Reinforcement Learning (RL) fits Interactive Information Retrieval (IIR) well as both of them center on accomplishing a goal during an interactive process. In RL, a machine agent maximizes its cumulative rewards collected during the course of interactions with an environment. In IIR, a search system satisfies an information need during the course of interactions with a user and a corpus. These commonalities have inspired approaches for IIR using RL solutions (Zhao et al., 2019; Tang and Yang, 2019). In these solutions, the search system is the RL agent; and both the user and text corpus the RL environment.
Training an RL agent could be difficult. First, it is expensive to train the agent. To learn some policy useful, the agent may need to take millions of steps to interact with the environment. It becomes even more expensive when real humans are involved in the interactions, as what we have here in IIR. Simulations have therefore been proposed to replace real human users to train interactive systems (Yang et al., 2017). Second, these simulators are usually created based on pre-annotated ground truth corpora. The corpora usually contain many irrelevant documents and a few relevant documents. Therefore the simulators would not be able to form a balanced training environment.
The unbalance in the forming of training environments would prevent the RL agent from learning good policies. It is because the rewards – relevant documents – are too few and the RL agent may not be able connect a long sequence of retrieval actions to a distant future reward, thus will never learn how to perform a task. This is known as the problem of “sparse rewards”. Trained with sparse rewards, when the RL agent is deployed in a real dynamic environment, such as in the Web, it would be likely to make wrong decisions, especially when the agent encounters documents never seen before.
In this paper, we propose a novel domain randomization method to enhance the training experiences of RL agents in interactive retrieval. Our method, Document Environment Generation (DEG), proposes to automatically generate positive learning environments as many as needed during a simulated training. DEG derives a stream of synthetic environments from available relevant documents by merging relevant segments within the documents with other irrelevant segments extracted from the corpus. In addition, our method dynamically restrains the changing rate of a policy to make sure that the RL agent adapts conservatively to the synthetic environments. We experiment our method on the Text REtrieval Conference (TREC) 2017 Dynamic Domain (DD) Track and the results show that the proposed method is able to statistically significantly boost the agents’ learning effectiveness.
2. Related Work
Reinforcement learning has been successfully applied in several applications in text domains. They include dialogue systems (Li et al., 2016), recommendation systems (Liu et al., 2018) and dynamic search (Tang and Yang, 2019). These systems aim to find items that match with user’s interests by modeling the interactions between a system and a human user. The required participation of real human users makes it quite costly to sample training trajectories for the system. Researcher have proposed to build simulators to reduce the sample complexity (Yang et al., 2017). However, the difference between a simulation and a real environment, known as their reality gap, makes it challenging to directly deploy an RL agent into the real world (Chebotar et al., 2019). Domain randomization has been proposed to alleviate this problem in robotics. For instance, (Tobin et al., 2017) randomized the objects and textures when training a robotic arm to grasp in cluttered environments. (Peng et al., 2018) randomized the mass and damping of robotic arms. (Sadeghi and Levine, 2016) randomized the floor plan when training a drone to fly indoor.
A similar technique, data augmentation, has been used in supervised learning to add more training data. In computer vision, it is proposed to augment image data by performing geometry transformation, such as flipping and cropping(Shorten and Khoshgoftaar, 2019), and photometric transformation, such as edge enhancement and color jittering (Taylor and Nitschke, 2017)
. In natural language processing, it is proposed to generate new text by various language generation methods. For instances,(Li et al., 2017) created noisy texts by substituting words with its synonyms. (Ebrahimi et al., 2018)
flipped the order of characters to improve the robustness of a text classifier.(Wei and Zou, 2019) also augmented training samples by random insertion and deletion.
In this work, we randomize a process to generate new texts by leveraging a unique characteristic in IR. The characteristic is that whether a document is relevant largely depends on if it contains the query keywords and only the matching parts in the document would determine the relevance of the document.
Training an RL agent in a series of different environments sometimes would result in “catastrophic forgetting” (Kirkpatrick et al., 2017). We handle this problem in our work too because our agent needs to learn from an enlarged and more diverse set of environments. (Rusu et al., 2016)
dealt with this problem by training an ensemble of neural networks, each for a separate task, and sharing weights among them.(Kirkpatrick et al., 2017)
augmented a loss function to slow down the learning on important parameters.(Berseth et al., 2018) transferred policies learned among different tasks with network distillation. Unlike them, DEG handles this problem by restraining the policy’s changing rate based on how much the newly generated environments differ from the original.
