Parametric Inference Maximum Likelihood Inference Exponential Families Expectation Maximization (EM) Bayesian Inference Statistical Decison Theory

Transcription

1 Statistical Inference Parametric Inference Maximum Likelihood Inference Exponential Families Expectation Maximization (EM) Bayesian Inference Statistical Decison Theory IP, José Bioucas Dias, IST,

2 Statistical Inference Statistics aims at retriving the causes (e.g., parameters of a pdf) from the observations (effects) Probability Statistics Statistical inference problems can thus be seen as Inverse Problems As a result of this perpective, at the eighteenth century (at the time of Bayes and Laplace) Statistics was often called Inverse Probability IP, José Bioucas Dias, IST,

3 Parametric Inference Consider the parametric model is the parameter space and The problem of inference reduces to the estimation of where is the parameter from ; i.e, Parameters of interest and nuisance parameters Let Sometimes we are only interested in some function that depends only on - parameter of interest; Example: - nuisance parameter IP, José Bioucas Dias, IST,

4 Parametric Inference (theoretical limits) The Cramer Rao Lower Bound (CRLB) Under under appropriate regularity conditions, the covariance matrix of any Unbiased estimator, satisfies where is the Fisher information matrix given by An unbiased estimator that attains the CRLB may be found iif For some function h. The estimator is IP, José Bioucas Dias, IST,

5 CRLB for the general Gaussian case Example: Parameter of a signal in white noise If Example: Known signal in unknown white noise IP, José Bioucas Dias, IST,

6 Maximum Likelihood Method is the likelihood function If for all f we can use the log-likelihood Example (Bernoulli) IP, José Bioucas Dias, IST,

7 Maximum Likelihood Example (Uniform) 1 1 IP, José Bioucas Dias, IST,

8 Maximum Likelihood Example (Gaussian) IID Sample mean Sample variace IP, José Bioucas Dias, IST,

9 Maximum Likelihood Example (Multivariate Gaussian) IID Sample mean Sample covariance IP, José Bioucas Dias, IST,

10 Maximum Likelihood (linear observation model) Example: Linear observation in Gaussian noise A is full rank IP, José Bioucas Dias, IST,

11 Example: Linear observation in Gaussian noise (cont.) MLE is equivalent to the LSE using the norm If,, is given by the Moore-Penrose Pseudo-Inverse is a projection matrix (SVD) If the noise is zero-mean but not Gaussian, the Best Linear Unbiased estimator (BLUE) is still given by IP, José Bioucas Dias, IST,

12 Maximum likelihood Linear observation in Gaussian noise MLE Properties (MLE is optimal for the linear model) Is the Minimum Variance Unbiased (MVU) estimator [ and is the minimum among all unbiased estimators] Is efficient (it attains the Camer Rao Lower Bound (CRLB)) Its PDF is IP, José Bioucas Dias, IST,

13 Maximum likelihood (characterization) Appealing properties of MLE Let A sequence of IID vectors in and 1. The MLE is consistent: ( denotes the true parameter) 2. The MLE is equivariant: if is the MLE estimate of, then is the MLE estimate of 3. The MLE (under appropriate regularity conditions) is asymptotically Normal and optimal or efficient: Fisher information matrix IP, José Bioucas Dias, IST,

14 The exponential Family Definition: the set dimension k if there there are functions such that an exponential family of is a sufficient statistic for f, i.e, Theorem: (Neyman-Fisher Factorization) f iif can be factored as is a sufficient statistic for IP, José Bioucas Dias, IST,

15 The exponential family Natural (or canonical) form Given an exponential family, it is always possible to introduce the change of variables and the reparemeterization such that Since is a PDF, it must integrate to one IP, José Bioucas Dias, IST,

16 The exponential family (The partition function) Computing moments from the derivatives of the partition function After some calculus IP, José Bioucas Dias, IST,

17 The exponential family (IID sequences) Let a member of an exponential family defined by The density of the IID sequence is belongs exponential family defined by IP, José Bioucas Dias, IST,

18 Examples of exponential families Many of the most common probabilistic models belong to exponential families; e.g., Gaussian, Poisson, Bernoulli, binomial, exponential, gamma, and beta. Example: Canonical form IP, José Bioucas Dias, IST,

