

_𝐾

∈ (0,1) where σ

_𝑖=1^𝐾

𝜇

_𝑖

= 1

 Dir(K, α): 𝑝(𝜇

₁

, … , 𝜇

_𝐾

|𝛼

₁

, … , 𝛼

_𝐾

)=

^Γ(σ^𝑖=1^𝐾 ^(𝛼^𝑖⁾⁾

ς_𝑖=1^𝐾 Γ(𝛼_𝑖)

𝜇

₁^𝛼¹⁻¹

… 𝜇

_𝐾^𝛼^𝐾⁻¹

 Mul(K, 𝜇): 𝑝(𝑥

₁

, … , 𝑥

_𝐾

|𝜇

₁

, … , 𝜇

_𝐾

)=

^𝑛!

𝑥₁!…𝑥_𝐾!

𝜇

₁^𝑥¹

… 𝜇

_𝐾^𝑥^𝐾

 Dir(K, c + α): 𝑝(𝜇|𝑥, 𝛼) ∝ 𝑝 𝑥 𝜇 𝑝(𝜇|𝛼)

where 𝑐 = (𝑐

₁

, … , 𝑐

_𝐾

) is number of occurrences

 𝐸[𝜇

_𝑘

] =

^𝑐^𝑘^+𝛼^𝑘

σ_𝑖=1^𝐾 (𝑐_𝑖+𝛼_𝑖)

1 3

5

0 0.02 0.04 0.06 0.08 0.1

1 2 3 4 5

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Markov Chain Monte Carlo (MCMC)

 Markov Chain Monte Carlo (MCMC) framework

Posteriors

Markov Blanket

i: 1, 2, 3, 4, 5, 6, 7, 8, 9 w: 1, 3, 2, 3, 3, 5, 4, 1, 6 z : 1, 2, 2, 2, 1, 1, 2, 1, 2 ℎ_𝜙(1,3): 1

ℎ_𝜙(2,3): 2

i: 1, 2, 3, 4, 5, 6, 7, 8, 9 w: 1, 3, 2, 3, 3, 5, 4, 1, 6 z : 1, 2, 2, 2, 1, 1, 2, 1, 2 ℎ_𝜃(d,2): 5

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Markov Chain Monte Carlo (MCMC)

Posteriors

Markov Blanket

w: 1, 3, 2, 3, 3, 5, 4, 1, 6 z : 1, 2, 2, 2, 1, 1, 2, 1, 2 ℎ_𝜙(1,3): 1

ℎ_𝜙(2,3): 2

w: 1, 3, 2, 3, 3, 5, 4, 1, 6 z : 1, 2, 2, 2, 1, 1, 2, 1, 2 ℎ_𝜃(d,2): 2

𝑝(𝑧|𝑤, 𝜙_𝑘, 𝛼) Gibbs sampling

𝑝(𝑧|𝑤, 𝜙_𝑘, 𝛼)

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Gibbs Sampling (Review)

1

2

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Collapsed Gibbs Sampling for LDA

 Latent Variables

• 𝜃 ∶ topic distribution in a document

• ∅ : word distribution in a topic

• 𝑧 : topic assignment to a word 𝑤

• 𝑝(𝜃, ∅, 𝑧|𝑤, 𝛼, 𝛽)

 Collapsed Gibbs Sampling

• 𝜃 and ∅ are induced by the association between 𝑧 and 𝑤

• 𝑧 is sufficient statistic to estimate 𝜃 and ∅

• Simpler algorithm can be used by sampling only 𝑧 after marginalizing 𝜃 and ∅.

• 𝑝 𝑧 𝑤, 𝛼, 𝛽 = ׭ 𝑝(𝜃, ∅, 𝑧|𝑤, 𝛼, 𝛽) 𝑑_𝜃𝑑_𝜙

𝑝(𝑧|𝑤, 𝛼, 𝛽) 𝑝(𝑧|𝑤, 𝜙_𝑘, 𝛼) Collapsed Gibbs Sampling

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Collapsed Gibbs Sampling for LDA

 The Gibbs sampling equation for LDA (Topic Labelling, Unsupervised Learning)

Not related with 𝑧_−𝑑𝑖

𝑝(𝑧|𝑤, 𝛼, 𝛽)

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Collapsed Gibbs Sampling for LDA

 Dirichlet and multinomial probability into 𝑝 𝜙 𝛽 and 𝑝(𝑤|𝑧, 𝜙)

where

Dir(K, α): 𝑝(𝜇₁, … , 𝜇_𝐾|𝛼₁, … , 𝛼_𝐾)=^Γ(σ^𝑖=1^𝐾 ^(𝛼^𝑖⁾⁾

ς_𝑖=1^𝐾 Γ(𝛼_𝑖) 𝜇₁^𝛼¹⁻¹ … 𝜇_𝐾^𝛼^𝐾⁻¹

Mul(K, 𝜇): 𝑝(𝑥₁, … , 𝑥_𝐾|𝜇₁, … , 𝜇_𝐾)= ^𝑛!

