1. 中文信息处理 Chinese NLP
Lecture 7

2. 词——词性标注（2） Part-of-Speech Tagging (2)
统计模型的训练（Training a statistical model）
马尔可夫链（Markov chain)
隐马尔可夫模型（Hidden Markov Model, or HMM）
隐马尔可夫标注算法（HMM POS tagging）

3. 统计模型的训练 Training a Statistical Model
Back to POS tagging
Given a word sequence 𝑤 1 𝑛 = 𝑤 1 … 𝑤 𝑛 , decide its best POS sequence 𝑡 1 𝑛 among all 𝑡 1 𝑛 = 𝑡 1 … 𝑡 𝑛 .
𝑡 1 𝑛 = 𝑎𝑟𝑔𝑚𝑎𝑥 𝑡 1 𝑛 𝑃 𝑡 1 𝑛 𝑤 1 𝑛
= 𝑎𝑟𝑔𝑚𝑎𝑥 𝑡 1 𝑛 𝑃 𝑤 1 𝑛 𝑡 1 𝑛 𝑃( 𝑡 1 𝑛 ) 𝑃( 𝑤 1 𝑛 )
= 𝑎𝑟𝑔𝑚𝑎𝑥 𝑡 1 𝑛 𝑃 𝑤 1 𝑛 𝑡 1 𝑛 𝑃( 𝑡 1 𝑛 )
Bayes Rule
Likelihood
Prior

4. Simplifying assumptions
Computing Probabilities
𝑃( 𝑤 1 𝑛 | 𝑡 1 𝑛 )≈ 𝑖=1 𝑛 𝑃( 𝑤 𝑖 | 𝑡 𝑖 )
𝑃( 𝑡 1 𝑛 )≈ 𝑖=1 𝑛 𝑃( 𝑡 𝑖 | 𝑡 𝑖−1 )
𝑡 1 𝑛 = 𝑎𝑟𝑔𝑚𝑎𝑥 𝑡 1 𝑛 𝑃 𝑤 1 𝑛 𝑡 1 𝑛 𝑃( 𝑡 1 𝑛 )
≈ 𝑎𝑟𝑔𝑚𝑎𝑥 𝑡 1 𝑛 𝑖=1 𝑛 𝑃( 𝑤 𝑖 | 𝑡 𝑖 ) 𝑃( 𝑡 𝑖 | 𝑡 𝑖−1 )
𝑃 𝑡 𝑖 𝑡 𝑖−1 = 𝐶𝑜𝑢𝑛𝑡( 𝑡 𝑖−1 , 𝑡 𝑖 ) 𝐶𝑜𝑢𝑛𝑡( 𝑡 𝑖−1 )
𝑃 𝑤 𝑖 𝑡 𝑖 = 𝐶𝑜𝑢𝑛𝑡( 𝑡 𝑖 , 𝑤 𝑖 ) 𝐶𝑜𝑢𝑛𝑡( 𝑡 𝑖 )
Simplifying assumptions
Counts from corpus
The above probability computation is oversimplified. Consult the textbook about deleted interpolation and other smoothing methods for better probability computation.

5. Using Probabilities for POS Tagging
Example
What POS is race in Secretariat is expected to race tomorrow?

6. Using Probabilities for POS Tagging
Example
Using the (87-tag) Brown corpus, we get
P(NN|TO) = P(VB|TO) = 0.83
P(race|NN) = P(race|VB) =
P(NR|VB) = P(NR|NN) =
Compare
P(VB|TO) P(NR|VB) P(race|VB) =
P(NN|TO) P(NR|NN) P(race|NN) =

7. 马尔可夫链 Markov Chain Definition
A Markov chain is a special case of a weighted automaton in which the input sequence uniquely determines which states the automaton will go through.

