A Tutorial on Variational Bayes

Transcription

1 A Tutorial on Variational Bayes Junhao Hua ( 华俊豪 ) Laboratory of Machine and Biological Intelligence, Department of Information Science & Electronic Engineering, ZheJiang University 204/3/27 huajh7@gmail.com For further information, see:

2 Outline Motivation The Variational Bayesian Framework Variational Free Energy Optimization Tech. Mean Field Approximation Exponential Family Bayesian Networks Example: VB for Mixture model Discussion Application Reference 2

3 A Problem: How Learn From Data? Typical, we use a complex statistical model, but how to learn its parameters and latent variables? Data: X Model: P(X \theta, Z) 3

4 Challenge Maximum Likelihood: Overfits the data Model Complexity Computational tractability Bayesian Framework: Arising intractable integrals: partition function posterior of unobserved variables Approximate Inference: Monte Carlo Sampling: e.g. MCMC, particle filter. Variational Bayes 4

5 Outline Motivation The Variational Bayesian Framework Variational Free Energy Optimization Tech. Mean Field Approximation Exponential Family Bayesian Networks Example: VB for Mixture model Discussion Application Reference 5

6 Variational Free energy Basic Idea: conditional independence is enforced as a functional constraint in the approximating distribution, and the best such approximation is found by minimization of a Kullback-Leibler divergence (KLD). Use a simpler variational distribution, Q(Z), to approximate the true posterior P(Z X) Two alternative explanations Minimize (reverse) Kullback-Leibler divergence QZ ( ) QZ ( ) DKL( Q P) = QZ ( ) log = QZ ( ) log + log PD ( ) PZ ( D) PZD (, ) Maximum variational free energy(lower bound) LQ ( ) = QZ ( ) log PZD (, ) QZ ( ) log QZ ( ) = E[log PZD (, )] + HQ ( ) Z Z Z Z Q 6

7 Optimization Techniques: Mean Field Approximation Originated in the statistical physics literature Conditional Independence assumption Decoupling: intractable distribution -> a product of tractable marginal distributions (tractable subgraph) Factorization: M QZ ( ) = qz ( i D) i= 7

8 Optimization Techniques: Variational methods Optimization Problem: Maximum the lower bound LQZ ( ( )) = E [ln PZD (, )] + HQZ ( ( )) Q( Z) QZ ( ) = Q( Z) Where i i i Subject to normalization constraints: i. Qi( Zi) dzi = Seek the extremum of a functional: Euler Lagrange equation 8

9 Derivation Consider the partition Consider Energy term, Q( Z) i i i We define normalization constant. {, },where = i i Z = i Z \ Zi Z Z Z E [ln PZD (, )] = ( Q ( Z))ln( ZDdZ, ) = Q ( Z ) Q ( Z )ln( Z, D) dz dz i i i i i i = Q ( Z ) ln( Z, D) dz i i Q ( Z ) i i i = Q( Z)ln exp ln( ZD, ) i i Q Q Z Q Z dz * = i( i)ln i ( i) i +lnc Q ( Z ) = exp ln( Z, D) * i i Q ( Z ) i i i C ( Z ) i dz i, where C is the 9

10 Derivation (cont.) Consider the entropy, = i Then we get the functional, H ( Q( Z)) ( Q ( Z ))ln Q ( Z ) dz i k k k i i = Q ( Z ) Q ( Z )ln Q ( Z ) dz dz i i i i i i i i i = Q ( Z )ln Q ( Z ) dz i i i i i i Q ( Z ) = Q ( Z )ln Q ( Z ) dz i i i i i L(Q(Z))= * Qi( Zi)ln Qi ( Zi) dzi + Qi( Zi)ln Qi( Zi) dzi + lnc i =( * Qi( Zi)ln Qi ( Zi) dzi Qi( Zi)ln Qi( Zi) dzi)+ Qk( Zk)ln Qk( Zk) dzk + lnc k i Q ( Z ) = Q ( Z )ln dz + Q ( Z )ln Q ( Z ) dz + lnc * i i i i i k k k k k Qi( Zi) k i * = D KL ( i ( i ) i ( i )) + [ i ( i )] + ln Q Z Q Z HQ Z C i i 0

11 Derivation (cont.) Maximizing energy functional L w.r.t. each Q_i could be achieved by Lagrange multipliers and functional differentiation i Q Z Q Z Q Z dz = *. { D KL[ i( i) i ( i)] λi( i( i) i )}: 0 Qi( Zi) A long algebraic derivation would then eventually lead to a Gibbs distribution; Fortunately, L will be maximized when the KL divergence is zero, Q Z = Q Z = PZ Z D * i( i) i ( i) exp ln ( i, i, ) Q ( Z ) i i C Where C is normalization constant.

