hw2-lighthouse problemstefan/kurs1447/kddl5_2011.pdf · 2011-02-11 · 6 probability ratio in mcmc...
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%HW2-Sivia N=10000; alphas=rand(N,1); alphas=(alphas-1/2)*pi; xs=tan(alphas); xs=sort(xs); figure; plot(xs) meanest=mean(xs) medest=median(xs) llh=[]; for x0=-1:0.01:1 llh=[llh sum(-log((1+(x-x0).^2)))]; end figure; plot(llh)
HW2-Lighthouse problem
meanest =-0.1099 medest =0.0047
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Statistical Data models, Non-parametrics,
Dynamics
Non-informative, proper and improper priors
• For real quantity bounded to interval, standard prior is uniform distribution
• For real quantity, unbounded, standard is uniform - but with what density?
• For real quantity on half-open interval, standard prior is f(s)=1/s - but integral diverges!
• Divergent priors are called improper - they can only be used with convergent likelihoods
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Dirichlet Distribution- prior for discrete distribution
Mean of Dirichlet - Laplaces estimator
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Occurence table probability
Occurence table probability Uniform prior:
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Non-parametric inference
• How to perform inference about a distribution without assuming a distribution family?
• A distribution over reals can be approximated by a piecewise uniform distribution a mixture of real distributions
• But how many parts? This is non-parametric inference
Non-parametric inference Change-points, Rao-Blackwell
• Given times for events (eg coal-mining disasters) Infer a piecewise constant intensity function (change-point problem)
• State is set of change-points with intensities inbetween • But how many pieces? This is non-parametric inference • MCMC: Given current state, propose change in segment
bounadry or intensity • But it is possible to integrate out intensities proposed
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Probability ratio in MCMC
For a proposed merge of intervals j and j+1, with sizes proportional to (α,1-α), were the counts and obtained by tossing a ‘coin’ with success probability or not? Compute model probability ratio as in HW1. Also, the total number of breakpoints has prior distribution Poisson with parameter (average) . Probability ratio in favor of split :
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n j
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n j+1
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Averging MCMC run, positions and number of breakpoints
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Averging MCMC run, positions with uniform test data
Mixture of Normals
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Mixture of Normals elimination of nuisance parameters
Mixture of Normals elimination of nuisance parameters
(integrate using normalization constant of Gaussian and Gamma distributions)
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Matlab Mixture of Normals, MCMC (AutoClass method)
function [lh,lab,trlpost,trm,trstd,trlab,trct,nbounc]= mmnonu1(x,N,k,labi,NN); %[lh,lab,trlpost,trm,trstd,trlab,trct,nbounc]= % MMNONU1(x,N,k,labi,NN); %inputs % 1D MCMC mixture modelling, % x - 1D data column vector % N - MCMC iterations. % k - number of components %lab,labi - component labelling of data vector) % NN - thinning (optional)
Matlab Mixture of Normals, MCMC
function [lab,trlh,trm,trstd,trlab,trct,nbounc]= mmnonu1(x,N,k,labi,NN); %[lh,lab,trlpost,trm,trstd,trlab,trct,nbounc]= % MMNONU1(x,N,k,labi,NN); %outputs %trlh - thinned trace of log probability (optional) %trm - thinned trace of means vector (optional) %trstd - thinned vector of standard deviations (optional) %trlab - thinned trace of labels vector (size(x,1) by N/NN (optional) %trct - thinned trace of mixing proportions
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Matlab Mixture of Normals, MCMC
N=10000; NN=100; x=[randn(100,1)-1;randn(100,1)*3;randn(100,1)+1]; % 3 components synthetic data k=2; labi=ceil(rand(size(x))*2); [llhc,lab2,trl,trm,trstd,trlab,trct,nbounc]= … mmnonu1(x,N,k,labi,NN); [llhc2,lab2,trl2,trm2,trstd2,trlab2,trct2,nbounc]=… mmnonu1(x,N,k,lab2,NN); … (k=3, 4, 5)
Matlab Mixture of Normals, MCMC
The three components and the joint empirical distr
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Matlab Mixture of Normals, MCMC Putting them
together makes the identification seem harder.