8. Graphical Model Representation
A set of N states: Q = q1q2 … qN
A transition probability matrix: A = a01a02 … an1 … ann
𝑗=1 𝑛 𝑎 𝑖𝑗 =1 ∀𝑖
A special start state and end state: q0, qF
Alternately, we use an initial probability distribution over states.
π = π1π2 … πN. πi is the probability that the Markov chain will start in state i 𝑖=1 𝑛 𝜋 𝑖 =1

9. What is Special about Markov Chains
A Markov chain can’t represent inherently ambiguous problems, so it is only useful for assigning probabilities to unambiguous sequences.
A Markov chain is not suitable for POS tagging, because the states (POS) cannot be directly observed.
Markov assumption:
𝑃 𝑞 𝑖 | 𝑞 1 … 𝑞 𝑖−1 =𝑃( 𝑞 𝑖 | 𝑞 𝑖−1 )

10. In-Class Exercise
Using the following Markov chain, compute the probability of the sequence: {cold hot cold hot}

11. 隐马尔可夫模型 Hidden Markov Model
Markov Chain vs HMM
A Markov chain is useful when we need to compute a probability for a sequence of events that we can observe in the world.
HMM allows us to talk about both observed events (like words that we see in the input) and hidden events (like POS tags).

12. HMM Components A set of N states: Q = q1q2 … qN
A transition probability matrix: A = a11a12 … an1 … ann
𝑗=1 𝑛 𝑎 𝑖𝑗 =1 ∀𝑖
A sequence of T observations: O = o1o2 … oT
A sequence of observation likelihoods, or emission probabilities:
B = bi(oT)
A special start state and end state: q0, qF
Alternately, we use an initial probability distribution over states.
π = π1π2 … πN. πi is the probability that the Markov chain will start in state i 𝑖=1 𝑛 𝜋 𝑖 =1

13. HMM Assumptions Fundamental Problems Markov assumption
𝑃 𝑞 𝑖 | 𝑞 1 … 𝑞 𝑖−1 =𝑃( 𝑞 𝑖 | 𝑞 𝑖−1 )
Output independence assumption
𝑃 𝑜 𝑖 | 𝑞 1 … 𝑞 𝑖 … 𝑞 𝑇 , 𝑜 1 … 𝑜 𝑖 … 𝑜 𝑇 =𝑃( 𝑜 𝑖 | 𝑞 𝑖 )
Fundamental Problems
Computing likelihood: Given an HMM λ = (A,B) and an observation sequence O, determine the likelihood P(O|λ).
Decoding: Given an observation sequence O and an HMM λ = (A,B), discover the best hidden state sequence Q.
Learning: Given an observation sequence O and the set of states in the HMM, learn the HMM parameters A and B.

14. A Running Example Jason eating ice creams on some day
There is some relation between weather states (hot, cold) and the number of ice creams eaten on that day.
An integer represents the number of ice creams eaten on a given day (observed), and a sequence of H and C designates the weather states (hidden) that caused Jason to eat the ice cream.

15. Computing Likelihood
Given an HMM model, what is the likelihood of {3, 1, 3}? Note that we do not know the hidden states (weather).
Forward algorithm (a kind of dynamic programming)
αt(j) represents the probability of being in state j after seeing the first t observations, given the model λ.
𝛼 𝑡 𝑗 =𝑃( 𝑜 1 , 𝑜 2 … 𝑜 𝑡 , 𝑞 𝑡 =𝑗|𝜆)
= 𝑖=1 𝑁 𝛼 𝑡−1 𝑖 𝑎 𝑖𝑗 𝑏 𝑗 ( 𝑜 𝑡 )
the state observation likelihood of the observation symbol ot given
the current state j
the previous forward path probability from the previous time step
the transition probability from previous state qi to current state qj

16. Computing Likelihood Algorithm Initialization
𝛼 1 𝑗 = 𝑎 0𝑗 𝑏 𝑗 𝑜 ≤𝑗≤𝑛
Recursion
𝛼 𝑡 𝑗 = 𝑖=1 𝑁 𝛼 𝑡−1 𝑖 𝑎 𝑖𝑗 𝑏 𝑗 ( 𝑜 𝑡 ) 1≤𝑗≤𝑛, 1<𝑡≤𝑇
Termination
𝑃 𝑂 𝜆 = 𝛼 𝑇 𝑞 𝐹 = 𝑖=1 𝑁 𝛼 𝑇 (𝑖) 𝑎 𝑖𝐹
forward[s, t] = 𝛼 𝑡 𝑠