𝑥₁!…𝑥_𝐾!𝜇₁^𝑥¹ … 𝜇_𝐾^𝑥^𝐾

𝛽 𝑣 − 1

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Collapsed Gibbs Sampling for LDA

 The number of times that the word 𝑤_𝑑𝑖 = 𝑣 is assigned to the topic 𝑧_𝑑𝑖 = 𝑘

where ℎ_𝜙(𝑘, 𝑣) ∈ 𝑁^𝐾×𝑉 denotes the histogram matrix which counts the number of times given by

w: 1, 2, 2, 3, 3, 5, 4, 1, 6 z : 1, 2, 2, 1, 1, 1, 2, 1, 2 h(2,3): 0

h(1,3): 2

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Collapsed Gibbs Sampling for LDA

 In consequence

𝛽 𝑣 − 1

𝛽 𝑣 − 1 𝛽 𝑣 − 1

Dir(K, α): 𝑝(𝜇₁, … , 𝜇_𝐾|𝛼₁, … , 𝛼_𝐾)=^Γ(σ^𝑖=1^𝐾 ^(𝛼^𝑖⁾⁾

ς_𝑖=1^𝐾 Γ(𝛼_𝑖) 𝜇₁^𝛼¹⁻¹ … 𝜇_𝐾^𝛼^𝐾⁻¹

Mul(K, 𝜇): 𝑝(𝑥₁, … , 𝑥_𝐾|𝜇₁, … , 𝜇_𝐾)= ^𝑛!

𝑥₁!…𝑥_𝐾!𝜇₁^𝑥¹ … 𝜇_𝐾^𝑥^𝐾

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Collapsed Gibbs Sampling for LDA

∝

 Integrating PDF

𝛽 𝑣 − 1

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Collapsed Gibbs Sampling for LDA

 In a similar manner Topic portion corresponding to a word

w: 1, 2, 2, 3, 3, 5, 4, 1, 6 z : 1, 2, 2, 1, 1, 1, 2, 1, 2 h(d,2): 4

𝛼 𝑘 − 1

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Collapsed Gibbs Sampling for LDA

 In a similar manner

𝛼 𝑘 − 1

𝛼 𝑘 − 1 𝛼 𝑘 − 1

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Collapsed Gibbs Sampling for LDA

 The joint distribution of words 𝑤 and topic assignments 𝑧 becomes

w: 1, 3, 2, 3, 3, 5, 4, 1, 6 z : 1, 2, 2, 2, 1, 1, 2, 1, 2 ℎ_𝜙(1,3): 1

ℎ_𝜙(2,3): 2

w: 1, 3, 2, 3, 3, 5, 4, 1, 6 z : 1, 2, 2, 2, 1, 1, 2, 1, 2 ℎ_𝜃(d,2): 5

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Collapsed Gibbs Sampling for LDA

 The Gibbs sampling equation for LDA

Not related with 𝑧_−𝑑𝑖

𝑝(𝑧|𝑤, 𝛼, 𝛽)

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Collapsed Gibbs Sampling for LDA

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Collapsed Gibbs Sampling for LDA

∝ ෑ

𝑘=1 𝐾

ෑ

𝑣=1 𝑉

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Collapsed Gibbs Sampling for LDA

𝑘 depends on the sampled 𝑧_𝑑𝑖

∝ ෑ

𝑑=1 𝐷

ෑ

𝑘=1 𝐾

= ෑ

𝑑=1 𝐷

ෑ

𝑘=1 𝐾

constant

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Collapsed Gibbs Sampling for LDA

 Sampling

𝑝(𝑧_𝑑𝑖)

1 2 3 4 5

෍ 𝑝(𝑧_𝑑𝑖)

1 2 3 4 5

ෑ

𝑣=1 𝑉

𝑝(𝑧_𝑑𝑖|𝑧_−𝑑𝑖, 𝑤, 𝛼, 𝛽) ∝

Tractable

∝ ෑ

𝑘=1 𝐾

ෑ

𝑣=1 𝑉

=

ෑ

𝑑=1 𝐷

ෑ

𝑘=1 𝐾

What is the inferred label of 𝑤_𝑑𝑖 ?