In IIR, a human user searches for relevant information within a text collection from an interactive search engine. The Text REtrieval Conference (TREC) Dynamic Domain (DD) Track (Yang et al., 2017) provides a platform to evaluate interactive search engines. In the Track, a simulated user starts an initial query. At each subsequent time step, a search system retrieves a set of documents and returns the top ( in TREC DD) to the simulated user. The simulated user provides explicit feedback regarding how relevant these documents are and then the search system adjusts its search algorithm based on the feedback. The process repeats until the search stops.
CE3 (Tang and Yang, 2020) is a state-of-the-art IIR system. It is based on the proximal policy optimization (PPO) algorithm (Schulman et al., 2017). In this paper, we improve this state-of-the-art system by incorporating domain randomization. Following CE3’s practice, DEG also uses a corpus-level state representation. To build the states, our method first segments each document into a fixed number (
) of segments. Each segment is on the same topic and maps into a separate doc2vec vector after compression. State at timeis formed like taking a snapshot of the entire corpus by stacking together the representations of all documents at . This global state representation at is expressed in the embedding function :
where is the text corpus and is the set of documents retrieved at time . The state representation’s dimension is , where is the size of the corpus, the number of segments per document, and the lower dimension ( vocabulary size) of the doc2vec vector after compression. The retrieved documents at time , i.e. , will be marked as ‘visited’ on this global representation and passed on to next run of retrieval.
Action at time , , is a weighting vector for document ranking. The ranking function calculates a relevance score between document and search topic (query) as the weighted sum over all segments :
where is the doc2vec representation of the segment in the document and is the action vector (weighting vector) at time .
Reward is derived from the relevance ratings annotated by human assessors. These relevance ratings are from 0 (irrelevant) to 4 (highly relevant). We define as the sum of all relevance ratings for the returned documents after duplicated results are removed:
where gets the relevance rating for by looking up and summing the ratings for all passages in in the ground truth.
4. Proposed Method
This section presents our method on generating more relevant documents to form more balanced training environments and using an adaptive clipping rate to constrain the policy from dramatic changes.
4.1. Generate Synthetic Environments
Information retrieval is a task driven by keyword matching. Users recognize relevant documents by recognizing query keywords that are present in the documents. As long as the keywords are kept in a document, the document is considered as relevant. We therefore propose to separate relevant segments from irrelevant segments in a relevant document and put them into different uses.
Our process to create new relevant documents is the following. First, DEG separates the corpus into three parts, relevant segments from relevant documents, irrelevant segments from relevant documents, and irrelevant segments from irrelevant documents. Note all segments in an irrelevant document are irrelevant. Second, an ‘irrelevant pool” of segments, , is formed by putting together all irrelevant segments from both relevant and irrelevant documents. Third, for each relevant document, DEG samples segments, , from the irrelevant pool :
is a uniform distribution andis a parameter to control the maximum “fake” level that the synthesized documents would be. Fourth, irrelevant segments replace the original irrelevant segments in the relevant document and form a synthetic relevant document. Each sampling can independently generate a new document from the same seed document. Fifth, documents synthesized from the same document share the same relevance rating as the seed document as as they all contain the same set of relevant segments. Last, the process repeats for every relevant document in the corpus. DEG can generate as many new relevant documents as possible from the same original document. Figure 1 illustrates DEG’s document generation process.
These generated documents are added into the corpus to form a new training environment. With more relevant documents now in the training environment, the RL agent is able to meet a reward (relevant document) more frequently when it interacts with the environment. It is then able to learn a policy with enough reward signals. Moreover, the agent can be exposed to many different training environments when we include different synthesized documents to form a new environment. The agent is thus trained with not only more data but more diverse data, which would help with its generalizing ability.
Note that the generation process needs knowledge about passage-level ground truth relevance, which may not be available in datasets outside of TREC DD. However, such knowledge may also be obtained from matching keywords with ground truth documents. The relevance can also be derived from implicit feedback such as clicks and dwell time. We think the method proposed here is applicable to other settings for interactive retrieval. The test environments used in this work do not use DEG because it would be unfair to know ground truth in a test environment.
