Expectation Maximization Algorithm

Transcription

1 Expectation Maximization Algorithm Vibhav Gogate The University of Texas at Dallas Slides adapted from Carlos Guestrin, Dan Klein, Luke Zettlemoyer and Dan Weld

2 The Evils of Hard Assignments? Clusters may overlap Some clusters may be wider than others Distances can be deceiving!

3 Probabilistic Clustering Try a probabilistic model! allows overlaps, clusters of different size, etc. Can tell a generative story for data P(X Y) P(Y) Challenge: we need to estimate model parameters without labeled Ys Y X 1 X 2?? ?? ?? ?? ??

4 The General GMM assumption P(Y): There are k components P(X Y): Each component generates data from a multivariate Gaussian with mean μ i and covariance matrix S i Each data point is sampled from a generative process: 1. Choose component i with probability P(y=i) 2. Generate datapoint ~ N(m i, S i ) Gaussian mixture model (GMM) m 1 m 2 m 3

5 What Model Should We Use? Depends on X! Here, maybe Gaussian Naïve Bayes? Multinomial over clusters Y Gaussian over each X i given Y Y X 1 X 2?? ?? ?? ?? ??

6 Could we make fewer assumptions? What if the X i co-vary? What if there are multiple peaks? Gaussian Mixture Models! P(Y) still multinomial P(X Y) is a multivariate Gaussian dist n P(X = x j Y = i) = 1 (2p) m/2 S i exp é - 1 1/2 ê 2 x -m j i ë ( ) T S 1 ī ( x j - m i ) ù ú û

7 The General GMM assumption 1.What s a Multivariate Gaussian? 2.What s a Mixture Model? m 1 m 2 m 3

8 Review: Gaussians

9 Learning Gaussian Parameters (given fully-observable data)

10 Geometry of the Multivariate Gaussian Eigenvalue, λ Eigenvector u i Covariance matrix, Σ, = degree to which x i vary together

11 Multivariate Gaussians Σ identity matrix 12

12 Multivariate Gaussians Σ = diagonal matrix X i are independent ala Gaussian NB 13

13 Multivariate Gaussians Σ = arbitrary (semidefinite) matrix specifies rotation (change of basis) eigenvalues specify relative elongation 14

14 The General GMM assumption 1.What s a Multivariate Gaussian? 2.What s a Mixture Model? m 1 m 2 m 3

15 Time to Eruption Mixtures of Gaussians (1) Old Faithful Data Set Duration of Last Eruption

16 Mixtures of Gaussians (1) Old Faithful Data Set Single Gaussian Mixture of two Gaussians

17 Mixtures of Gaussians (2) Combine simple models into a complex model: Component Mixing coefficient K=3

18 Mixtures of Gaussians (3)

19 Eliminating Hard Assignments to Clusters Model data as mixture of multivariate Gaussians

20 Eliminating Hard Assignments to Clusters Model data as mixture of multivariate Gaussians

21 Eliminating Hard Assignments to Clusters Model data as mixture of multivariate Gaussians π i = probability point was generated from i th Gaussian

22 Detour/Review: Supervised MLE for GMM How do we estimate parameters for Gaussian Mixtures with fully supervised data? Have to define objective and solve optimization problem. P(y = i, x j ) = 1 (2p) m/2 S i exp é - 1 1/2 ê 2 x -m j i ë ( ) T S 1 ī ( x j -m i ) ù úp(y = i) û For example, MLE estimate has closed form solution: m ML = 1 n n åx n j=1 S ML = 1 n n å( x j - m )( ML x j - m ) T ML j=1

23 Univariate Gaussian Compare Mixture of Multivariate Gaussians m ML = 1 n n åx n j=1 S ML = 1 n n å( x j - m )( ML x j - m ) T ML j=1

24 That was easy! MLE: But what if unobserved data? argmax θ j P(y j,x j ) θ: all model parameters eg, class probs, means, and variance for naïve Bayes But we don t know y j s!!! Maximize marginal likelihood: argmax θ j P(x j ) = argmax j i=1 k P(y j =i,x j )

25 How do we optimize? Closed Form? Maximize marginal likelihood: argmax θ j P(x j ) = argmax j i=1 k P(y j =i,x j ) Almost always a hard problem! Usually no closed form solution Even when P(X,Y) is convex, P(X) generally isn t For all but the simplest P(X), we will have to do gradient ascent, in a big messy space with lots of local optimum

26 Learning general mixtures of Gaussian P(y i x j ) 1 (2) m / 2 S i exp 1 1/ 2 2 x m j i T S 1 i x j m i P(y i) Marginal likelihood: m P(x j ) P(x j,y i) j1 m k j1 m i1 k j1 i1 1 (2) m / 2 S i exp 1 1/ 2 2 x m j i T S 1 i x j m i P(y i) Need to differentiate and solve for μ i, Σ i, and P(Y=i) for i=1..k There will be no closed form solution, gradient is complex, lots of local optimum Wouldn t it be nice if there was a better way!?!

27 Expectation Maximization

28 The EM Algorithm A clever method for maximizing marginal likelihood: argmax θ j P(x j ) = argmax θ j i=1 k P(y j =i,x j ) A type of gradient ascent that can be easy to implement (eg, no line search, learning rates, etc.) Alternate between two steps: Compute an expectation Compute a maximization Not magic: still optimizing a non-convex function with lots of local optima The computations are just easier (often, significantly so!)