12 Challenge Q Z = Q Z = PZ Z D * i( i) i ( i) exp ln ( i, i, ) Q ( Z ) i i C The expectation can be intractable. We need pick a family of distributions Q that allow for exact inference Then Find Q' Q that maximizes the functional energy. 2

13 Challenge The expectation can be intractable. We need pick a family of distributions Q that allow for exact inference Q' Q Then Find functional energy. that maximizes the Exponential Family 3

14 Why Exponential Family? Principle of maximum entropy Density function: Log partition function: canonical parameters Mean parameters 4

15 Properties of Exponential family Mean parameters: θ various statistical computations, among them marginalization and maximum likelihood estimation, can be understood as transforming from one parameterization to the other. All realizable mean parameters Always a convex subset of Forward mapping From canonical parameters Backward mapping From mean parameters d φ( x) to the mean parameters θ to the canonical parameters φ( x) θ 5

16 Properties of partition function A 6

17 Conjugate Duality: Maximum Likelihood and Maximum Entropy The variational representation of log partition function The conjugate dual function to A 7

18 Nonconvexity for Naïve Mean Field Mean field optimization is always nonconvex for any exponential family in which the state space is finite. It is a strict subset of M(G) Contains all of the extreme points of polytope 8

19 Outline Motivation The Variational Bayesian Framework Variational Free Energy Optimization Tech. Mean Field Approximation Exponential Family Bayesian Networks Example: VB for Mixture model Discussion Application Reference 9

20 Inference in Bayesian Networks Variational Message Passing (Winn, Bishop, 2003.) Message from parents: my X uy Message to parents: = φ Update natural parameter vector : =,{ } ( ) m u m ({ m } ) i pa X Y XY X i X i cp Y φ = φ + m * Y Y i Y j Y Y j ch Y PX ( φ) = exp[ φ T ux ( ) + f( X) + g ( φ)] 20

21 Summary of VB Mean-field Assumption Variational Methods QZ ( ) exp ln PZ (, Z, D) C i i i Q ( Z ) orq ( mb ( Z )) i i Conjugate-exponential family Forward, backward mapping 2

22 Outline Motivation The Variational Bayesian Framework Variational Free Energy Optimization Tech. Mean Field Approximation Exponential Family Bayesian Networks Example VB for Mixture model Discussion Application Reference 22

23 Mixture of Gaussian (MoG) N K z (, µ, Λ ) = Nx ( n µ k, Λk ) nk n= k= px Z pxz (,, π, µ, Λ ) = px ( Z, µ, Λ) pz ( π) p( π ) p( µ Λ) p( Λ) 23

24 Infinite Student s t-mixture DP( α, G ) 0 Dirichlet Process G = π j ( V) δθ j= Dirichlet Process Mixture Stick-Breaking prior j j π ( V) = V ( V) j j i i= V ~ Beta(, α) j p( α) = Gam( α η, η ) 2 N p( X ) = π ( V ) St( x µ, Λ, v ) n= j= j n j j j 24

25 Latent Dirichlet Allocation (LDA) 25

26 Outline Motivation The Variational Bayesian Framework Variational Free Energy Optimization Tech. Mean Field Approximation Exponential Family Bayesian Networks Example: VB for Mixture model Discussion Application Reference 26