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Collapsed Gibbs Sampling for LDA

 Estimation of the multinomial parameters 𝜃 and 𝜙

w: 1, 3, 2, 3, 3, 5, 4, 1, 6 z : 1, 2, 2, 2, 1, 1, 2, 1, 2 ℎ_𝜙(1,3): 1

ℎ_𝜙(2,3): 2

w: 1, 3, 2, 3, 3, 5, 4, 1, 6 z : 1, 2, 2, 2, 1, 1, 2, 1, 2 ℎ_𝜃(d,2): 5

w: 1, 2, 2, 3, 3, 5, 4, 1, 6 z : 1, 2, 2, 1, 1, 1, 2, 1, 2 ℎ_𝜃(d,2): 4

← 2

← 1

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Interim Summary

 LDA Model (Topic Modelling)

 Inference of LDA Model

 Markov Chain Monte Carlo (MCMC)

 Gibbs Sampling

 Collapsed Gibbs Sampling for LDA

 Estimation of Multinomial Parameters via Dirichlet Prior

(27)

Inference of Bayesian Network:

Variational Inference

Jin Young Choi

1

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Outline

 What is variational inference ?

 Kullback–Leibler divergence (KL-divergence) formulation

 Dual of KL-divergence

 Variational Inference for LDA

 Estimating variational parameters

 Estimating LDA parameters

 Application of VI to Generative Image Modeling (VAE)

 Application of LDA toTraffic Pattern Analysis

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LDA Model (Topic Modelling)

𝐿𝑖𝑘𝑒𝑙𝑖ℎ𝑜𝑜𝑑: 𝑝(𝑤|𝑧, 𝜃, ∅, 𝛼, 𝛽) 𝑃𝑜𝑠𝑡𝑒𝑟𝑖𝑜𝑟𝑖: 𝑝(𝑧, 𝜃, ∅|𝑤, 𝛼, 𝛽)

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Markov Chain Monte Carlo (MCMC)

Posteriors

Markov Blanket

w: 1, 3, 2, 3, 3, 5, 4, 1, 6 z : 1, 2, 2, 2, 1, 1, 2, 1, 2 ℎ_𝜙(1,3): 1

ℎ_𝜙(2,3): 2

w: 1, 3, 2, 3, 3, 5, 4, 1, 6 z : 1, 2, 2, 2, 1, 1, 2, 1, 2 ℎ_𝜃(d,2): 5

𝑝(𝑧|𝑤, 𝜙_𝑘, 𝜃_𝑘)

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Collapsed Gibbs Sampling for LDA

 Sampling

𝑝(𝑧_𝑑𝑖)

1 2 3 4 5

෍ 𝑝(𝑧_𝑑𝑖)

1 2 3 4 5

ෑ

𝑣=1 𝑉

𝑝(𝑧_𝑑𝑖|𝑧_−𝑑𝑖, 𝑤, 𝛼, 𝛽) ∝

Tractable

∝ ෑ

𝑘=1 𝐾

ෑ

𝑣=1 𝑉

=

ෑ

𝑑=1 𝐷

ෑ

𝑘=1 𝐾

What is the inferred label of 𝑤_𝑑𝑖 ?

(32)

Variational Inference (VI)

 Approximating posteriors in an tractable probabilistic model

 MCMC: stochastic inference

 VI: deterministic inference

 The posterior distribution 𝑝 𝑧 𝑥 can be approximated by a variational distribution 𝑞(𝑧)

where 𝑞(𝑧) should belong to a family of simpler form than 𝑝 𝑧 𝑥

 Minimizing Kullback–Leibler divergence (KL-divergence)

𝐿𝑖𝑘𝑒𝑙𝑖ℎ𝑜𝑜𝑑: 𝑝(𝑤|𝑧, 𝜃, ∅, 𝛼, 𝛽) 𝑃𝑜𝑠𝑡𝑒𝑟𝑖𝑜𝑟𝑖: 𝑝(𝑧, 𝜃, ∅|𝑤, 𝛼, 𝛽) 𝑝 𝜙 𝑤, 𝑧 ∝ 𝑝 𝑤 𝜙, 𝑧 𝑝(𝜙|𝛼) ≈ 𝑞 𝜙|𝛾

