department of computer science csci 5622: machine learning ... · •a general way to model count...

Department of Computer ScienceCSCI 5622: Machine Learning

Chenhao TanLecture 20: Topic modeling and variational inferrence

Slides adapted from Jordan Boyd-Graber, Chris Ketelsen

1

Administrivia

• Poster printing (stay tuned!)• HW 5 (final homework) is due next Friday!• Midpoint feedback

2

Learning Objectives

• Learn about latent Dirichlet allocation

• Understand the inituion behind variational inference

3

Topic models

• Discrete count data

4

Topic models

5

• Suppose you have a huge number of documents

• Want to know what's going on• Can't read them all (e.g. every New

York Times article from the 90's)• Topic models offer a way to get a

corpus-level view of major themes• Unsupervised

Why should you care?

• Neat way to explore/understand corpus collections• E-discovery• Social media• Scientific data

• NLP Applications• Word sense disambiguation• Discourse segmentation

• Psychology: word meaning, polysemy• A general way to model count data and a general inference

algorithm6

Conceptual approach

• Input: a text corpus and number of topics K • Output:

• K topics, each topic is a list of words• Topic assignment for each document

7

Forget the Bootleg, Just Download the Movie LegallyMultiplex Heralded As

Linchpin To GrowthThe Shape of Cinema, Transformed At the Click of

a MouseA Peaceful Crew Puts

Muppets Where Its Mouth IsStock Trades: A Better Deal For Investors Isn't SimpleThe three big Internet portals begin to distinguish

among themselves as shopping malls

Red Light, Green Light: A 2-Tone L.E.D. to Simplify Screens

Corpus

Conceptual approach

8

computer,

technology,

system,

service, site,

phone,

internet,

machine

play, film,

movie, theater,

production,

star, director,

stage

sell, sale,

store, product,

business,

advertising,

market,

consumer

TOPIC 1 TOPIC 2 TOPIC 3

• K topics, each topic is a list of words

Conceptual approach

9

• Topic assignment for each document

Forget the Bootleg, Just Download the Movie Legally

Multiplex Heralded As Linchpin To

GrowthThe Shape of

Cinema, Transformed At the Click of a

Mouse A Peaceful Crew Puts Muppets

Where Its Mouth Is

Stock Trades: A Better Deal For Investors Isn't

Simple

Internet portals begin to distinguish among themselves as shopping malls

Red Light, Green Light: A

2-Tone L.E.D. to Simplify Screens

TOPIC 2"BUSINESS"

TOPIC 3"ENTERTAINMENT"

TOPIC 1"TECHNOLOGY"

Topics from Science

10

Topic models

• Discrete count data• Gaussian distributions are not appropriate

11

Generative model: Latent Dirichlet Allocation• Generate a document, or a bag of words• Blei, Ng, Jordan. Latent Dirichlet Allocation. JMLR, 2003.

12

Generative model: Latent Dirichlet Allocation• Generate a document, or a bag

of words• Multinomial distribution

• Distribution over discrete outcomes

• Represented by non-negative vector that sums to one

• Picture representation

13

(1,0,0) (0,0,1)

(1/2,1/2,0)(1/3,1/3,1/3) (1/4,1/4,1/2)

(0,1,0)

Generative model: Latent Dirichlet Allocation• Generate a document, or a bag

of words• Multinomial distribution

• Distribution over discrete outcomes

• Represented by non-negative vector that sums to one

• Picture representation• Come from a Dirichlet distribution

14

(1,0,0) (0,0,1)

(1/2,1/2,0)(1/3,1/3,1/3) (1/4,1/4,1/2)

(0,1,0)

Generative story

15

computer,

technology,

system,

service, site,

phone,

internet,

machine

play, film,

movie, theater,

production,

star, director,

stage

sell, sale,

store, product,

business,

advertising,

market,

consumer

TOPIC 1

TOPIC 2

TOPIC 3

Generative story

16

Forget the Bootleg, Just Download the Movie Legally

Multiplex Heralded As Linchpin To Growth

The Shape of Cinema, Transformed At the Click of

a Mouse

A Peaceful Crew Puts Muppets Where Its Mouth Is

Stock Trades: A Better Deal For Investors Isn't Simple

The three big Internet portals begin to distinguish

among themselves as shopping mallsRed Light, Green Light: A

2-Tone L.E.D. to Simplify Screens

TOPIC 2

TOPIC 3

TOPIC 1

Generative story

17

Hollywood studios are preparing to let people

download and buy electronic copies of movies over

the Internet, much as record labels now sell songs for

99 cents through Apple Computer's iTunes music store

and other online services ...

computer, technology,

system, service, site,

phone, internet, machine

play, film, movie, theater,

production, star, director,

stage

sell, sale, store, product,

business, advertising,

market, consumer

Generative story

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stage



market, consumer

Generative story

19











stage



market, consumer

Generative story

20











stage



market, consumer

Missing component: how to generate a multinomial distribution

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What are Topic Models?