27 步骤一 : 选择无信息先验分布 选择先验分布原则 : 共轭分布,Jefferys 原则, 最大熵原则等 一般要求先验分布应取共轭分布 (conjugate distribution) 才合适, 即先验分布 h(θ) 与后验分布 h(θ x) 属于同一分布类型 π Λ z i=,..., k 0 i=,..., k 0 0 i=,..., k ~ Nm ( 0,( 0Λi) ) i=,..., N X ~ SymDir( K, α ) ~ W( w, υ ) µ β ~ Mult(, π ) N( µ ) i=,..., N z 说明 : K: 单高斯分布个数,N: 样本个数 SymDir() :K 维对称 Dirichlet 分布 ; 它是分类分布 (categorical) 或多项式分布的共轭先验分布 W() 表示 Wishart 分布 ; 对多元高斯分布, 它是 Precision 矩阵 ( 逆协方差矩阵 ) 的共轭先验 Mult() 表示多项分布 ; 是二项式分布的推广, 表示在一个 K 维向量中只有一项为, 其它都为 0. N() 为多元高斯分布 X = { x,..., x } 是 N个训练样本, 每项都是服从多元高斯分布的 K维向量 ; Z = { z,..., z } 是一组潜在变量, 每项 z = { z, z } 表示对应的样本 x 属于哪个混合部分 ; N k k,... nk k π = { π,..., π } 表示每个单高斯分布混合比例 ; µ N K 和 Λ 分别表示每个单高斯分布参数的均值和精度 ; i=,..., k i=,..., k K, α, β, w, υ, m 称为超参数 ( hyperparameter), 都为已知量

28 用 盘子表示法 (plate notation) 表示多元高斯混合模型, 如图所示 小正方形表示不变的超参数, 如 β 0,ν 0,α 0,µ 0,W 0 ; 圆圈表示随机变量, 如 π, zi, xi, µ k, Λk ; 圆圈内的值为已知量, 其中 [K],[D] 表示 K D 维的向量,[D,D] 表示 DxD 的矩阵 ; 单个 K 表示一个有 K 个值的 categorical 变量 ; 波浪线和开关表示变量 x i 通过一个 K 维向量 z i 来选择其他传入的变量 (µ k, Ʌ k ) 28

29 步骤二 : 写出联合概率密度函数 假设各参数与潜在变量条件独立, 则联合概率密度函数可以表示为 pxz (,, π, µ, Λ ) = px ( Z, µ, Λ) pz ( π) p( π ) p( µ Λ) p( Λ) 每个因子为 : 其中, N K z (, µ, Λ ) = Nx ( n µ k, Λk ) n= k= px Z pz ( π) = n= k= Γ( Kα0) p( π) = Γ K ( α ) 0 k = α0 k ( ) ( k 0,( 0 k) ) p( Λ ) = W( Λ w, ν ) N k K π K z k p µ Λ = N µ m β Λ nk π 0 0 T N( x µ, ) = exp ( ) ( ) D/2 /2 x µ x µ (2 π ) 2 ( ν D )/2 W ( Λ w, ν) = B( w, ν) Λ exp( Tr( w Λ)) 2 D ν/2 νd/2 D( D )/4 ν + i Bw (, ν) = w (2 π Γ( )) 2 i= nk 29

30 步骤三 : 计算边缘密度 (VB- marginal) () 计算 Z 的边缘密度, 根据平均场假设, 有 qz (, π, µ, Λ ) = qzq ( ) ( π, µ, Λ) * ln q( Z) = E [ln pxz (,, π, µ, Λ )] + const 其中 π, µ, Λ = E [ln px ( Z, µ, Λ) pz ( π) p( π ) p( µ Λ) p( Λ )] + const π, µ, Λ = E[ln pz ( π )] + E [ln px ( Z, µ, Λ )] + const π µ, Λ N K = z ln ρ + const n= k= * znk 两边分别取对数可得, q ( Z) ρnk * znk 归一化, 得 q ( Z) r, 其中 nk nk D T ln ρnk = E[ln πk] + E[ln Λk ] ln(2 π ) Eµ, [( x ) ( )] k Λk n µ k Λk xn µ k N K = n= k= nk N K n= k= r 可见 q * ( Z) 是多个单观测多项式分布 (single-observation multinomial distribution) 的乘积 更进一步, 根据 categorical 分布, 有 Ez [ ] = r nk ρ nk = K j = nk ρ nk nj 30

31 K (2) 计算 π 的概率密度, q( π, µ, Λ ) = q( π ) q( µ k, Λk) * ln ( π) Z, µ, Λ[ (, π, µ, )] N * r nk α0 n 两边取对数 q ( π)~ π + = k, 可见 q * ( 是 π ) Dirichlet 分布, 其中 α = α0 + Nk, Nk = rnk. 0 k = q = E p X Z Λ + const q * ( π)~ Dir( α) = ln p( π) + E [ln p( Z π)] + const Z K N K =( α -) lnπ + r lnπ + const k nk k k= n= k= K n= N n= 3