Matlab Mixture of Normals, MCMC
K=2:
std
mean
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Matlab Mixture of Normals, MCMC
K=3:
std
mean
Burn in progressing
Matlab Mixture of Normals, MCMC
K=3:
std
mean
Burnt in
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Matlab Mixture of Normals, MCMC
K=4: Low prob
std
mean
No focus- No interpretation as 4 clusters
Matlab Mixture of Normals, MCMC
K=5: Low prob
std
mean
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Matlab Mixture of Normals, MCMC
X sample: 1-100 : (-1 1) 101:200: (0 3) 201:300: (1 1)
Trace of state labels
Unsorted sample label trace sorted
Mixtures of multivariate normals
• This works the same way, but instead of a Gamma distribution for the variance we use the Wishart distribution, a matrix-valued distribution over covariance matrices.
• Competes well with both clustering and Expectation Maximization, which are prone to overfitting (clustering cannot handle overlapping components)
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Dynamic Systems, time series
• An abundance of linear prediction models exists
• For non-linear and Chaotic systems, method was developed in 1990:s (Santa Fe)
• Gershenfeld, Weigend: The Future of Time Series
• Online/offline: prediction/retrodiction
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Berry and Linoff have eloquently stated their preferences with ���the often quoted sentence: "Neural networks are a good choice for most classification problems when the results of the model are more important than understanding how the model works".��� “Neural networks typically give the right answer”
Dynamic Systems and Taken’s Theorem
• Lag vectors (xi,x(i-1),…x(i-T), for all i, occupy a submanifold of E^T, if T is large enough
• This manifold is ‘diffeomorphic’ to original state space and can be used to create a good dynamic model
• Taken’s theorem assumes no noise and must be empirically verified.
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Dynamic Systems and Taken’s Theorem
Santa Fe 1992 Competition
Unstable Laser
Intensive Care Unit Data, Apnea
Exchange rate Data
Synthetic series with drift
White Dwarf Star Data
Bach’s unfinished Fugue
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Stereoscopic 3D view of state space manifold, series A (Laser) The points seem to lie on a surface, which means that a lag-vector of 3 gives good prediction of the time series. The surface is either produced for a training batch, or produced on-the-fly from neighboring data points (possibly downweighing very old points)
Figure in book misleading: Origin where surface touches ground
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Variational Bayes
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True trajectory in state space (Valpola-Karhunen 2002)
Reconstructed trajectory in inferred state space
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Hidden Markov Models
• Given a sequence of discrete signals xi • Is there a model likely to have produced xi
from a sequence of states si of a Finite Markov Chain?
• P(.|s) - transition probability in state s • S(.|s) - signal probability in state s • Speech Recognition, Bioinformatics, …
Hidden Markov Models function [Pn,Sn,stn,trP,trS,trst,tll]=… hmmsim(A,N,n,s,prop,Po,So,sto,NN); %[Pn,Sn,stn,trP,trS,trst]=HMMSIM(A,N,n,s,prop,Po,So,sto,NN); % Compute trace of posterior for hmm parameters % A - the sequence of signals % N - the length of trace % n - number of states in Markov chain % s - number of signal values % prop - proposal stepsize % optional inputs: % Po - starting transition matrix (each of n columns a discrete pdf % in n-vector % So - starting signal matrix (each of n columns a discrete pdf
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Hidden Markov Models function [Pn,Sn,stn,trP,trS,trst,tll]=… hmmsim(A,N,n,s,prop,Po,So,sto,NN); % in s-vector % sto - starting state sequence (congruent to vector A) % NN - thining of trace, default 10 % outputs % Pn - last transition matrix in trace % Sn - last signal emission matrix % stn - last hidden state vector (congruent to A) % trP - trace of transition matrices % trS - trace of signal matrices % trace of hidden state vectors
Hidden Markov Models
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Hidden Markov Models
Hidden Markov Models
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Hidden Markov Models Over 100000 iterations, burnin is visible 2 states, 2 signals P-transition matrix S-signaling
Chapman Kolmogorov version of Bayes’ rule
f (!t | Dt ) " f (dt | !t)# f (!t |!t$1) f (!t$1 | Dt$1 )d!t$1
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Chapman Kolmogorov version of Bayes’ rule
f (!t | Dt ) " f (dt | !t)# f (!t |!t$1) f (!t$1 | Dt$1 )d!t$1
Observation and video based particle filter tracking
Defence: tracking with heterogeneous observations
Crowd analysis: tracking from video
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Cycle in Particle filter
Importance (weighted) sample Resampled ordinary sample Diffused sample Weighted by likelihood X- state Z - Observation
Time step cycle
Particle filter- general tracking