Avoiding Your Teacher's Mistakes: Training Neural Networks with Controlled Weak Supervision

This post is about the project I've done in collaboration with Aliaksei Severyn, Sascha Rothe, and Jaap Kamps, during my internship at Google Research.

Deep neural networks have shown impressive results in a lot of tasks in computer vision, natural language processing, and information retrieval. However, their success is conditioned on the availability of exhaustive amounts of labeled data, while for many tasks such a data is not available.  Hence, unsupervised and semi-supervised methods are becoming increasingly attractive.

Using weak or noisy supervision is a straightforward approach to increase the size of the training data.  In one of my previous post, I've talked about how to beat your teacher, which provides an insight on how to train a neural network model using only the output of a heuristic model as supervision signal which eventually works better than that heuristic model. Assuming that most of the time, besides a lot of unlabeled (or weakly labeled) data there is a small amount of training data with strong (true) labels, i.e. a semi-supervised setup, here I'll talk about how to learn from a weak teacher and avoid his mistakes.

This is usually done by pre-training the network on weak data and fine-tuning it with true labels.  However, these two independent stages do not leverage the full capacity of information from true labels.  For instance, in the pre-training stage, there is no handle to control the extent to which the data with weak labels contribute in the learning process, while they can be of different quality.

In this post, I'm going to talk about our proposed idea which is a semi-supervised method that leverages a small amount of data with true labels along with a large amount of data with weak labels.  Our proposed method has three main components:

  • A weak annotator, which can be a heuristic model, a weak classifier, or even human via crowdsourcing and it is employed to annotate massive amount of unlabeled data.
  • A target network which uses a large set of weakly annotated instances by weak annotator to learn the main task
  • A confidence network which is trained on a small human-labeled set to estimate confidence scores for instances annotated by the weak annotator. We train the target network and confidence in a multi-task fashion.

In a joint learning process, target network and confidence network try to learn a suitable representation of the data and this layer is shared between them as a two-way communication channel.  The target network tries to learn to predict the label of the given input under the supervision of the weak annotator. In the same time, the output of the confidence network, which are the confidence scores, define the magnitude of the weight updates to the target network with respect to the loss computed based on labels from weak annotator, during the back-propagation phase of the target network. This way, the confidence network helps the target network to avoid mistakes of her teacher, i.e.weak annotator, by down-weighting the weight updates from weak labels that do not look reliable to the confidence network.

From a meta-learning perspective, the goal of the confidence network trained jointly with the target network is to calibrate the learning rate for each instance in the batch. I.e., the weights \pmb{w} of the target network  f_w at step t+1 are updated as follows:

(1)   \begin{equation*} \pmb{w}_{t+1} = \pmb{w}_t + \frac{l_r}{b}\sum_{i=1}^b c_{\theta}(\pmb{\tau}_i, \tilde{y}_i)\mathcal{L}(f_w(\pmb{\tau}_i), \tilde{y_i}) \end{equation*}

where l_r is the global learning rate, b is the batch size, \mathcal{L}(\cdot) the loss of predicting \hat{y}=f_w(\pmb{\tau}) for an input \pmb{\tau} when the target label is \tilde{y}; c_\theta(\cdot) is a scoring function learned by the confidence network taking input instance \pmb{\tau}_i and its noisy label \tilde{y}_i. Thus, we can effectively control the contribution to the parameter updates for the target network from weakly labeled instances based on how reliable their labels are according to the confidence network (learned on a small supervised data).

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Toward Document Understanding for Information Retrieval

Document understanding for the purpose of assessing the relevance of a document or passage to a query based only on the document content appears to be a familiar goal for information retrieval community, however, this problem has remained largely intractable, despite repeated attacks over many years.  This is while people are able to assess the relevance quite well, though unfamiliar topics and complex documents can defeat them.  This assessment may require the ability to understand language, images, document structure, videos, audio, and functional elements.  In turn, understanding of these elements is built on background information about the world, such as human behavior patterns, and even more fundamental truths such as the existence of time, space, and people.  All this comes naturally to people, but not to computers!

Recently, large-scale machine learning has altered the landscape.  Deep learning has greatly advanced machine understanding of images and language.  Since document and query understanding incorporate these elements, deep learning can hold great promise.  But it comes with a drawback:  general purpose representations (like CNNs for images) have proved somewhat elusive for text.  In particular, embeddings act as a distributed representation not just of semantic information but also application-specific learnings, which are hard to transfer. In short, conditions seem right for a renewed attempt on the fundamental document understanding problem.

What is document understanding?

In order to think about the way we can approach this problem, I think we should first answer some questions: Can we understand documents?  What is "understanding"?  Getting at the true meaning of a document?  Okay, but then what is "meaning"?  How do we even approach such an ill-defined goal?

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Learning Useful Document Representations for Search

With the emergence of deep learning, representation learning has been a hot topic in recent years. There are a large variety of approaches to representation learning in general, but the basic idea is to learn some set of features from data and then using these features for some task. Generating representation for documents is still one the key challenges in information retrieval.  In search, the main task is to determine whether a document is relevant to a query or not.  So we want to process the document in order to produce a representation of it that preserves our ability to judge relevance while stripping away nonessential data.

Most of the time, due to efficiency reasons, this is done as a preprocessing stage where we generate query-independent document representation and it has one straightforward motivation: processing documents online would be terribly expensive. Note that this is not to suggest that query-dependent document representations are not useful, for instance, we can train a neural network which predicts the relevance of a query and a document based on words near hits which could be both efficient and effective.