17. Computing Likelihood

18. Decoding
Given an HMM model and an ice cream sequence {3, 1, 3}, what is the hidden weather states?
Viterbi algorithm (a kind of dynamic programming)
vt(j) represents the probability that the HMM is in state j after seeing the first t observations and passing through the most probable state sequence q0,q1, ...,qt−1,, given the model λ.
𝑣 𝑡 𝑗 = max 𝑞 0 , 𝑞 1 … 𝑞 𝑡−1 𝑃( 𝑞 0 , 𝑞 1 … 𝑞 𝑡−1 , 𝑜 1 , 𝑜 2 … 𝑜 𝑡 , 𝑞 𝑡 =𝑗|𝜆)
= max 1≤𝑖≤𝑁 𝑣 𝑡−1 (𝑖) 𝑎 𝑖𝑗 𝑏 𝑗 ( 𝑜 𝑡 )
the state observation likelihood of the observation symbol ot given
the current state j
the previous Viterbi path probability from the previous time step
the transition probability from previous state qi to current state qj

19. Decoding Algorithm Initialization 𝑣 1 𝑗 = 𝑎 0𝑗 𝑏 𝑗 𝑜 1 1≤𝑗≤𝑛 𝑏𝑡 1 𝑗 =0
𝑣 1 𝑗 = 𝑎 0𝑗 𝑏 𝑗 𝑜 ≤𝑗≤𝑛
𝑏𝑡 1 𝑗 =0
Recursion
𝑣 𝑡 𝑗 = max 1≤𝑖≤𝑁 𝑣 𝑡−1 𝑖 𝑎 𝑖𝑗 𝑏 𝑗 ( 𝑜 𝑡 ), 1≤𝑗≤𝑛, 1<𝑡≤𝑇
𝑏𝑡 𝑡 𝑗 = argmax 1≤𝑖≤𝑁 𝑣 𝑡−1 𝑖 𝑎 𝑖𝑗 𝑏 𝑗 ( 𝑜 𝑡 ), 1≤𝑗≤𝑛, 1<𝑡≤𝑇
Termination
𝑃∗= 𝑣 𝑇 𝑞 𝐹 = max 1≤𝑖≤𝑁 𝑣 𝑇 𝑖 𝑎 𝑖𝐹
𝑞 𝑇 ∗= 𝑏𝑡 𝑇 𝑞 𝐹 = 𝑎𝑟𝑔max 1≤𝑖≤𝑁 𝑣 𝑇 𝑖 𝑎 𝑖𝐹

20. Decoding
Algorithm

21. Decoding

22. Learning
Given an ice cream sequence {3, 1, 3} and the set of possible weather states {H, C}, what are the HMM parameters (A and B)?
Forward-Backward algorithm (a kind of Expectation Maximization)
βt(j) represents the probability of seeing the observations from time t+1 to the end, given that we are in state j at time t, and the model λ.
𝛽 𝑡 𝑗 =𝑃( 𝑜 𝑡+1 , 𝑜 𝑡+2 … 𝑜 𝑇 | 𝑞 𝑡 =𝑗, 𝜆)
Initialization
𝛽 𝑇 𝑖 = 𝑎 𝑖𝐹 ≤𝑖≤𝑛
Recursion
𝛽 𝑡 𝑖 = 𝑗=1 𝑁 𝑎 𝑖𝑗 𝑏 𝑗 ( 𝑜 𝑡+1 )𝛽 𝑡+1 𝑗 1≤𝑗≤𝑛, 1≤𝑡≤𝑇
Termination
𝑃 𝑂 𝜆 = 𝛼 𝑇 𝑞 𝐹 = 𝛽 1 0 = 𝑗=1 𝑁 𝛼 0𝑗 𝑏 𝑗 𝑜 1 𝛽 1 (𝑗)

23. the probability of being in state j at time t
Learning
Algorithm
the probability of being in state j at time t
the probability of being in state i at
time t and state j at time t+1
Solving the learning problem is the most complicated. Consult your textbook to find more details.