𝐶𝜙₁^{ℎ 1 +𝛼}¹𝜙₂^ℎ(2)+𝛼² …𝜙_𝑣^{ℎ(𝑣)+𝛼}^𝑣 ≈ 𝐶𝜙₁^𝛾¹𝜙₂^𝛾² …𝜙_𝑣^𝛾^𝑣

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Variational Inference (VI)

 Kullback–Leibler divergence (KL-divergence)

constant Minimize −

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Variational Inference (VI)

 Dual of KL-divergence

entropy of 𝑞 𝑧

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Variational Inference (VI)

 Choose a variational distribution 𝑞 𝑧

 An approximation (Parisi, 1988) was proposed

where 𝜆_𝑖 is a variational parameter for each hidden variable 𝑧_𝑖

 Estimation of 𝜆 መ𝜆 = max

𝜆 ℒ 𝜆ҧ

𝛻_𝜆ℒ 𝜆 = 0ҧ

where 𝑞 . would be designed for ҧℒ 𝜆 to be convex.

𝑝 𝜙 𝑤, 𝑧 ∝ 𝑝 𝑤 𝜙, 𝑧 𝑝(𝜙|𝛼) ≈ 𝑞 𝜙|𝛾

𝐶𝜙₁^{ℎ 1 +𝛼}¹𝜙₂^ℎ(2)+𝛼² …𝜙_𝑉^{ℎ(𝑉)+𝛼}^𝑣 ≈ 𝐶𝜙₁^𝛾¹𝜙₂^𝛾² …𝜙_𝑉^𝛾^𝑉 𝑝 𝜃 𝑧 ∝ 𝑝 𝑧 𝜃 𝑝(𝜃|𝛼) ≈ 𝑞 𝜃|𝜇

𝐶𝜃₁^{ℎ 1 +𝛼}¹𝜃₂^ℎ(2)+𝛼² …𝜃_𝐾^{ℎ(𝐾)+𝛼}^𝑣 ≈ 𝐶𝜃₁^𝜇¹𝜃₂^𝜇² …𝜃_𝐾^𝜇^𝐾

ҧℒ 𝛾 ҧℒ 𝜇 𝑧 → 𝜙

𝑥 → 𝑤 𝑧 → θ 𝑥 → 𝑧

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Variational Inference for LDA

 Objective function and joint probability of LDA

Maximization is intractable

→ Variational Inference

≈

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Variational Inference for LDA

 A simpler variational distribution

where 𝜆, 𝛾, 𝜑 are the variational parameters used for approximate inference of 𝜙, 𝜃, 𝑧 , respectively.

𝑝 𝜙 𝑤, 𝑧 ∝ 𝑝 𝑤 𝜙, 𝑧 𝑝(𝜙|𝛽) ≈ 𝑞 𝜙|𝜆

𝐶𝜙₁^{ℎ 1 +𝛽}¹𝜙₂^ℎ(2)+𝛽² …𝜙_𝑉^{ℎ(𝑉)+𝛽}^𝑉 ≈ 𝐶𝜙₁^𝜆¹𝜙₂^𝜆² …𝜙_𝑉^𝜆^𝑉 𝑝 𝜃 𝑧 ∝ 𝑝 𝑧 𝜃 𝑝(𝜃|𝛼) ≈ 𝑞 𝜃|𝛾

𝐶𝜃₁^{ℎ 1 +𝛼}¹𝜃₂^ℎ(2)+𝛼² …𝜃_𝐾^{ℎ(𝐾)+𝛼}^𝐾 ≈ 𝐶𝜃₁^𝛾¹𝜃₂^𝛾² …𝜃_𝐾^𝛾^𝐾

, 𝑤_𝑑𝑖)

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Variational Inference for LDA

 Optimal values of the variational parameters

𝑝 𝜙 𝑤, 𝑧 ∝ 𝑝 𝑤 𝜙, 𝑧 𝑝(𝜙|𝛽) ≈ 𝑞 𝜙|𝜆

𝐶𝜙₁^{ℎ 1 +𝛽}¹𝜙₂^ℎ(2)+𝛽² …𝜙_𝑉^{ℎ(𝑉)+𝛽}^𝑉 ≈ 𝐶𝜙₁^𝜆¹𝜙₂^𝜆² …𝜙_𝑉^𝜆^𝑉 𝑝 𝜃 𝑧 ∝ 𝑝 𝑧 𝜃 𝑝(𝜃|𝛼) ≈ 𝑞 𝜃|𝛾