Since deep learning turned to be a tool using which we can renew our attempts for representing documents, it is good to think about this question again that "What does make for useful document representations?". Of course representations that lead to better results in search are appreciated, but it is not enough. Here, in this write-up, I'm going to shortly talk about some characteristics that I think a useful query-independent document representation for the task of search should possess.

Good representations of documents should ideally satisfy the following properties for maximal utility in information retrieval:

  1. Semantic: The representation should be the same or similar even if the text is rewritten in a different manner as long as it has the same meaning. In other words, it should be resilient to paraphrases. (more…)

Some Highlights of MILA Deep Learning and Reinforcement Learning Summer Schools 2017

A couple weeks a go, I attended MILLA Deep Learning summer school (DLSS) from June 26th to July 1st and Reinforcement Learning summer school (RLSS) from  July 3rd to 5th, 2017, organized by Yoshua Bengio and Aaron Courville. You can find information about the lectures here. In the following, I will share some of "my" highlights from the summer schools.

Different types of learning problems

The first day of summer school was on general topics of machine learning and neural networks. Doina Precup gave a talk which was a gentle refreshing of the general concepts of Machine Learning and then Hugo Larochelle covered the basics of neural networks. In the second part of his talk, Hugo started by dividing learning problems into different types, based on the data and settings of the problem during training and inference. Based on his grouping, each learning problem can be classified to one of these categories:

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Learning to Attend, Copy, and Generate for Session-Based Query Suggestion

Our paper "Learning to Attend, Copy, and Generate for Session-Based Query Suggestion", with Sascha Rothe, Enrique Alfonseca, and  Pascal Fleury, has been accepted as a long paper at the international Conference on Information and Knowledge Management (CIKM'17). This paper is on the outcome of my internship at Google Research. \o/

Users interact with search engines during search sessions and try to direct their search by submitting a sequence of queries. Based on these interactions, search engines provide a prominent feature, in which they assist their users to formulate their queries to better represent their intent during Web search by providing suggestions for the next query.

 

Query suggestion might address the need for disambiguation of the user queries to make the direction of the search more clear for both, the user and the search engine.
It might help users by providing a precise and succinct query when they are not familiar with the specific terminology or when they lack understanding of the internal vocabulary and structures in order to be able to formulate an effective query. It has been shown that in general, query suggestion accelerates search satisfaction by either diving deeper into the current search direction or by moving to a different aspect of a search task.

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Share your Model instead of your Data!

Our paper "Share your Model instead of your Data: Privacy Preserving Mimic Learning for Ranking", with Hosein Azarbonyad, Jaap Kamps, and Maarten de Rijke, has been accepted at Neu-IR: SIGIR Workshop on Neural Information Retrieval (NeuIR'17). \o/

In this paper, we aim to lay the groundwork for the idea of sharing a privacy preserving model instead of sensitive data in IR applications. This suggests researchers from industry share the knowledge learned from actual users’ data with the academic community that leads to a better collaboration of all researchers in the field.

Deep neural networks demonstrate undeniable success in several fields and employing them is taking o for information retrieval problems. It has been shown that supervised neural network models perform be er as the training dataset grows bigger and becomes more diverse. Information retrieval is an experimental and empirical discipline, thus, having access to large-scale real datasets is essential for designing effective IR systems. However, in many information retrieval tasks, due to the sensitivity of the data from users and privacy issues, not all researchers have access to large-scale datasets for training their models.

Much research has been done on the general problem of pre- serving the privacy of sensitive data in IR applications, where the question is how should we design effective IR systems without damaging users’ privacy?

One of the solutions so far is to anonymize the data and try to hide the identity of users. However, there is no guarantee that the anonymized data will be as effective as the original data.

Using machine learning-based approaches, sharing the trained model instead of the original data has turned out to be an option for transferring knowledge. The idea of mimic learning is to use a model that is trained based on the signals from the original training data to annotate a large set of unlabeled data and use these labels as training signals for training a new model. It has been shown, for many tasks in computer vision and natural language processing, that we can transfer knowledge this way and the newly trained models perform as well as the model trained on the original training data.

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On Search Powered Navigation

Our paper "On Search Powered Navigation", with Glorianna Jagfeld, Hosein Azarbonyad, Alex Olieman, Jaap Kamps, Maarten Marx, has been accepted as a short paper at . \o/
Knowledge graphs and other hierarchical domain ontologies hold great promise for complex information seeking tasks, yet their massive size defies the standard and effective way smaller hierarchies are used as a static navigation structure in faceted search or standard website navigation. As a result, we see the only limited use of knowledge bases in entity surfacing for navigational queries, and fail to realize their full potential to empower search. Seeking information in structured environments consists of two main activities: exploratory browsing and focused searching.
Exploratory browsing refers to activities aimed at better defining the information need and increasing the level of understanding of the information space, while focused searching includes activities such as query rewriting and comparison of results, which are performed after the information need has been made more concrete. Based on the interplay of these two actions, a search system is supposed to provide a connected space of information for the users to navigate, as well as search to adjust the focus of their browsing towards useful content.

In our paper, we introduce the concept of Search Powered Navigation (SPN), which enables users to combine navigation with the query based searching in a structured information space, and offers a way to find a balance between exploration and exploitation. We hypothesize that SPN enables users to exploit the semantic structure of a large knowledge base in an effective way. We test this hypothesis by conducting a user study in which users are engaged in exploratory search activities and investigate the effect of SPN on the variability in users’ behavior and experience. We employed an exploratory search system on parliamentary data in two modes, pure navigation and search powered navigation, and tested two types of tasks, broad- and focused-topic tasks.
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