Parsimonious Unsupervised and Semi-Supervised Domain Adaptation with Good Similarity Functions

International audienceIn this paper, we address the problem of domain adaptation for binary classification. This problem arises when the distributions generating the source learning data and target test data are somewhat different. From a theoretical standpoint, a classifier has better generalization guarantees when the two domain marginal distributions of the input space are close. Classical approaches try mainly to build new projection spaces or to reweight the source data with the objective of moving closer the two distributions. We study an original direction based on a recent framework introduced by Balcan et al. enabling one to learn linear classifiers in an explicit projection space based on a similarity function, not necessarily symmetric nor positive semi-definite. We propose a well founded general method for learning a low-error classifier on target data which is effective with the help of an iterative procedure compatible with Balcan et al.'s framework. A reweighting scheme of the similarity function is then introduced in order to move closer the distri- butions in a new projection space. The hyperparameters and the reweighting quality are controlled by a reverse validation procedure. Our approach is based on a linear programming formulation and shows good adaptation performances with very sparse models. We first consider the challenging unsupervised case where no target label is accessible, which can be helpful when no manual annotation is possible. We also propose a generalization to the semi-supervised case allowing us to consider some few target labels when available. Finally, we evaluate our method on a synthetic problem and on a real image annotation task

Domain Adaptation of Majority Votes via Perturbed Variation-based Label Transfer

We tackle the PAC-Bayesian Domain Adaptation (DA) problem. This arrives when one desires to learn, from a source distribution, a good weighted majority vote (over a set of classifiers) on a different target distribution. In this context, the disagreement between classifiers is known crucial to control. In non-DA supervised setting, a theoretical bound - the C-bound - involves this disagreement and leads to a majority vote learning algorithm: MinCq. In this work, we extend MinCq to DA by taking advantage of an elegant divergence between distribution called the Perturbed Varation (PV). Firstly, justified by a new formulation of the C-bound, we provide to MinCq a target sample labeled thanks to a PV-based self-labeling focused on regions where the source and target marginal distributions are closer. Secondly, we propose an original process for tuning the hyperparameters. Our framework shows very promising results on a toy problem

Domain adaptation of weighted majority votes via perturbed variation-based self-labeling

In machine learning, the domain adaptation problem arrives when the test (target) and the train (source) data are generated from different distributions. A key applied issue is thus the design of algorithms able to generalize on a new distribution, for which we have no label information. We focus on learning classification models defined as a weighted majority vote over a set of real-val ued functions. In this context, Germain et al. (2013) have shown that a measure of disagreement between these functions is crucial to control. The core of this measure is a theoretical bound--the C-bound (Lacasse et al., 2007)--which involves the disagreement and leads to a well performing majority vote learning algorithm in usual non-adaptative supervised setting: MinCq. In this work, we propose a framework to extend MinCq to a domain adaptation scenario. This procedure takes advantage of the recent perturbed variation divergence between distributions proposed by Harel and Mannor (2012). Justified by a theoretical bound on the target risk of the vote, we provide to MinCq a target sample labeled thanks to a perturbed variation-based self-labeling focused on the regions where the source and target marginals appear similar. We also study the influence of our self-labeling, from which we deduce an original process for tuning the hyperparameters. Finally, our framework called PV-MinCq shows very promising results on a rotation and translation synthetic problem

A New PAC-Bayesian Perspective on Domain Adaptation

We study the issue of PAC-Bayesian domain adaptation: We want to learn, from a source domain, a majority vote model dedicated to a target one. Our theoretical contribution brings a new perspective by deriving an upper-bound on the target risk where the distributions' divergence---expressed as a ratio---controls the trade-off between a source error measure and the target voters' disagreement. Our bound suggests that one has to focus on regions where the source data is informative.From this result, we derive a PAC-Bayesian generalization bound, and specialize it to linear classifiers. Then, we infer a learning algorithmand perform experiments on real data.Comment: Published at ICML 201

PAC-Bayes and Domain Adaptation

We provide two main contributions in PAC-Bayesian theory for domain adaptation where the objective is to learn, from a source distribution, a well-performing majority vote on a different, but related, target distribution. Firstly, we propose an improvement of the previous approach we proposed in Germain et al. (2013), which relies on a novel distribution pseudodistance based on a disagreement averaging, allowing us to derive a new tighter domain adaptation bound for the target risk. While this bound stands in the spirit of common domain adaptation works, we derive a second bound (introduced in Germain et al., 2016) that brings a new perspective on domain adaptation by deriving an upper bound on the target risk where the distributions' divergence-expressed as a ratio-controls the trade-off between a source error measure and the target voters' disagreement. We discuss and compare both results, from which we obtain PAC-Bayesian generalization bounds. Furthermore, from the PAC-Bayesian specialization to linear classifiers, we infer two learning algorithms, and we evaluate them on real data.Comment: Neurocomputing, Elsevier, 2019. arXiv admin note: substantial text overlap with arXiv:1503.0694

A survey on domain adaptation theory: learning bounds and theoretical guarantees

All famous machine learning algorithms that comprise both supervised and semi-supervised learning work well only under a common assumption: the training and test data follow the same distribution. When the distribution changes, most statistical models must be reconstructed from newly collected data, which for some applications can be costly or impossible to obtain. Therefore, it has become necessary to develop approaches that reduce the need and the effort to obtain new labeled samples by exploiting data that are available in related areas, and using these further across similar fields. This has given rise to a new machine learning framework known as transfer learning: a learning setting inspired by the capability of a human being to extrapolate knowledge across tasks to learn more efficiently. Despite a large amount of different transfer learning scenarios, the main objective of this survey is to provide an overview of the state-of-the-art theoretical results in a specific, and arguably the most popular, sub-field of transfer learning, called domain adaptation. In this sub-field, the data distribution is assumed to change across the training and the test data, while the learning task remains the same. We provide a first up-to-date description of existing results related to domain adaptation problem that cover learning bounds based on different statistical learning frameworks

Apprentissage de vote de majorité pour la classification supervisée et l'adaptation de domaine : approches PAC-Bayésiennes et combinaison de similarités

Nowadays, due to the expansion of the web a plenty of data are available and many applications need to make use of supervised machine learning methods able to take into account different information sources. For instance, for multimedia semantic indexing applications, one have to efficiently take advantage of information about color, textual, texture or sound sources of the document. Most of the existing methods try to combine these multimodal informations, either by directly fusionning the descriptors or by combining similarities or classifiers, in order to produce a classification model more reliable for the considered task. Usually, these multimodal facets imply two main issues. On the one hand, one have to be able to correctly make use of all the a priori information available. On the other hand, the data, on which the model will be applied, does not come from the same probability distribution than the data used during the learning step. In this context, we have to adapt the model on new data, which is known as domain adaptation. In this thesis, we propose several theoretically-founded contributions for tackle these issues. A first serie of contributions studies the problem of learning a weighted majority vote over a set of voters in a supervised classification setting.These results fall within the context of the PAC-Bayesian theory allowing to derive generalization abilities for such a vote by assuming an a priori on the relevance of the voters. Our first contribution aims at extending a recent algorithm, MinCq, minimizing a bound over the error of the majority vote in binary classification. This extension can take into account an a priori belief on the performances of the voters. This belief is expressed as an aligned distribution. We illustrate its usefulness for combining nearest neighbor classifiers, and for classifier fusion on a multimedia semantic indexing task. Then, we propose a theoretical contribution for multiclass classification tasks. Our approach is based on an original PAC-Bayesian analysis considering the operator norm of the confusion matrix as an error measure. Our second series of contributions relates to domain adaptation. In this situation we present our third result for combining similarities in order to infer a representation space for moving closer the learning distribution and the testing distribution. This contribution is based on the theory of learning from (epsilon,gamma,tau)-good similarity functions and is justified by the minimization of an usual bound in domain adaptation. For our last contribution, we propose the first PAC-Bayesian analysis for domain adaptation. This analysis is based on a consistent divergence measure between distributions allowing us to derive a generalization bound for learning majority votes in binary classification. Moreover, we propose a first algorithm specialized to linear classifiers and able to directly minimize our bound.De nos jours, avec l'expansion d'Internet, l'abondance et la diversité des données accessibles qui en résulte, de nombreuses applications requièrent l'utilisation de méthodes d'apprentissage automatique supervisé capables de prendre en considération différentes sources d'informations. Par exemple, pour des applications relevant de l'indexation sémantique de documents multimédia, il s'agit de pouvoir efficacement tirer bénéfice d'informations liées à la couleur, au texte, à la texture ou au son des documents à traiter. La plupart des méthodes existantes proposent de combiner ces informations multimodales, soit en fusionnant directement les descriptions, soit en combinant des similarités ou des classifieurs, avec pour objectif de construire un modèle de classification automatique plus fiable pour la tâche visée. Ces aspects multimodaux induisent généralement deux types de difficultés. D'une part, il faut être capable d'utiliser au mieux toute l'information a priori disponible sur les objets à combiner. D'autre part, les données sur lesquelles le modèle doit être appliqué ne suivent nécessairement pas la même distribution de probabilité que les données utilisées lors de la phase d'apprentissage. Dans ce contexte, il faut être à même d'adapter le modèle à de nouvelles données, ce qui relève de l'adaptation de domaine. Dans cette thèse, nous proposons plusieurs contributions fondées théoriquement et répondant à ces problématiques. Une première série de contributions s'intéresse à l'apprentissage de votes de majorité pondérés sur un ensemble de votants dans le cadre de la classification supervisée. Ces contributions s'inscrivent dans le contexte de la théorie PAC-Bayésienne permettant d'étudier les capacités en généralisation de tels votes de majorité en supposant un a priori sur la pertinence des votants. Notre première contribution vise à étendre un algorithme récent, MinCq, minimisant une borne sur l'erreur du vote de majorité en classification binaire. Cette extension permet de prendre en compte une connaissance a priori sur les performances des votants à combiner sous la forme d'une distribution alignée. Nous illustrons son intérêt dans une optique de combinaison de classifieurs de type plus proches voisins, puis dans une perspective de fusion de classifieurs pour l'indexation sémantique de documents multimédia. Nous proposons ensuite une contribution théorique pour des problèmes de classification multiclasse. Cette approche repose sur une analyse PAC-Bayésienne originale en considérant la norme opérateur de la matrice de confusion comme mesure de risque. Notre seconde série de contributions concerne la problématique de l'adaptation de domaine. Dans cette situation, nous présentons notre troisième apport visant à combiner des similarités permettant d'inférer un espace de représentation de manière à rapprocher les distributions des données d'apprentissage et des données à traiter. Cette contribution se base sur la théorie des fonctions de similarités (epsilon,gamma,tau)-bonnes et se justifie par la minimisation d'une borne classique en adaptation de domaine. Pour notre quatrième et dernière contribution, nous proposons la première analyse PAC-Bayésienne appropriée à l'adaptation de domaine. Cette analyse se base sur une mesure consistante de divergence entre distributions permettant de dériver une borne en généralisation pour l'apprentissage de votes de majorité en classification binaire. Elle nous permet également de proposer un algorithme adapté aux classifieurs linéaires capable de minimiser cette borne de manière directe