32 最后同时考虑 µ, Λ, 对于每一个单高斯分布有, µ Λ = µ Λ µ Λ * ln q( k, k) EZ, π, µ, Λ [ln px ( Z, k, k) p( k, k)] i k N = ln ( µ k, k) [ nk]ln ( n µ k, k ) n= 经过一系列重组化解将得到 Gaussian-Wishart 分布, i k p Λ + E z N x Λ + const q ( µ, Λ ) = N( µ m,( β Λ ) ) W( Λ w, ν ) * k k k k k k k k k 其中 βk = β0 + Nk, mk = ( β0m0 + Nx k k ), βk β0nk T wk = w0 + NkSk + ( xk m0)( xk m0), β0 + Nk vk = v0 + Nk, N xk = rnk xn, Nk n= N T Sk = rnk( xk xk)( xk xk). Nk n= 32

33 步骤四 : 迭代收敛 最后, 注意到对 π,µ,ʌ 的边缘概率都需要且只需要 r nk ; 另一方面,r nk 的 T 计算需要 ρ nk, 而这又是基于 E[ln π k], E[ln Λk ], Eµ, [( ) ( )], k Λ x k n µ k Λk xn µ k 即需要知道 π,µ,ʌ 的值 不难确定这三个期望的一般表达式为 : 这些结果能导出, K 且 r =. ( K ) i= ln πk E[ln πk ] = ψα ( k) ψ αi D vk + i ln Λ k E[ln Λ k ] = ψ Dln 2 ln i= k Λ Eµ k, Λ x Λ x = D + x m W x m k k = nk T T [( n µ k) k( n µ k)] βk νk( n k) k( n k) D ν /2 T k rnk π kλ k exp ( xn mk) Wk( xn mk) 2βk 2 33

34 Summary: Variational Inference for GMM q * ( π)~ Dir( α) α = α + N 0 k N k N = r n= nk q ( µ, Λ ) = N( µ m,( β Λ ) ) W( Λ w, ν ) * k k k k k k k k k N β = β + N, m = ( β m+ Nx), v = v+ N, x = r x k 0 k k 0 0 k k k 0 k k nk n βk Nk n= β N w w N S ( x m )( x m ), S r ( x x )( x x ) N 0 k T k = 0 + k k + k 0 k 0 k = nk k n k n β0 + Nk Nk n= VBM-Step Latent variable Soft-count or ESS T N K * z q Z = rnk n= k= ( ) nk VBE-Step D ν /2 T k rnk π kλ k exp ( xn mk) Wk( xn mk) 2βk 2 34

35 Outline Motivation The Variational Bayesian Framework Variational Free Energy Optimization Tech. Mean Field Approximation Exponential Family Bayesian Networks Example: VB for Mixture model Discussion Application Reference 35

36 EM vs. VB 36

37 The Accuracy-vs-Complexity trade-off 37

38 Application Matrix Factorization: Probabilistic PCA, Mixtures of PPCA, Independent Factor Analysis(IFA), nonlinear ICA/IFA/SSM, Mixture of Bayesian ICA, Bayesian Mixture of Factor Analyzers, etc. Time Series: Bayesian HMMs, variational Kalman filtering, Switching State-space models, etc. Topic model: Latent Dirichlet Allocation(LDA), (Hierarchical) Dirichlet Process (Mixture) Model, Bayesian Nonparametrical Models, etc. Variational Gaussian Process Classifiers Sparse Bayesian Learning Variational Bayesian Filtering, etc. 38

39 Reference Neal, Radford M., and Geoffrey E. Hinton. "A view of the EM algorithm that justifies incremental, sparse, and other variants." Learning in graphical models. Springer Netherlands, Attias, Hagai. "Inferring parameters and structure of latent variable models by variational Bayes." Proceedings of the Fifteenth conference on Uncertainty in artificial intelligence. Morgan Kaufmann Publishers Inc., 999. Winn, John, Christopher M. Bishop, and Tommi Jaakkola. "Variational Message Passing." Journal of Machine Learning Research 6.4 (2005). Wainwright, Martin J., and Michael I. Jordan. "Graphical models, exponential families, and variational inference." Foundations and Trends in Machine Learning.-2 (2008): Šmídl, Václav, and Anthony Quinn. The variational Bayes method in signal processing. Springer, Wikipedia, Variational Bayesian methods, 39

40 Any Question? 40