# Teg Grenager NLP Group Lunch February 24, 2005

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Teg Grenager NLP Group Lunch February 24, 2005
Chinese Restaurants and Stick-Breaking: An Introduction to the Dirichlet Process Teg Grenager NLP Group Lunch February 24, 2005

Agenda Motivation Mixture Models Dirichlet Process Gibbs Sampling
Applications

Clustering Goal: learn a partition of the data, such that:
Data within classes are “similar” Classes are “different” from each other Two very different approaches: Agglomerative: build up clusters by iteratively sticking similar things together Mixture Model: learn a generative model over the data, treating the classes as hidden variables

Agglomerative Clustering
Num Clusters Max Distance 20 19 5 18 5 17 5 16 8 15 8 14 8 13 8 12 8 11 9 10 9 9 Pros: Doesn’t need generative model (number of clusters, parametric distribution) Cons: Ad-hoc, no probabilistic foundation, intractable for large data sets 8 10 7 10 6 10 5 10 4 12 3 12 2 15 1 16

Mixture Model Clustering
Examples: K-means, mixture of Gaussians, Naïve Bayes Pros: Sound probabilistic foundation, efficient even for large data sets Cons: Requires generative model, including number of clusters (mixture components)

Problem How many clusters are there here?
Can use agglomerative to get a whole cluster tree Can run K-means with diff numbers of clusters to see how likelihood changes But what’s the right number? Would be nice to let the data decide somehow…

Big Idea Want to use a generative model, but don’t want to decide number of clusters in advance Suggestion: put each datum in its own cluster Problem: probability of 2 clusters colliding is zero under any density function, no “stickiness” Solution: instead of a density function, use a statistical process where the probability of two clusters falling together is non-negative Best of both worlds: stickiness with variable number of clusters

Finite Mixture Model p  ci xi N c x p  ci xij N M c x1 x2 xM
Gaussian p ci xi N c x Naïve Bayes p ci xij N M c x1 x2 xM

Dirichlet Priors (Review)
A distribution over possible parameter vectors of the multinomial distribution Thus values must lie in the k-dimensional simplex Beta distribution is the 2-parameter special case Expectation A conjugate prior to the multinomial Explicit formulation is ugly! xi N

Infinite Mixture Model
G0 p ci xi N

Chinese Restaurant Process
CRP is a distribution on partitions that captures the clustering effect of the DP

DP Mixture Model ci xi N p   G0 xi N G i  G0
So far we have presented the DP mixture model, let’s now define the more general DP Remember we’re going to put each datum in its own cluster, but draw the cluster parameters from a DP In DP the prob of sampling the same point twice is positive G0 is called the base measure of G, and alpha is called the concentration Parameter G is a random probability measure distributed according to the DP

Stick-breaking Process
0.4 0.6 0.5 0.3 0.3 0.8 0.24 What is G? - A sample from the DP The theta params for each datum are drawn from it Because prob of drawing the same theta twice is positive, it must be discrete Depends somehow on theta and G_0 Stick breaking process G0

Properties of the DP Let (,) be a measurable space, G0 be a probability measure on the space, and  be a positive real number A Dirichlet process is any distribution of a random probability measure G over (,) such that, for all finite partitions (A1,…,Ar) of , Draws G from DP are generally not distinct The number of distinct values grows with O(log n)

Infinite Exchangeability
In general, an infinite set of random variables is said to be infinitely exchangeable if for every finite subset {xi,…,xn} and for any permutation  we have Note that infinite exchangeability is not the same as being independent and identically distributed (i.i.d.)! Using DeFinetti’s theorem, it is possible to show that our draws  are infinitely exchangeable Thus the mixture components may be sampled in any order

Mixture Model Inference
We want to find a clustering of the data: an assignment of values to the hidden class variable Sometimes we also want the component parameters In most finite mixture models, this can be found with EM The Dirichlet process is a non-parametric prior, and doesn’t permit EM We use Gibbs sampling instead

Gibbs Sampling 1 xi N G i  G0 xi N i  G0
Algorithm 1: integrate out G, and sample the i directly, conditioned on everything else This is inefficient, because we update cluster information for one datum at a time xi N G i G0 xi N i G0

Gibbs Sampling 2 xi N G i  G0 xi N ci  G0  c Algorithm 2:
Reintroduce a cluster variable ci which takes on values that are the names c of the clusters Store the parameters that are shared by all data in class c in a new variable c xi N G i G0 xi N ci G0 c

Gibbs Sampling 2 (cont.) Works well
Algorithm 2: For i = 1,…,N sample ci from where H-i,c is the posterior distribution of c based on the prior G0 and all observations for which ji and cj=c Repeat Works well Note: can also use variational methods (other than EM)

NLP Applications Clustering Sequence modeling: the “infinite HMM”
Document clustering for topic, genre, sentiment,… Word clustering for POS, WSD, synonymy,… Topic clustering across documents (see Blei et. al., 2004 and Teh et. al., 2004) Noun coreference: don’t know how many entities there are Other identity uncertainty problems: deduping, etc. Grammar induction Sequence modeling: the “infinite HMM” Topic segmentation (see Grenager et. al., 2005) Sequence models for POS tagging Others? Useful anytime you want to cluster or do unsup learning without specifying the number fo clusters

Nested CRP Used for modeling topic hierarchies by Blei et. al., 2004. Day 1 Day 2 Day 3

Nested CRP (cont.) To generate a document given a tree with L levels
Choose a path from the root of the tree to a leaf Draw a vector  of topic mixing proportions from an L-dimensional Dirichlet Generate the words in the document from a mixture of the topics along the path, with mixing proportions 

Nested CRP (cont.) A topic hierarchy estimated from 1717 abstracts from NIPS01 through NIPS12.

References Seminal: Foundational: NLP:
T.S. Ferguson. A Bayesian analysis of some nonparametric problems. Annals of Statistics 1: , 1973. C.E. Antoniak. Mixtures of Dirichlet processes with applications to Bayesian nonparametric problems. Annals of Statistics 2: , 1974. Foundational: M.D. Escobar and M. West. Bayesian density estimation and inference using mixtures. Journal of the American Statistical Association, 90: , 1995. S.N. MacEachern and P. Muller. Estimating mixture of Dirichlet process models. Journal of Computational and Graphical Statistics, 7: , 1998. R.M. Neal. Markov chain sampling methods for Dirichlet process mixture models. Journal of Computational and Graphical Statistics, 9: , 2000. C.E. Rasmussen. The Infinite Gaussian Mixture Model. NIPS, 2000. H. Ishwaran and L. James. Gibbs sampling methods for stick-breaking priors. Journal of the American Statistical Association, 96: , 2001. NLP: D.M. Blei, T.L. Griffiths, M.I. Jordan, and J.B. Tenenbaum. Hierarchical topic models and the nested Chinese restaurant process. NIPS, 2004. Y.W. Teh, M.I. Jordan, M.J. Beal, and D.M. Blei. Hierarchical Dirichlet processes. NIPS, 2004.

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