Learning to Transform, Combine, and Reason in Open-Domain Question Answering

Mostafa Dehghani 
Our paper "Learning to Transform, Combine, and Reason in Open-Domain Question Answering", with Hosein Azarbonyad, Jaap Kamps, and  Maarten de Rijke, has been accepted as a long paper at 12th ACM International Conference on Web Search and Data Mining (WSDM 2019).\o/

We have all come to expect getting direct answers to complex questions from search systems on large open-domain knowledge sources like the Web. Open-domain question answering is a critical task that needs to be solved for building systems that help address our complex information needs.

To be precise open-domain question answering is the task of answering a user's question in the form of short texts rather than a list of relevant documents, using open and available external sources.

Most open-domain question answering systems described in the literature first retrieve relevant documents or passages, select one or a few of them as the context, and then feed the question and the context to a machine reading comprehension system to extract the answer.

However, the information needed to answer complex questions is not always contained in a single, directly relevant document that is ranked high. In many cases, there is a need to take a broader context into account, e.g., by considering low-ranked documents that are not immediately relevant, combining information from multiple documents, and reasoning over multiple facts from these documents to infer the answer.

Why should we take a broader context into account?

In order to better understand why taking a broader context into account can be necessary or useful, let's consider an example. Assume that a user asks this question: "Who is the Spanish artist, sculptor and draughtsman famous for co-founding the Cubist movement?"

We can use a search engine to retrieve the top-k relevant documents. The figure below shows the question along with a couple of retrieved documents.

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SIGIR2018 Workshop on Learning From Noisy/Limited Data for IR

Mostafa Dehghani

We are organizing the "Learning From Noisy/Limited Data for Information Retrieval" workshop which is co-located with SIGIR 2018. This is the first edition of this workshop and The goal of the workshop is to bring together researchers from industry, where data is plentiful but noisy, with researchers from academia, where data is sparse but clean, to discuss solutions to these related problems.

We invited contributions relevant to this topics:

  • Learning from noisy data for IR
    • Learning from automatically constructed data
    • Learning from implicit feedback data, e.g., click data
  • Distant or weak supervision and learning from IR heuristics
  • Unsupervised and semi-supervised learning for IR
  • Transfer learning for IR
  • Incorporating expert/domain knowledge to improve learning-based IR models
    • Learning from labeled features
    • Incorporating IR axioms to improve machine learning models

Marc Najork is going to give a fantastic keynote on "Using biased data for learning-to-rank" and we have a set of fantastic papers  (including mine :P) that are going to be presented at the workshop and a great discussion panel with wonderful panelist from both industry and academia.

Save the date on your calendar!

Fidelity-Weighted Learning

Mostafa Dehghani 
Our paper "Fidelity-Weighted Learning", with Arash Mehrjou, Stephan Gouws, Jaap Kamps, Bernhard Schölkopf, has been accepted at Sixth International Conference on Learning Representations (ICLR2018). \o/

tl;dr

Fidelity-weighted learning (FWL) is a semi-supervised student-teacher approach for training deep neural networks using weakly-labeled data. It modulates the parameter updates to a student network which trained on the task we care about, on a per-sample basis according to the posterior confidence of its label-quality estimated by a Bayesian teacher, who has access to a rather small amount of high-quality labels.

The success of deep neural networks to date depends strongly on the availability of labeled data which is costly and not always easy to obtain. Usually, it is much easier to obtain small quantities of high-quality labeled data and large quantities of unlabeled data. The problem of how to best integrate these two different sources of information during training is an active pursuit in the field of semi-supervised learning and here, with FWL, we propose an idea to address this question.

Learning from samples of variable quality

For a large class of tasks, it is also easy to define one or more so-called “weak annotators”, additional (albeit noisy) sources of weak supervision based on heuristics or “weaker”, biased classifiers trained on e.g. non-expert crowd-sourced data or data from different domains that are related. While easy and cheap to generate, it is not immediately clear if and how these additional weakly-labeled data can be used to train a stronger classifier for the task we care about. More generally, in almost all practical applications machine learning systems have to deal with data samples of variable quality. For example, in a large dataset of images only a small fraction of samples may be labeled by experts and the rest may be crowd-sourced using e.g. Amazon Mechanical Turk. In addition, in some applications, labels are intentionally perturbed due to privacy issues.