𝐶𝜃₁^{ℎ 1 +𝛼}¹𝜃₂^ℎ(2)+𝛼² …𝜃_𝐾^{ℎ(𝐾)+𝛼}^𝐾 ≈ 𝐶𝜃₁^𝛾¹𝜃₂^𝛾² …𝜃_𝐾^𝛾^𝐾

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Variational Inference for LDA

 Optimal values of the variational parameters

𝑝 𝜙, 𝜃, 𝑧 𝑤, 𝛼, 𝛽 = 𝑝 𝜙, 𝜃, 𝑧, 𝑤 𝛼, 𝛽 /𝑝(𝑤)

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Variational Inference for LDA

 Dual of KL divergence

ℒ 𝑞 = log 𝑝 𝑤 𝛼, 𝛽 − 𝐷[𝑞(𝜙, 𝜃, 𝑧|𝜆, 𝛾, 𝜑)||𝑝( 𝜙, 𝜃, 𝑧|𝑤, 𝛼, 𝛽)]

 Yielding the optimal value of each variational parameter

𝑓(𝛾_𝑑) 𝑓(𝜆_𝑘)

, 𝑤_𝑑𝑖 )

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Variational Inference for LDA

 Expectation

where Ψ(. ) is the digamma function (Blei et al. 2003) given by

 The iterative updates of the variational parameters are guaranteed to converge into a stationary point

 For the iteration, 𝜑 are updated with λ and 𝛾 fixed, and λ and 𝛾 are updated given the fixed 𝜑

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Variational Inference for LDA

 The final distribution results 𝜙 𝑎𝑛𝑑 𝜃

𝑝(𝑧_𝑑𝑖|𝜑_𝑑𝑖)

1 2 3 4 5

, 𝑤_𝑑𝑖 )

(43)

References for LDA and VI

 Blei, D. M., Ng, A. Y., & Jordan, M. I. (2003). Latent dirichlet allocation.

JML. Res., 3, 993–1022.

 Blei, D. M. (2012). Probabilistic topic models. Commun. ACM, 55(4), 77–84.

 Blei, D. M. (2014). Build, compute, critique, repeat: Data analysis with latent variable models. Annual Review of Statistics and Its Application, 1(1), 203–232.

 Jordan, M. I., Ghahramani, Z., Jaakkola, T. S., & Saul, L. K. (1999). An introduction to variational methods for graphical models. Mach. Learn., 37(2), 183–233.

URL http://dx.doi.org/10.1023/A:1007665907178

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Perception and Intelligence Lab.

Ex) Image Generation

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Generative Image Modeling

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Goal: Model the distribution p(x)

 cf) Discriminative approach

20

Generative Image Modeling

Issue: High Dimensional low resolution (64x64x3 = 12,288)

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Goal: Model the distribution p(x)

 Using Deep Neural Networks (CNNs)

21

Generative Image Modeling

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Two prominent approaches

 Variational Auto-encoder (VAE)

 Generative Adversarial Networks (GAN)

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Deep Generative Models

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Variational Auto-encoder (VAE)

Encoder Decoder

x

z

𝑳𝒐𝒔𝒔 = −𝒍𝒐𝒈𝑷_𝜽 𝒙 𝒛 + 𝑫_𝑲𝑳(𝒒_𝝋 𝒛 𝒙 ||𝑷_𝜽(𝒛))

Reconstruction Loss

𝒑_𝜽 𝒙 𝒛 : a multivariate Gaussian (real-valued data) 𝒑_𝜽 𝒙 𝒛 : a Bernoulli (binary-valued data)

Variational Inference

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24

Variational Auto-encoder (VAE)

Encoder Decoder

x

z

𝑳𝒐𝒔𝒔 = −𝒍𝒐𝒈𝑷_𝜽 𝒙 𝒛 + 𝑫_𝑲𝑳(𝒒_𝝋 𝒛 𝒙 ||𝑷_𝜽(𝒛))

Variational Inference

𝒑_𝜽 𝒛 ~𝑵(𝟎, 𝑰)

Decoder x

z

𝑺𝒂𝒎𝒑𝒍𝒊𝒏𝒈 𝒛 𝒇𝒓𝒐𝒎 𝑵(𝟎, 𝑰)

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