4.2. Adaptive Training
Training an RL agent on vastly different environments may result in a problem known as “catastrophic forgetting” of the original environment (Kirkpatrick et al., 2017). In our case, the synthetic environment could be very different from the original and causes this problem. We propose to control the policy derivations within a bound based on how different the synthesized environment is from the original. We call this strategy “adaptive training”.
The RL framework we use is based on PPO (Schulman et al., 2017; Tang and Yang, 2020), which optimizes the following objective: , where is a ratio denoting the change of action distribution. The function limits the change ratio within , where is the maximum change of the action distributions and usually set to a fixed value, such as 0.2. This formulation has already been able to prevent drastic policy change.
Our work goes one step further to use a dynamically defined , instead of a fixed value. In DEG, the synthetic environment differs from the original environment in terms of the documents it consists of. Thus, we calculate based on how much the synthetic documents differ from the original document. Their difference is measured by Euclidean distance in the embedding space. For the synthetic environment, is calculated as
where is the original environment, is the synthetic environment, is the embedding function shown in Eq. 1, is the Euclidean distance function, and and are hyper parameters.
For environments that differ more from the original environment, puts a tighter restraint on the change rate, which slows down the change of the policy. In this way, DEG prevents the agent from catastrophic forgetting when trained with more diverse environments.
We experiment on the TREC 2017 Dynamic Domain Track (Yang et al., 2017). It uses the New York Times corpus (Sandhaus, 2008). Sixty search topics were created for the Track and each is a distinct learning environment. We compare the performance of the following IIR methods: CE3 (Tang and Yang, 2020), a PPO-based RL method without domain randomization. DEG, the proposed method with domain randomization and adaptive training; , , and . DEG (fixed), a variant of DEG with a fixed ; is set to as it works the best for many topics.
5.1. Effectiveness under Unseen Situations
To understand how well a trained agent can perform interactive retrieval under unseen situations, we split the TREC DD 2017 dataset into non-overlapping training and test sets. The test dataset is considered as unseen environments to the agents. For each search topic, we train RL agents using CE3, DEG or DEG (fixed) on the training set and test them on the test set. We compare the agents’ performance by measuring the rewards they obtain in the first 100 runs of interactions in the test set. Figure 2 reports the reward of CE3, DEG, and DEG (fixed), averaged over test search topics.
We observe that both DEG runs achieve a significant 5% absolute and 22% relative (¡
, double-tailed t-test) improvement over CE3. It shows that agents trained with domain randomization work much better than trained without it. We think it is because DEG generates a lot more relevant documents to form more balanced training environments and makes the agents generalize better. In addition, we found that although on average DEG and DEG (fixed) work comparable, DEG demonstrates much lower variance in the rewards that it can obtain. The proposed use of adaptive training contributes to a more conservative policy update, which results in more stable rewards. This is because DEG is less likely to take a big step to fall into a trajectory that is either too good or too bad.
5.2. Case Studies
Figure 3 shows two representative examples, topics 49 and 21. In Topic 49, CE3 gets nearly zero rewards in unseen environments, which indeed works very poor. On the other hand, DEG variants much better adjust to unseen environments and can both get impressive rewards. Domain randomization seems to help the agents develop much better generalization ability. In Topic 21, we can see that DEG (fixed)’s rewards fluctuate a lot as the interactions go on while DEG produces much more stable rewards. Although DEG may miss some high-value steps, it too avoids the wrong moves, which is desirable for a task that cares much about precision at top positions.
This paper proposes a domain randomization method to enhance reinforcement learning in interactive retrieval. Our method generates new relevant documents from existing documents in a collection to increase frequency of meeting relevant documents during RL training. The experiments show that this strategy can significantly boost an RL agent’s performance in interactive retrieval under unseen situations. The proposed adaptive training method also helps the RL agent explore more steadily with stable rewards.
Acknowledgements.This research was supported by U.S. National Science Foundation Grant no. IIS-145374. Any opinions, findings, conclusions, or recommendations expressed in this paper are of the authors, and do not necessarily reflect those of the sponsor.
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