Sequence Modelling with Features: Linear-Chain Conditional Random Fields. COMP-599 Oct 6, 2015
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1 Sequence Modelling with Features: Linear-Chain Conditional Random Fields COMP-599 Oct 6, 2015
2 Announcement A2 is out. Due Oct 20 at 1pm. 2
3 Outline Hidden Markov models: shortcomings Generative vs. discriminative models Linear-chain CRFs Inference and learning algorithms with linear-chain CRFs 3
4 Hidden Markov Model Graph specifies how join probability decomposes Q 1 Q 2 Q 3 Q 4 Q 5 O 1 O 2 O 3 O 4 O 5 T 1 P(O, Q) = P Q 1 P(Q t+1 Q t ) Initial state probability t=1 State transition probabilities T t=1 P(O t Q t ) Emission probabilities 4
5 Shortcomings of Standard HMMs How do we add more features to HMMs? Might be useful for POS tagging: Word position within sentence (1 st, 2 nd, last ) Capitalization Word prefix and suffixes (-tion, -ed, -ly, -er, re-, de-) Features that depend on more than the current word or the previous words. 5
6 Possible to Do with HMMs Add more emissions at each timestep Q 1 O 12 O 11 O 13 O 14 Clunky Word identity Capitalization Prefix feature Word position Is there a better way to do this? 6
7 Discriminative Models HMMs are generative models Build a model of the joint probability distribution P(O, Q), Let s rename the variables Generative models specify P(X, Y; θ gen ) If we are only interested in POS tagging, we can instead train a discriminative model Model specifies P(Y X; θ disc ) Now a task-specific model for sequence labelling; cannot use it for generating new samples of word and POS sequences 7
8 Generative or Discriminative? Naive Bayes P y x = P y i P x i y / P( x) Logistic regression P(y x) = 1 Z ea 1x 1 + a 2 x a n x n + b 8
9 Discriminative Sequence Model The parallel to an HMM in the discriminative case: linear-chain conditional random fields (linear-chain CRFs) (Lafferty et al., 2001) P Y X = 1 Z X exp t Z(X) is a normalization constant: k θ k f k (y t, y t 1, x t ) sum over all features sum over all time-steps Z X = y exp t k θ k f k (y t, y t 1, x t ) sum over all possible sequences of hidden states 9
10 Intuition Standard HMM: product of probabilities; these probabilities are defined over the identity of the states and words Transition from state DT to NN: P(y t+1 = NN y t = DT) Emit word the from state DT: P(x t = the y t = DT) Linear-chain CRF: replace the products by numbers that are NOT probabilities, but linear combinations of weights and feature values. 10
11 Features in CRFs Standard HMM probabilities as CRF features: Transition from state DT to state NN f DT NN (y t, y t 1, x t ) = 1(y t 1 = DT) 1(y t = NN) Emit the from state DT f DT the (y t, y t 1, x t ) = 1(y t = DT) 1(x t = the) Initial state is DT f DT y 1, x 1 = 1 y 1 = DT Indicator function: Let 1 condition = 1 if condition is true 0 otherwise 11
12 Features in CRFs Additional features that may be useful Word is capitalized f cap (y t, y t 1, x t ) = 1(y t =? )1(x t is capitalized) Word ends in ed f ed (y t, y t 1, x t ) = 1(y t =? )1(x t ends with ed) Exercise: propose more features 12
13 Inference with LC-CRFs Dynamic programming still works modify the forward and the Viterbi algorithms to work with the weightfeature products. HMM LC-CRF Forward algorithm P X θ Z(X) Viterbi algorithm argmax Y P(X, Y θ) argmax Y P(Y X, θ) 13
14 Forward Algorithm for HMMs Create trellis α i (t) for i = 1 N, t = 1 T α j 1 = π j b j (O 1 ) for j = 1 N for t = 2 T: for j = 1 N: N α j t = α i t 1 a ij b j (O t ) i=1 N P O θ = α j (T) j=1 Runtime: O(N 2 T) 14
15 Forward Algorithm for LC-CRFs Create trellis α i (t) for i = 1 N, t = 1 T α j 1 = exp k θ k init f k init (y 1 = j, x 1 ) for j = 1 N for t = 2 T: for j = 1 N: Z(X) = α j t = N j=1 N i=1 α j (T) Runtime: O(N 2 T) α i t 1 exp k θ k f k (y t = j, y t 1, x t ) Transition and emission probabilities replaced by exponent of weighted sums of features. Having Z(X) allows us to compute P(Y X) 15
16 Viterbi Algorithm for HMMs Create trellis δ i (t) for i = 1 N, t = 1 T δ j 1 = π j b j (O 1 ) for j = 1 N for t = 2 T: for j = 1 N: δ j t = max δ i t 1 a ij b j (O t ) i Take max δ i T i Runtime: O(N 2 T) 16
17 Viterbi Algorithm for LC-CRFs Create trellis δ i (t) for i = 1 N, t = 1 T δ j 1 = exp k θ init k f init k (y 1 = j, x 1 )for j = 1 N for t = 2 T: for j = 1 N: Take max i δ j t = max i δ i T δ i t 1 exp k θ k f k (y t = j, y t 1, x t ) Runtime: O(N 2 T) Remember that we need backpointers. 17
18 Training LC-CRFs Unlike for HMMs, no analytic MLE solution Use iterative method to improve data likelihood Gradient descent A version of Newton s method to find where the gradient is 0 18
19 Convexity Fortunately, l θ is a concave function (equivalently, its negation is a convex function). That means that we will find the global maximum of l θ with gradient descent. 19
20 Gradient of Log-Likelihood Find the gradient of the log likelihood of the training corpus: l θ = log i P(Y i X i ) 20
21 Regularization To avoid overfitting, we can encourage the weights to be close to zero. Add term to corpus log likelihood: l θ = log i P(Y i X i ) exp σ controls the amount of regularization k θ k 2 2σ 2 Results in extra term in gradient: θ k σ 2 21
22 Gradient Ascent Walk in the direction of the gradient to maximize l(θ) a.k.a., gradient descent on a loss function θ new = θ old + γ l(θ) γis a learning rate that specifies how large a step to take. There are more sophisticated ways to do this update: Conjugate gradient L-BFGS (approximates using second derivative) 22
23 Gradient Descent Summary Initialize θ = θ 1, θ 2,, θ k Do for a while: randomly Compute l(θ), which will require dynamic programming (i.e., forward algorithm) θ θ + γ l(θ) 23
24 Interpretation of Gradient Overall gradient is the difference between: and: i t f k (y i i t, y t 1, x i t ) the empirical distribution of feature f k in the training corpus i t y,y f k y, y, x t i P(y, y X i ) the expected distribution of f k as predicted by the current model 24
25 Interpretation of Gradient When the corpus likelihood is maximized, the gradient is zero, so the difference is zero. Intuitively, this means that finding MLE by gradient descent is equivalent to telling our model to predict the features in such a way that they are found in the same distribution as in the gold standard. 25
26 Stochastic Gradient Descent In the standard version of the algorithm, the gradient is computed over the entire training corpus. Weight update only once per iteration through training corpus. Alternative: calculate gradient over a small mini-batch of the training corpus and update weights then Many weight updates per iteration through training corpus Usually results in much faster convergence to final solution, without loss in performance 26
27 Stochastic Gradient Descent Initialize θ = θ 1, θ 2,, θ k Do for a while: randomly Randomize order of samples in training corpus For each mini-batch in the training corpus: Compute l(θ) over this mini-batch θ θ + γ l(θ) 27
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