19 Examples of exponential families (Gaussian) Example: Canonical form IP, José Bioucas Dias, IST,

20 Computing maximum likelihood estimates Very often the MLE can not be found analytically. Commonly used numerical methods: 1. Newton-Raphson 2. Scoring 3. Expectation Maximization (EM) Newton-Raphson method Scoring method Can be computed off-line IP, José Bioucas Dias, IST,

21 Computing maximum likelihood estimates (EM) Expectation Maximization (EM) [Dempster, Laird, and Rubin, 1977] Suppose that is hard to maximize But we can find a vector z such that is easy to maximze and Idea: iterate between two steps: E-step: Fill in z in M-step: Maximize Terminology Observed data Missing data Complete data IP, José Bioucas Dias, IST,

22 Expectation maximization The EM algorithm 1. Pick up a starting vector : repeat steps 2. and E-step: Calculate 3. M-step Alternatively (GEM) IP, José Bioucas Dias, IST,

23 Expectation maximization The EM (GEM) algorithm always increases the likelihood. Define Kulback Leibler distance 4. KL distance maximization IP, José Bioucas Dias, IST,

24 Expectation maximization (why does it work?) IP, José Bioucas Dias, IST,

25 EM: Mixtures of densities Let be the random variable that selects the active mode: where and IP, José Bioucas Dias, IST,

26 EM: Mixtures of densities Consider now that is a sequence of IID random variables Let be IID random variables, where selects the active mode in the sample : IP, José Bioucas Dias, IST,

27 EM: Mixtures of densities Equivalent Q Where is the sample mean of x, i.e., IP, José Bioucas Dias, IST,

28 EM: Mixtures of densities E-step: M-step: IP, José Bioucas Dias, IST,

29 EM: Mixtures of densities E-step: M-step: IP, José Bioucas Dias, IST,

30 EM: Mixtures of Gaussian densities (MOGs) E-step: M-step: Weighted sample mean Weighted sample covariance IP, José Bioucas Dias, IST,

31 EM: Mixtures of Gaussian densities. 1D Example loglikelihood L(f k ) p = 3 N = IP, José Bioucas Dias, IST,

32 EM: Mixtures of Gaussian Densities (MOGs) Example 1D p = 3 N = hist hist 0.3 est MOG true MOG 0.3 est modes true modes IP, José Bioucas Dias, IST,

33 EM: Mixtures of Gaussian Densities: 2D Example MOG with determination of the number of modes [M. Figueiredo, 2002] k= IP, José Bioucas Dias, IST,

34 Bayesian Estimation IP, José Bioucas Dias, IST,

35 The Bayesian Philosophy ([Wasserman, 2004]) Bayesian Inference B1 Probabilities describe degrees of belief, not limiting relative frequency B2 We can make probability statements about parameters, even though they are fixed parameters B3 We make inferences about a parameter by producing a probalility distribution for Frequencist or Classical Inference F1 Probabilities refer to limiting relative frequencies and are objective properties of the real world F2 Parameters are fixed unknown parameters F3 The criteria for obtaining statistical procedures are based on long run frequency properties. IP, José Bioucas Dias, IST,

36 The Bayesian Philosophy unknown Observation model observation Prior knowledge Bayesian Inference Classical Inference describes degrees of belief (subjective), not limiting frequency IP, José Bioucas Dias, IST,

37 The Bayesian method 1. Choose a prior density, called the prior (or a priori) distribution that expresses our beliefs about f, before we see any data 2. Choose the observation model that reflects our belief about g given f 3. Calculate the posterior (or a posteriori) distribution using the Bayes law: where is the marginal on g (other names: evidence, unconditional, predictive) 4. Any inference should be based on the posterior IP, José Bioucas Dias, IST,

38 The Bayesian method Example: Let IID and = =0.5 = =1 = =2 = =10 for = >1, towards 1/2 pulls IP, José Bioucas Dias, IST,

39 Example (cont.): (Bernoulli observations, Beta prior) Observation model Prior Posterior Thus, IP, José Bioucas Dias, IST,

40 Example (cont.): (Bernoulli observations, Beta prior) Maximum a posteriori estimate (MAP) Total ignorance: flat prior = =1 Note that for large values of n The von Mises Theorem If the prior is continuous and not zero at the location of the MLestimate, then, IP, José Bioucas Dias, IST,