Assuming we can obtain a large set of weakly-labeled data in addition to a much smaller training set of “strong” labels, the simplest approach is to expand the training set by including the weakly-supervised samples (all samples are equal). Alternatively, one may pretrain on the weak data and then fine-tune on observations from the true function or distribution (which we call strong data). Indeed,  a small amount of expert-labeled data can be augmented in such a way by a large set of raw data, with labels coming from a heuristic function, to train a more accurate neural ranking model. The downside is that such approaches are oblivious to the amount or source of noise in the labels.

All labels are equal, but some labels are more equal than others, just like animals.

 

Inspired by George, Animal Farm, 1945.

We argue that treating weakly-labeled samples uniformly (i.e. each weak sample contributes equally to the final classifier) ignores potentially valuable information of the label quality. Instead, we propose Fidelity-Weighted Learning (FWL), a Bayesian semi-supervised approach that leverages a small amount of data with true labels to generate a larger training set with confidence-weighted weakly-labeled samples, which can then be used to modulate the fine-tuning process based on the fidelity (or quality) of each weak sample. By directly modeling the inaccuracies introduced by the weak annotator in this way, we can control the extent to which we make use of this additional source of weak supervision: more for confidently-labeled weak samples close to the true observed data, and less for uncertain samples further away from the observed data.

How fidelity-weighted learning works?

We propose a setting consisting of two main modules:

  1. One is called the student and is in charge of learning a suitable data representation and performing the main prediction task,
  2. The other is the teacher which modulates the learning process by modeling the inaccuracies in the labels.
Illustration of Fidelity-Weighted Learning: Step 1: Pre-train student on weak data, Step 2: Fit teacher to observations from the true function, and Step 3: Fine-tune student on labels generated by the teacher, taking the confidence into account. Red dotted borders and blue solid borders depict components with trainable and non-trainable parameters, respectively.

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Learning to Learn from Weak Supervision by Full Supervision

Mostafa Dehghani 
Our paper "Learning to Learn from Weak Supervision by Full Supervision", with Sascha Rothe, and  Jaap Kamps, has been accepted at NIPS2017 Workshop on Meta-Learning (MetaLearn 2017). \o/

Using weak or noisy supervision is a straightforward approach to increase the size of the training data and it has been shown that the output of heuristic methods can be used as weak or noisy signals along with a small amount of labeled data to train neural networks. 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 and using noisy labels of lower quality often brings little to no improvement. This issue is tackled by noise-aware models where denoising the weak signal is part of the learning process.

We propose a meta-learning approach in which we train two networks: a target network, which plays the role of the learner and it uses a large set of weakly annotated instances to learn the main task, and a confidence network which plays the role of the meta-learner and it is trained on a small human-labeled set to estimate confidence scores. These scores define the magnitude of the weight updates to the target network during the back-propagation phase. The goal of the confidence network trained jointly with the target network is to calibrate the learning rate of the target network 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:

\pmb{w}_{t+1} = \pmb{w}_t - \frac{\eta_t}{b}\sum_{i=1}^b c_{\theta}(x_i, \tilde{y}_i) \nabla \mathcal{L}(f_{\pmb{w_t}}(x_i), \tilde{y_i}) %+ \nabla \mathcal{R}(\pmb{w_t})

where \eta_t is the global learning rate, \mathcal{L}(\cdot) is the loss of predicting \hat{y}=f_w(x_i) for an input x_i when the label is \tilde{y}; c_\theta(\cdot) is a scoring function learned by the confidence network taking input instance x_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.

Our setup requires running a weak annotator to label a large amount of unlabeled data, which is done at pre-processing time. For many tasks, it is possible to use a simple heuristic to generate weak labels. This set is then used to train the target network. In contrast, a small human-labeled set is used to train the confidence network.  The general architecture of the model is illustrated in the figure below:

Our proposed multi-task network for learning a target task using a large amount of weakly labeled data and a small amount of data with true labels. Faded parts of the network are disabled during the training in the corresponding mode. Red-dotted arrows show gradient propagation. Parameters of the parts of the network in red frames get updated in the backward pass, while parameters of the network in blue frames are fixed during the training.

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

Mostafa Dehghani 
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.

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