Probabilité a posteriori | EPFL Graph Search

Dans le théorème de Bayes, la probabilité a posteriori désigne la probabilité recalculée ou remesurée qu'un évènement ait lieu en prenant en considération une nouvelle information. Autrement dit, la probabilité a posteriori est la probabilité qu'un évènement A ait lieu étant donné que l'évènement B a eu lieu. Elle s'oppose à la probabilité a priori dans l'inférence bayésienne. Définition La loi a priori p(\theta) qu'un évènement X ait lieu avec vraisemblance est p(X|\theta). La probabilité a posteriori se définit comme : :p(\theta|X) = \frac{p(X|\theta)p(\theta)}{p(X)}. La probabilité a posteriori peut s'écrire : \text{Probabilité a posteriori}\propto\text{Vraisemblance}\times\text{Probabilité a prori}. Calcul La distribution d'une probabilité a posteriori d'une variable aléatoire étant donné la valeur d'une autre peut être calculée avec le théorème de Bayes en multipliant la distribution de la

On Vacuous and Non-Vacuous Generalization Bounds for Deep Neural Networks.

Konstantinos Pitas

Deep neural networks have been empirically successful in a variety of tasks, however their theoretical understanding is still poor. In particular, modern deep neural networks have many more parameters than training data. Thus, in principle they should overfit the training samples and exhibit poor generalization to the complete data distribution. Counter intuitively however, they manage to achieve both high training accuracy and high testing accuracy. One can prove generalization using a validation set, however this can be difficult when training samples are limited and at the same time we do not obtain any information about why deep neural networks generalize well. Another approach is to estimate the complexity of the deep neural network. The hypothesis is that if a network with high training accuracy has high complexity it will have memorized the data, while if it has low complexity it will have learned generalizable patterns. In the first part of this thesis we explore Spectral Complexity, a measure of complexity that depends on combinations of norms of the weight matrices of the deep neural network. For a dataset that is difficult to classify, with no underlying model and/or no recurring pattern, for example one where the labels have been chosen randomly, spectral complexity has a large value, reflecting that the network needs to memorize the labels, and will not generalize well. Putting back the real labels, the spectral complexity becomes lower reflecting that some structure is present and the network has learned patterns that might generalize to unseen data. Spectral complexity results in vacuous estimates of the generalization error (the difference between the training and testing accuracy), and we show that it can lead to counterintuitive results when comparing the generalization error of different architectures. In the second part of the thesis we explore non-vacuous estimates of the generalization error. In Chapter 2 we analyze the case of PAC-Bayes where a posterior distribution over the weights of a deep neural network is learned using stochastic variational inference, and the generalization error is the KL divergence between this posterior and a prior distribution. We find that a common approximation where the posterior is constrained to be Gaussian with diagonal covariance, known as the mean-field approximation, limits significantly any gains in bound tightness. We find that, if we choose the prior mean to be the random network initialization, the generalization error estimate tightens significantly. In Chapter 3 we explore an existing approach to learning the prior mean, in PAC-Bayes, from the training set. Specifically, we explore differential privacy, which ensures that the training samples contribute only a limited amount of information to the prior, making it distribution and not training set dependent. In this way the prior should generalize well to unseen data (as it hasn't memorized individual samples) and at the same time any posterior distribution that is close to it in terms of the KL divergence will also exhibit good generalization.

EPFL2020

Stratégies de compromis pour l'estimation des paramètres de régression et pour la classification floue

Nicolas Fournier

Model specification is an integral part of any statistical inference problem. Several model selection techniques have been developed in order to determine which model is the best one among a list of possible candidates. Another way to deal with this question is the so-called model averaging, and in particular the frequentist approach. An estimation of the parameters of interest is obtained by constructing a weighted average of the estimates of these quantities under each candidate model. We develop compromise frequentist strategies for the estimation of regression parameters, as well as for the probabilistic clustering problem. In the regression context, we construct compromise strategies based on the Pitman estimators associated with various underlying errors distributions. The weight given to each model is equal to its profile likelihood, which gives a measure of the goodness-of-fit. Asymptotic properties of both Pitman estimators and profile likelihood allow us to define a minimax strategy for choosing the distributions of the compromise, involving a notion of distance between distributions. Performances of such estimators are then compared to other usual and robust procedures. In the second part of the thesis, we develop compromise strategies in the probabilistic clustering context. Although this clustering method is based on mixtures of distributions, our compromise strategies are not applied directly to the estimates of the parameters, but on the posterior probabilities of membership. Two types of compromise are presented, and the performances of resulting classification rules are investigated.

EPFL2011

On Vacuous and Non-Vacuous Generalization Bounds for Deep Neural Networks.

Konstantinos Pitas

EPFL2020