41 Conjugate priors In the previous example, the prior and the posterior are both Beta distributed. We say that the prior is conjugate with respect to the model Formally, let and be two parametrized families of priors and observation models, respectively is a conjugate family for if for some Very often, prior information about f is very small, allowing to select conjugate priors Conjugate priors why? Computing the posterior density simply consists in updating the parameters of the prior IP, José Bioucas Dias, IST,

42 Conjugate priors (Gaussian observation, Gaussian prior) Gaussian observations Gaussian prior The posterior distribution is Gaussian 1. The mean of is in the simplex defined by {g, } 2. The variance of is the parallel of variances and IP, José Bioucas Dias, IST,

43 Conjugate priors (Gaussian IID observations, Gaussian prior) Gaussian IID observations Gaussian prior The posterior distribution is Gaussian 1. The mean of is in the simplex defined by 2. The variance of is the parallel of variances and IP, José Bioucas Dias, IST,

44 Conjugate Priors (Gaussian IID observations, Gaussian prior) IP, José Bioucas Dias, IST,

45 Conjugate Priors (multivariate Gaussian: observation and prior) (g,f) jointly Gaussian distributed: Then a) b) c) IP, José Bioucas Dias, IST,

46 Conjugate Priors (multivariate Gaussian: observation and prior) Linear observation model (f and w independent) Posterior IP, José Bioucas Dias, IST,

47 Conjugate Priors (multivariate Gaussian: observation and prior) Linear observation model (f and w independent) Using the matrix inversion lemma is the solution of the following regularized LS problem e.g., penalize oscillatory solutions IP, José Bioucas Dias, IST,

48 Improper Priors Assume that p(f)=k on given domain Even if the domain of f is unbounded, and, thus, the posterior is well defined. In a sense, improper priors account for a state of total ignorance. This raises no issues to the Bayesian framework, as far as the posterior is proper. IP, José Bioucas Dias, IST,

49 Bayes Estimators IP, José Bioucas Dias, IST,

50 Bayes estimators Ingredients of Statistical Decision Theory: posterior distribution conveys all knowledge about f, given the observation g loss function measures the discrepancy between and. a posteriori expected loss optimal Bayes estimator IP, José Bioucas Dias, IST,

51 Bayesian framework Nuisance Parameter Let and Nuisance parameter The posterior risk depends only on the marginal on In a pure Bayesian framework, nuisance parameters are integrated out IP, José Bioucas Dias, IST,

52 Bayes estimators: Maximum a posteriori probability (MAP) Zero-one, 0/1, loss Volume of an -ball Maximum a posteriori probability A discrete domain leads to the MAP estimator as well IP, José Bioucas Dias, IST,

53 Bayes Estimators: Posterior Mean (PM) Quadratic loss: Q is symmetric and positive definite Only this term Depends on Posterior mean may be hard to compute Valid for any is additive. If Q diagonal the loss function IP, José Bioucas Dias, IST,

54 Bayes estimators: Additive loss Let Then, the minimization is decoupled Each component of minimizes the corresponding marginal a posteriori expected loss IP, José Bioucas Dias, IST,

55 Bayes Estimators: Additive Loss Additive 0/1 loss: is the maximizer of the posterior marginal Additive quadratic loss: The additive quadratic loss is a quadratic loss with Q=I. Therefore, The corresponding Bayes estimator is the posterior mean IP, José Bioucas Dias, IST,

56 Example (Gaussian IID observations, Gaussian prior) Gaussian IID observations Gaussian prior The posterior distribution is Gaussian as IP, José Bioucas Dias, IST,

57 Example (Gaussian observation, Laplacian prior) MAP estimate Strictly concave IP, José Bioucas Dias, IST,

58 Example (Gaussian observation, Laplacian prior) MAP estimate IP, José Bioucas Dias, IST,

59 Example (Gaussian observation, Laplacian prior) PM estimate No closed form expressions Resort to numerical procedures IP, José Bioucas Dias, IST,

60 Example (Gaussian observation, Laplacian prior) IP, José Bioucas Dias, IST,

61 Example (Gaussian observation, Laplacian prior) IP, José Bioucas Dias, IST,

62 Example (Multivariate Gaussian: observation and prior) Linear observation model (f and w independent) Posterior is called the Wiener filter If all the eigenvectors of C approaches infinite, then which is the Moore-Penrose pseudo (or generalized) inverse of A IP, José Bioucas Dias, IST,