24. 隐马尔可夫标注算法 HMM POS Tagging
Using Viterbi to solve the decoding problem
An English example
I want to race.
Transition probabilities A
Emission probabilities B

25. An English Example

26. In-Class Exercise
Compute V3(3) on the previous page, using the given probabilities. Note that you need to first compute all the V2(*).

27. Other Tagging Methods CLAWS (a brute-force algorithm)
In a word span (the beginning and end words have unique POS’s), calculate all the path possibilities and choose the maximum.
VOLSUNGA (a greedy algorithm)
As an improvement on CLAWS, it finds the optimal path step by step. In each step, it only considers the best path so far found. The ultimate optimal path is simply the sum of parts.

28. A Chinese Example
In implementation, we often use log probabilities to prevent numerical underflows due to small probability products. If we take the negative log probabilities, finding maximum product becomes finding minimum sum.
，报道新闻了，
Transition probabilities A / Transition Costs TC
Emission probabilities B / Emission Costs EC

29. The result is 报道/v 新闻/n 了/v
A Chinese Example
CLAWS
Path 1：w-n-n-u-w
Cost1 = TC[w,n]+TC[n,n]+TC[n,u]+TC[u,w]= =8.47
Path 2：w-n-n-v-w
Cost2 = TC[w,n]+TC[n,n]+TC[n,v]+TC[v,w]= =7.41
Path 3：w-n-n-y-w
Cost3 = TC[w,n]+TC[n,n]+TC[n,y]+TC[y,w]= =9.03
Path 4：w-v-n-u-w
Cost4 = TC[w,v]+TC[v,n]+TC[n,u]+TC[u,w]= =8.24
Path 5：w-v-n-v-w
Cost5 = TC[w,v]+TC[v,n]+TC[n,v]+TC[v,w]= =7.18
Path 6：w-v-n-y-w
Cost6 = TC[w,v]+TC[v,n]+TC[n,y]+TC[y,w]= =8.80
The result is 报道/v 新闻/n 了/v

30. The result is 报道/v 新闻/n 了/u
A Chinese Example
VOLSUNGA
Step1：min{TC[w,n]+EC[报道|n], TC[w,v]+EC[报道|v]}
= min{ , }
T[1] = v
Step2：min{TC[v,n]+EC[新闻|n]}
= min{ }
T[2] = n
Step3：min{TC[n,u]+EC[了|u], TC[n,v]+EC[了|v], TC[n,y]+EC[了|y]}
= min{ , , }
T[3] = u
The result is 报道/v 新闻/n 了/u

31. The result is 报道/v 新闻/n 了/y
A Chinese Example
Viterbi
Step1：k = 1
Cost[1, 报道/n] = (Cost[0, w] + TC[w,n]) + EC[报道|n] = 10.31
Cost[1, 报道/v] = (Cost[0, w] + TC[w,n]) + EC[报道|v] = 7.59
Step2：k = 2
Cost[2, 新闻/n] = min{(Cost[1, v] + TC[v,n]), (Cost[1, n] + TC[n,n])} EC[新闻|n] = = 15.86
Step3：k = 3
Cost[3, 了/u] = (Cost[2, n] + TC[n,u]) + EC[了|u] = 20.24
Cost[3, 了/v] = (Cost[2, n] + TC[n,v]) + EC[了|v] = 25.33
Cost[3, 了/y] = (Cost[2, n] + TC[n,y]) + EC[了|y] = 21.34
Step4: k = 4
Cost[4, ，/w] = min{(Cost[3, u] + TC[u,w]), (Cost[3, v] + TC[v,w]), (Cost[3, y] + TC[y,w])}+ EC[，|w] = = 21.42
Cost[k, t] comes from the negative log probability:
Cost[k, t] = min{Cost[k-1, s] + TC[s, t]} + EC[wk|t]
The result is 报道/v 新闻/n 了/y

32. Wrap-Up 统计模型的训练 马尔可夫链 隐马尔可夫标注算法 隐马尔可夫模型 Computing Likelihood
Computing Probabilities
马尔可夫链
Definition
Graphical Representation
隐马尔可夫模型
Components
Assumptions
Computing Likelihood
Decoding
Learning
隐马尔可夫标注算法
Viterbi
Examples
Vs Other Methods