Doug Downey, adapted from Bryan Pardo,Northwestern University Machine Learning Hidden Markov Models.

transcript

Doug Downey, adapted from Bryan Pardo,Northwestern University

Machine Learning

Hidden Markov Models

The Markov Property

A stochastic process has the Markov property if the conditional probability of future states of the process, depends only upon the present state.

i.e. what I’m likely to do next depends only on where I am now, NOT on how I got here.

P(qt | qt-1,…,q1) = P(qt | qt-1)

Which processes have the Markov property?

Markov model for Dow Jones

The Dishonest Casino

A casino has two dice:• Fair die

P(1) = P(2) =…= P(5) = P(6) = 1/6• Loaded die

P(1) = P(2) =…= P(5) = 1/10; P(6) = ½

I think the casino switches back and forth between fair and loaded die once every 20 turns, on average

My dishonest casino model

FAIR LOADED

0.950.95

P(1|F) = 1/6P(2|F) = 1/6P(3|F) = 1/6P(4|F) = 1/6P(5|F) = 1/6P(6|F) = 1/6

P(1|L) = 1/10P(2|L) = 1/10P(3|L) = 1/10P(4|L) = 1/10P(5|L) = 1/10P(6|L) = 1/2

This is a hidden Markov model (HMM)

Elements of a Hidden Markov Model

• A finite set of states Q = { q1, ..., qK }

• A set of transition probabilities between states, A…each aij, in A is the prob. of going from state i to state j

• The probability of starting in each state 1, …, K} …each K in is the probability of starting in state k

• A set of emission probabilities, B…where each bi(oj) in B is the probability of observing output oj when in state i

My dishonest casino model

FAIR LOADED

0.950.95

This is a HIDDEN Markov model because the states are not directly observable.

If the fair die were red and the unfair die were blue, then the Markov model would NOT be hidden.

HMMs are good for…

• Speech Recognition• Gene Sequence Matching• Text Processing

– Part of speech tagging– Information extraction– Handwriting recognition

The Three Basic Problems for HMMs

• Given: observation sequence O=(o1o2…oT), of events from the alphabet , and HMM model = (A,B,)…

• Problem 1 (Evaluation): What is P(O| ), the probability of the observation sequence,

given the model

• Problem 2 (Decoding): What sequence of states Q=(q1q2…qT) best explains the

observations

• Problem 3 (Learning): How do we adjust the model parameters = (A,B,) to

maximize P(O| )?

The Evaluation Problem

• Given observation sequence O and HMM , compute P(O| )• Helps us pick which model is the best one

FAIR LOADED

0.950.95

FAIR LOADED

0.950.05

O = 1,6,6,2,6,3,6,6

Computing P(O|)

• Naïve: Try every path through the model

• Sum the probabilities of all possible paths

• This can be intractable. O(NT)• What we do instead:

– The Forward Algorithm. O(N2T)FAIR LOADED

0.950.05

The Forward Algorithm

The inductive step,

• Computation of t(j) by summing all previous values t-1(i) for all i

t-1(i) t(j)

A hidden state at time t-1 transitionprobability

Forward Algorithm Example

Observation sequence = 1,6,6,2

Model =P(1|F) = 1/6P(2|F) = 1/6P(3|F) = 1/6P(4|F) = 1/6P(5|F) = 1/6P(6|F) = 1/6

P(1|L) = 1/10P(2|L) = 1/10P(3|L) = 1/10P(4|L) = 1/10P(5|L) = 1/10P(6|L) = 1/2

P (fair) = .7P (loaded) = .3

FAIR LOADED

0.950.05

State 1 (fair)

State 2 (loaded)

2(i)1(i) 3(i) 4(i)

Start prob

0.7*1/6

0.3*1/10

1(1)*0.05*1/6+1(2)*0.05*1/6

1(1)*0.95*1/2+1(2)*0.95*1/2

2(1)*0.05*1/6+2(2)*0.05*1/6

3(1)*0.05*1/6+3(2)*0.05*1/6

2(1)*0.95*1/2+2(2)*0.95*1/2

3(1)*0.95*1/10+3(2)*0.95*1/10

Markov model for Dow Jones

Forward trellis for Dow Jones

The Decoding Problem

• What sequence of states Q=(q1q2…qT) best explains the observation sequence O=(o1o2…oT)?

• Helps us find the path through a model.

ART N V ADV

The dog sat quietly

The Decoding Problem

What sequence of states Q=(q1q2…qT) best explains the observation sequence O=(o1o2…oT)?

• Viterbi Decoding: – slight modification of the forward algorithm– the major difference is the maximization over previous

states

Note: Most likely state sequence is not the same as the sequence of most likely states

The Viterbi Algorithm

The Forward inductive step

• Computation of t(j)

t-1(j)

)()()( 1 tjiji

tt obakj

The Viterbi inductive step

• Computation of vt(j)

vt-1(i)

))()((max)( 1 tjkjti

t obaivjv

Keep track of who the predecessor was at each step.

Viterbi for Dow Jones

The Learning Problem

• Given O, how do we adjust the model parameters = (A,B,) to maximize P(O| )?

• In other words: How do we make a hidden Markov Model that best models the what we observe?

Baum-Welch Local Maximization

• 1st step: You determine– The number of hidden states, N– The emission (observation alphabet)

• 2nd step: randomly assign values to…A - the transition probabilitiesB - the observation (emission) probabilities - the starting state probabilities

• 3rd step: Let the machine re-estimateA, B,

Estimation Formulae

expected frequency of state at time 1

expected num transitions from state to state

expected num of transitions from state

expected num of observations of symbol in state ( )

expected

k jb k

number of times in state j

Learning transitions…

),|,(),( 1 OSqSqPji jtitt

Math…

lttlklt

ttjijtt

ttijtt

jObaiji

OSqSqPji

)()()(

)()()(),(

),|,(),(

Estimation of starting probs.

1 at time i state offrequency expected

This is number of transitions from i at time t

Estimation Formulae

expected num transitions from state to state

expected num of transitions from state

Estimation Formulae

1Such that

expected num of observations of symbol in state ( )

expected number of times in state

( )( )

O vj T

k jb k

What are we maximizing again?

is... model dreestimateOur

is... modelcurrent The

The game is…

• EITHER the current model is at a local maximum and…

reestimate = current model

• OR our reestimate will be slightly better and…

reestimate != current model

• SO we feed in the reestimate as the current model, over and over until we can’t improve any more.

Caveats

• This is a kind of hill-climbing technique– Often has serious problems with local

maxima– You don’t know when you’re done

So…how else could we do this?

• Standard gradient descent techniques?– Hill climb?– Beam search?– Genetic Algorithm?

Back to the fundamental question

• Which processes have the Markov property?– What if a hidden state variable is

included?(an in an HMM)

Doug Downey, adapted from Bryan Pardo,Northwestern University Machine Learning Hidden Markov Models.

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