Dirichlet Distribution

Machine Learning: Jordan Boyd-Graber | Boulder Topic Models | 12 of 26


23

Conjugacy of Dirichlet and Multinomial

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• If � ⇠ Dir(↵), w ⇠ Mult(�), and nk = |{wi : wi = k}| then

p(�|↵,w) / p(w |�)p(�|↵) (1)

/Y

k

�nkY

k

�↵k�1 (2)

/Y

k

�↵k+nk�1 (3)

• Conjugacy: this posterior has the same form as the prior


Conjugacy of Dirichlet and Multinomial

25



• If � ⇠ Dir(↵), w ⇠ Mult(�), and nk = |{wi : wi = k}| then

p(�|↵,w) / p(w |�)p(�|↵) (1)

/Y

k

�nkY

k

�↵k�1 (2)

/Y

k

�↵k+nk�1 (3)

• Conjugacy: this posterior has the same form as the prior


Making the generative story formal

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Generative Model Approach

MNθd zn wn

Kβk

α

λ

• For each topic k 2 {1, . . . ,K}, draw a multinomial distribution �kfrom a Dirichlet distribution with parameter �

• For each document d 2 {1, . . . ,M}, draw a multinomialdistribution ✓d from a Dirichlet distribution with parameter ↵

• For each word position n 2 {1, . . . ,N}, select a hidden topic znfrom the multinomial distribution parameterized by ✓.

• Choose the observed word wn from the distribution �zn .


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MNθd zn wn

Kβk

α

λ







28

Making the generative story formalWhat are Topic Models?


MNθd zn wn

Kβk

α

λ






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MNθd zn wn

Kβk

α

λ




• Choose the observed word wn from the distribution �zn .Machine Learning: Jordan Boyd-Graber | Boulder Topic Models | 16 of 26

Topic models: What’s important

• Topic models (latent variables)• Topics to word types—multinomial distribution• Documents to topics—multinomial distribution

• Modeling & Algorithm• Model: story of how your data came to be• Latent variables: missing pieces of your story• Statistical inference: filling in those missing pieces

• We use latent Dirichlet allocation (LDA), a fully Bayesian version of pLSI, probabilistic version of LSA

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31

Which variables are hidden?

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Size of Variable

Joint distribution

33

Joint distribution

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Posterior distribution

35

Variational inference

36

KL divergence and evidence lower bound

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KL divergence and evidence lower bound

38

A different way to get ELBO

• Jensen’s inequality

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Evidence Lower Bound

40

Evidence Lower Bound

41

Variational inference

• Propose variational distribution q• Find ELBO (evidence lower bound) using q• Set derivatives to 0 and update variables

42

Variational distribution for LDA

43

Overall Algorithm

44

Updates to Maximize ELBO

45

Homework 5

• The original algorithm also updates alphas• Not required for the homework

46

Evaluation

Example

• Three topics

� =

2

664

cat dog hamburger iron pig.26 .185 .185 .185 .185.185 .185 .26 .185 .185.185 .185 .185 .26 .185

3

775 (4)

• Assume uniform �: (2.0, 2.0, 2.0)

• Compute update for �

�ni

/ �iv

exp

0

@ (�

i

) �

0

@X

j

�j

1

A

1

A (5)

• For the first word (dog) in the document: dog cat cat pig


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Evaluation

Update � for dog

� =

2

664


3

775

�ni

/

�iv

exp

0

@ (�

i

) �

0

@X

j

�j

1

A

1

A

• � = (2.000, 2.000, 2.000)

• �(0) / 0.185 ⇥ exp ( (2.000) � (2.000 + 2.000 + 2.000)) =

0.051• �(1) / 0.185 ⇥ exp ( (2.000) � (2.000 + 2.000 + 2.000)) =

0.051• �(2) / 0.185 ⇥ exp ( (2.000) � (2.000 + 2.000 + 2.000)) =

0.051• After normalization: {0.333, 0.333, 0.333}


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Evaluation

Update � for pig

� =

2

664


3

775

�ni

/

�iv

exp

0

@ (�

i

) �

0

@X

j

�j

1

A

1

A

• � = (2.000, 2.000, 2.000)

• �(0) / 0.185 ⇥ exp ( (2.000) � (2.000 + 2.000 + 2.000)) =

0.051• �(1) / 0.185 ⇥ exp ( (2.000) � (2.000 + 2.000 + 2.000)) =

0.051• �(2) / 0.185 ⇥ exp ( (2.000) � (2.000 + 2.000 + 2.000)) =



Evaluation

Update � for cat

� =

2

664


3

775

�ni

/

�iv

exp

0

@ (�

i

) �

0

@X

j

�j

1

A

1

A

• � = (2.000, 2.000, 2.000)

• �(0) / 0.260 ⇥ exp ( (2.000) � (2.000 + 2.000 + 2.000)) =

0.072• �(1) / 0.185 ⇥ exp ( (2.000) � (2.000 + 2.000 + 2.000)) =

0.051• �(2) / 0.185 ⇥ exp ( (2.000) � (2.000 + 2.000 + 2.000)) =



50

Evaluation

Update �

• Document: dog cat cat pig• Update equation

�i

= ↵i

+

X

n

�ni

(6)

• Assume ↵ = (.1, .1, .1)

�0 �1 �2dog .333 .333 .333cat .413 .294 .294 x2pig .333 .333 .333↵ 0.1 0.1 0.1

sum 1.592 1.354 1.354

• Note: do not normalize!


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Evaluation

Update �

• Count up all of the � across all documents• For each topic, divide by total• Corresponds to maximum likelihood of expected counts

• Unlike Gibbs sampling, no Dirichlet prior


52

Recap

• Topic models: a neat way to model discrete count data• Variational inference converts intractable optimization to

maximizing ELBO

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