When robust machine learning meets noisy supervision : mechanism, optimization and generalization

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Modern machine learning is migrating to the era of complex models (i.e., deep neural networks), which requires a plethora of well-annotated data. Crowdsourcing is a promising tool to achieve this goal, since a plethora of labels that can be efficiently collected from crowdsourcing services at very low cost. However, existing crowdsourcing approaches barely acquire a sufficient amount of high-quality labels. This brings the first question: How to design the robust mechanism to improve the label quality? Without such robust mechanism, labels annotated by crowdsourced workers are often noisy, which inevitably degrades the performance of largescale optimizations, including the prevalent stochastic gradient descent (SGD). Specifically, these noisy labels adversely affect updates of the primal variable in conventional SGD. This bring the second question: How to optimize the training model robustly under noisy labels? Without such robust optimization, it is challenging to train deep neural networks robustly with noisy labels, as the learning capacity of deep neural networks is so high that they can totally memorize and over-fit on these noisy labels. This brings the third question: How to acquire the robust model with good generalization under noisy labels? Therefore, in this thesis, we aim to develop a series of robust machine learning approaches, so that they can perfectly handle the difficult from noisy supervision. Our works are summarized as follows: Chapter 2 answers the first question. Motivated by the “Guess-with-Hints” answer strategy from the Millionaire game show, we introduce the hint-guided approach into crowdsourcing to deal with this challenge. Our approach encourages workers to get help from hints when they are unsure of questions. Specifically, we propose a hybrid-stage setting, consisting of the main stage and the hint stage. When workers face any uncertain question on the main stage, they are allowed to enter the hint stage and look up hints before making any answer. A unique payment mechanism that meets two important design principles is developed. Besides, the proposed mechanism further encourages high-quality workers less using hints, which helps identify and assigns larger possible payment to them. Experiments are performed on Amazon Mechanical Turk, which show that our approach ensures a sufficient number of high-quality labels with low expenditure and detects high-quality workers. Chapter 3 answers the second question. We propose a robust SGD mechanism called PrOgressive STochAstic Learning (POSTAL), which naturally integrates the learning regime of curriculum learning (CL) with the update process of vanilla SGD. Our inspiration comes from the progressive learning process of CL, namely learning from “easy” tasks to “complex” tasks. Through the robust learning process of CL, POSTAL aims to yield robust updates of the primal variable on an ordered label sequence, namely from “reliable” labels to “noisy” labels. To realize POSTAL mechanism, we design a cluster of “screening losses”, which sorts all labels from the reliable region to the noisy region. We derive the convergence rate of POSTAL realized by screening losses. Meanwhile, we provide the robustness analysis of representative screening losses. Experiments on benchmark datasets show that POSTAL using screening losses is more effective and robust than several existing baselines. Chapter 4 answers the third question. Motivated by the memorization effects of deep networks, which shows networks fit clean instances first and then noisy ones, we present a new paradigm called “𝘊𝘰-𝘵𝘦𝘢𝘤𝘩𝘪𝘯𝘨” combating with noisy labels. We train two networks simultaneously. First, in each mini-batch data, each network filters noisy instances based on memorization effects. Then, it teaches the remained instances as the useful knowledge to its peer network for updating the parameters. Empirical results on three benchmark datasets demonstrate that, the robustness of deep learning models trained by Co-teaching approach is much superior than that of state-of-the-art methods.
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