29 EM: Two Easy Steps Objective: argmax θ j i=1 k P(y j =i,x j θ) = j log i=1 k P(y j =i,x j θ) Data: {x j j=1.. n} Notation a bit inconsistent Parameters = = E-step: Compute expectations to fill in missing y values according to current parameters, For all examples j and values i for y, compute: P(y j =i x j, θ) M-step: Re-estimate the parameters with weighted MLE estimates Set θ = argmax θ j i P(y j =i x j, θ) log P(y j =i,x j θ) Especially useful when the E and M steps have closed form solutions!!!

30 EM algorithm: Pictorial View Given a set of Parameters and training data Supervised learning problem Relearn the parameters based on the new training data Class assignment is probabilistic or weighted (soft EM) Class assignment is hard (hard EM) Estimate the class of each training example using the parameters yielding new (weighted) training data

31 Simple example: learn means only! Consider: 1D data Mixture of k=2 Gaussians Variances fixed to σ=1 Dist n over classes is uniform Just need to estimate μ 1 and μ m k m k é Õ åp(x, y = i) µ Õåexpê- 1 ë 2s 2 j=1 i=1 j=1 i=1 x - m i 2 ù úp(y = i) û

32 EM for GMMs: only learning means Iterate: On the t th iteration let our estimates be t = { μ 1 (t), μ 2 (t) μ k (t) } E-step Compute expected classes of all datapoints M-step py i x j,m 1...m k exp x j m i Compute most likely new μs given class expectations m i m j1 m j1 Py i x j Py i x j x j 2 Py i

33 E.M. for General GMMs Iterate: On the t th iteration let our estimates be t = { μ 1 (t), μ 2 (t) μ k (t), S 1 (t), S 2 (t) S k (t), p 1 (t), p 2 (t) p k (t) } E-step Compute expected classes of all datapoints for each class ) ( ) ( ) (, p, P t i t i j t i t j x p x i y S m p i (t) is shorthand for estimate of P(y=i) on t th iteration M-step Compute weighted MLE for μ given expected classes above j t j j j t j t i x i y x x i y, P, P μ 1, P, P S j t j T t i j t i j j t j t i x i y x x x i y m m m x i y p j t j t i, P 1) ( m = #training examples Just evaluate a Gaussian at x j

34 Gaussian Mixture Example: Start

35 After first iteration

36 After 2nd iteration

37 After 3rd iteration

38 After 4th iteration

39 After 5th iteration

40 After 6th iteration

41 After 20th iteration

42 What if we do hard assignments? Iterate: On the t th iteration let our estimates be θ t = { μ 1 (t), μ 2 (t) μ k (t) } E-step Compute expected classes of all datapoints py i x j,m 1...m k exp 1 x 2 2 j m i M-step Compute most likely new μs given class expectations m i m j1 m j1 Py i x j Py i x j x j m i = d y = i, x j m å j=1 2 Py i ( ) x j ( ) d y = i, x j δ represents hard assignment to most likely or nearest cluster Equivalent to k-means clustering algorithm!!!

43 Lets look at the math behind the magic! We will argue that EM: Optimizes a bound on the likelihood Is a type of coordinate ascent Is guaranteed to converge to a (often local) optima

44 The general learning problem with missing data Marginal likelihood: x is observed, z (eg class labels, y) is missing: Objective: Find argmax θ l(θ:data)

45 A Key Computation: E-step x is observed, z is missing Compute probability of missing data given current choice of Q(z x j ) for each x j e.g., probability computed during classification step corresponds to classification step in K-means

46 Properties of EM We will prove that EM converges to a local minima Each iteration improves the log-likelihood How? (Same as k-means) E-step can never decrease likelihood M-step can never decrease likelihood

47 Jensen s inequality Theorem: log z P(z) f(z) z P(z) log f(z) e.g., Binary case for convex function f:

48 Applying Jensen s inequality Use: log z P(z) f(z) z P(z) log f(z)

49 The M-step Lower bound: Maximization step: We are optimizing a lower bound! Use expected counts to do weighted learning: If learning requires Count(x,z) Use E Q(t+1) [Count(x,z)] Looks a bit like boosting!!!

50 Convergence of EM Define: potential function F(,Q): lower bound from Jensen s inequality EM is coordinate ascent on F! Thus, maximizes lower bound on marginal log likelihood

51 M-step can t decrease F(θ,Q): by definition! We are maximizing F directly, by ignoring a constant!

52 E-step: more work to show that F(θ,Q) doesn t decrease KL-divergence: measures distance between distributions KL=zero if and only if Q=P

53 E-step also doesn t decrease F: Step 1 Fix to (t), take a max over Q:

54 E-step also doesn t decrease F: Step 2 Fixing to (t) : Now, the max over Q yields: Q(z x j ) P(z x j, (t) ) Why? The likelihood term is a constant; the KL term is zero iff the arguments are the same distribution!! So, the E-step is actually a maximization / tightening of the bound. It ensures that:

55 EM is coordinate ascent M-step: Fix Q, maximize F over (a lower bound on ): E-step: Fix, maximize F over Q: Realigns F with likelihood:

56 What you should know K-means for clustering: algorithm converges because it s coordinate ascent Know what agglomerative clustering is EM for mixture of Gaussians: Also coordinate ascent How to learn maximum likelihood parameters (locally max. like.) in the case of unlabeled data Relation to K-means Hard / soft clustering Probabilistic model Remember, E.M. can get stuck in local minima, And empirically it DOES

57 Acknowledgements K-means & Gaussian mixture models presentation contains material from excellent tutorial by Andrew Moore: K-means Applet: /tutorial_html/appletkm.html Gaussian mixture models Applet: