Robust Statistics and Interpretability

Jun 12, 2019



Do ImageNet Classifiers Generalize to ImageNet? Generalization is the main goal in machine learning, but few researchers systematically investigate how well models perform on truly unseen data. This raises the danger that the community may be overfitting to excessively re-used test sets. To investigate this question, we conduct a novel reproducibility experiment on CIFAR-10 and ImageNet by assembling new test sets and then evaluating a wide range of classification models. Despite our careful efforts to match the distribution of the original datasets, the accuracy of many models drops around 10%. However, accuracy gains on the original test sets translate to larger gains on the new test sets. Our results show that the accuracy drops are likely not caused by adaptive overfitting, but by the models' inability to generalize reliably to slightly "harder" images than those found in the original test set. Exploring the Landscape of Spatial Robustness The study of adversarial examples has so far largely focused on the lp setting. However, neural networks turn out to be also vulnerable to other, very natural classes of perturbations such as translations and rotations. Unfortunately, the standard methods effective in remedying lp vulnerabilities are not as effective in this new regime. With the goal of classifier robustness, we thoroughly investigate the vulnerabilities of neural network--based classifiers to rotations and translations. We uncover that while data augmentation generally helps very little, using ideas from robust optimization and test-time input aggregation we can significantly improve robustness. In our exploration we find that, in contrast to the lp case, first-order methods cannot reliably find fooling inputs. This highlights fundamental differences in spatial robustness as compared to lp robustness, and suggests that we need a more comprehensive understanding of robustness in general. Sever: A Robust Meta-Algorithm for Stochastic Optimization In high dimensions, most machine learning methods are brittle to even a small fraction of structured outliers. To address this, we introduce a new meta-algorithm that can take in a base learner such as least squares or stochastic gradient descent, and harden the learner to be resistant to outliers. Our method, Sever, possesses strong theoretical guarantees yet is also highly scalable -- beyond running the base learner itself, it only requires computing the top singular vector of a certain n×d matrix. We apply Sever on a drug design dataset and a spam classification dataset, and find that in both cases it has substantially greater robustness than several baselines. On the spam dataset, with 1% corruptions, we achieved 7.4% test error, compared to 13.4%−20.5% for the baselines, and 3% error on the uncorrupted dataset. Similarly, on the drug design dataset, with 10% corruptions, we achieved 1.42 mean-squared error test error, compared to 1.51-2.33 for the baselines, and 1.23 error on the uncorrupted dataset. Analyzing Federated Learning through an Adversarial Lens Federated learning distributes model training among a multitude of agents, who, guided by privacy concerns, perform training using their local data but share only model parameter updates, for iterative aggregation at the server to train an overall global model. In this work, we explore how the federated learning setting gives rise to a new threat, namely model poisoning, which differs from traditional data poisoning. Model poisoning is carried out by an adversary controlling a small number of malicious agents (usually 1) with the aim of causing the global model to misclassify a set of chosen inputs with high confidence. We explore a number of strategies to carry out this attack on deep neural networks, starting with targeted model poisoning using a simple boosting of the malicious agent’s update to overcome the effects of other agents. We also propose two critical notions of stealth to detect malicious updates. We bypass these by including them in the adversarial objective to carry out stealthy model poisoning. We improve its stealth with the use of an alternating minimization strategy which alternately optimizes for stealth and the adversarial objective. We also empirically demonstrate that Byzantine-resilient aggregation strategies are not robust to our attacks. Our results indicate that highly constrained adversaries can carry out model poisoning attacks while maintaining stealth, thus highlighting the vulnerability of the federated learning setting and the need to develop effective defense strategies. Fairwashing: the risk of rationalization Black-box explanation is the problem of explaining how a machine learning model -- whose internal logic is hidden to the auditor and generally complex -- produces its outcomes. Current approaches for solving this problem include model explanation, outcome explanation as well as model inspection. While these techniques can be beneficial by providing interpretability, they can be used in a negative manner to perform fairwashing, which we define as promoting the perception that a machine learning model respects some ethical values while it might not be the case. In particular, we demonstrate that it is possible to systematically rationalize decisions taken by an unfair black-box model using the model explanation as well as the outcome explanation approaches with a given fairness metric. Our solution, LaundryML, is based on a regularized rule list enumeration algorithm whose objective is to search for fair rule lists approximating an unfair black-box model. We empirically evaluate our rationalization technique on black-box models trained on real-world datasets and show that one can obtain rule lists with high fidelity to the black-box model while being considerably less unfair at the same time. Understanding the Origins of Bias in Word Embeddings Popular word embedding algorithms exhibit stereotypical biases, such as gender bias. The widespread use of these algorithms in machine learning systems can amplify stereotypes in important contexts. Although some methods have been developed to mitigate this problem, how word embedding biases arise during training is poorly understood. In this work we develop a technique to address this question. Given a word embedding, our method reveals how perturbing the training corpus would affect the resulting embedding bias. By tracing the origins of word embedding bias back to the original training documents, one can identify subsets of documents whose removal would most reduce bias. We demonstrate our methodology on Wikipedia and New York Times corpora, and find it to be very accurate. Bias Also Matters: Bias Attribution for Deep Neural Network Explanation The gradient of a deep neural network (DNN) w.r.t. the input provides information that can be used to explain the output prediction in terms of the input features and has been widely studied to assist in interpreting DNNs. In a linear model (i.e., g(x)=wx+b), the gradient corresponds solely to the weights w. Such a model can reasonably locally linearly approximate a smooth nonlinear DNN, and hence the weights of this local model are the gradient. The other part, however, of a local linear model, i.e., the bias b, is usually overlooked in attribution methods since it is not part of the gradient. In this paper, we observe that since the bias in a DNN also has a non-negligible contribution to the correctness of predictions, it can also play a significant role in understanding DNN behaviors. In particular, we study how to attribute a DNN's bias to its input features. We propose a backpropagation-type algorithm bias back-propagation (BBp)'' that starts at the output layer and iteratively attributes the bias of each layer to its input nodes as well as combining the resulting bias term of the previous layer. This process stops at the input layer, where summing up the attributions over all the input features exactly recovers b. Together with the backpropagation of the gradient generating w, we can fully recover the locally linear model g(x)=wx+b. Hence, the attribution of the DNN outputs to its inputs is decomposed into two parts, the gradient w and the bias attribution, providing separate and complementary explanations. We study several possible attribution methods applied to the bias of each layer in BBp. In experiments, we show that BBp can generate complementary and highly interpretable explanations of DNNs in addition to gradient-based attributions. Interpreting Adversarially Trained Convolutional Neural Networks We attempt to interpret how adversarially trained convolutional neural networks (AT-CNNs) recognize objects. We design systematic approaches to interpret AT-CNNs in both qualitative and quantitative ways, and compare them with normally trained models. Surprisingly, we find that adversarial training alleviates the texture bias of standard CNNs when trained on object recognition tasks, and helps CNNs learn a more shape-biased representation. We validate our hypothesis from two aspects. First, we compare the salience maps of AT-CNNs and standard CNNs on clean images and image under different transformations. The comparison could visually show that the predictions of the two types of CNNs are sensitive to dramatically different types of features. Second, to achieve quantitative verification, we construct additional test datasets that destroy either textures or shapes, such as style-transferred version of clean data, saturated images and patch-shuffled ones, and then evaluate the classification accuracy of AT-CNNs and normal CNNs on these datasets. Our findings shed some light on why AT-CNNs are more robust than those normally trained ones and contribute to a better understanding of adversarial training over CNNs from an interpretation perspective. The code for reproducibility is provided in the Supplementary Materials. Counterfactual Visual Explanations A counterfactual query is typically of the form For situation X, why was the outcome Y and not Z?''. A counterfactual explanation (or response to such a query) is of the formIf X was X*, then the outcome would have been Z rather than Y.'' In this work, we develop a technique to produce counterfactual visual explanations. Given a query' image $I$ for which a vision system predicts class $c$, a counterfactual visual explanation identifies how $I$ could change such that the system would output a different specified class $c'$. To do this, we select adistractor' image I′ that the system predicts as class c′ and identify spatial regions in I and I′ such that replacing the identified region in I with the identified region in I′ would push the system towards classifying I as c′. We apply our approach to multiple image classification datasets generating qualitative results showcasing the intepretability and discriminativeness of our counterfactual explanations. To explore the effectiveness of our explanations in teaching humans, we present machine teaching experiments for the task of fine-grained bird classification. We find that users trained to distinguish bird species fare better when given access to counterfactual explanations in addition to training examples. Data Poisoning Attacks on Stochastic Bandits Stochastic multi-armed bandits form a class of online learning problems that have important applications in online recommendation systems, adaptive medical treatment, and many others. Even though potential attacks against these learning algorithms may hijack their behavior, causing catastrophic loss in real-world applications, little is known about adversarial attacks on bandit algorithms. In this paper, we propose a framework of offline attacks on bandit algorithms and study convex optimization based attacks on several popular bandit algorithms. We show that the attacker can force the bandit algorithm to pull a target arm with high probability by a slight manipulation of the rewards in the data. Then we study a form of online attacks on bandit algorithms and propose an adaptive attack strategy against any bandit algorithm without the knowledge of the bandit algorithm. Our adaptive attack strategy can hijack the behavior of the bandit algorithm to suffer a linear regret with only a logarithmic cost to the attacker. Our results demonstrate a significant security threat to stochastic bandits.



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