Training Neural Response Selection for Task-Oriented Dialogue Systems

06/04/2019 ∙ by Matthew Henderson, et al. ∙ PolyAI 0

Despite their popularity in the chatbot literature, retrieval-based models have had modest impact on task-oriented dialogue systems, with the main obstacle to their application being the low-data regime of most task-oriented dialogue tasks. Inspired by the recent success of pretraining in language modelling, we propose an effective method for deploying response selection in task-oriented dialogue. To train response selection models for task-oriented dialogue tasks, we propose a novel method which: 1) pretrains the response selection model on large general-domain conversational corpora; and then 2) fine-tunes the pretrained model for the target dialogue domain, relying only on the small in-domain dataset to capture the nuances of the given dialogue domain. Our evaluation on six diverse application domains, ranging from e-commerce to banking, demonstrates the effectiveness of the proposed training method.

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1 Introduction

Retrieval-based dialogue systems conduct conversations by selecting the most appropriate system response given the dialogue history and the input user utterance (i.e., the full dialogue context). A typical retrieval-based approach to dialogue encodes the input and a large set of responses in a joint semantic space. When framed as an ad-hoc retrieval task Deerwester et al. (1990); Ji et al. (2014); Kannan et al. (2016); Henderson et al. (2017), the system treats each input utterance as a query and retrieves the most relevant response from a large response collection by computing semantic similarity between the query representation and the encoding of each response in the collection. This task is referred to as response selection Wang et al. (2013); Al-Rfou et al. (2016); Yang et al. (2018); Du and Black (2018); Chaudhuri et al. (2018); Weston et al. (2018), as illustrated in Figure 1.

Formulating dialogue as a response selection task stands in contrast with other data-driven dialogue modeling paradigms such as modular and end-to-end task-based dialogue systems Young (2010); Wen et al. (2017b); Liu and Perez (2017); Li et al. (2017); Bordes et al. (2017). Unlike standard task-based systems, response selection does not rely on explicit task-tailored semantics in the form of domain ontologies, which are hand-crafted for each task by domain experts Henderson et al. (2014a, b); Mrkšić et al. (2015). Response selection also differs from chatbot-style systems which generate new responses by generalising over training data, their main deficiency being the tendency towards generating universal but irrelevant responses such as “I don’t know” or “Thanks” Vinyals and Le (2015); Li et al. (2016); Serban et al. (2016); Song et al. (2018). Therefore, response selection removes the need to engineer structured domain ontologies, and to solve the difficult task of general language generation. Furthermore, it is also much easier to constrain or combine the output of response selection models. This design also bypasses the construction of dedicated decision-making policy modules.

Figure 1: The conversational response selection task: given the input sentence, the goal is to identify the relevant response from a large collection of candidates.

Although conceptually attractive, retrieval-based dialogue systems still suffer from data scarcity, as deployment to a new domain requires a sufficiently large in-domain dataset for training the response selection model. Procuring such data is expensive and labour-intensive, with annotated datasets for task-based dialogue still few and far between, as well as limited in size.111For instance, the recently published MultiWOZ dataset Budzianowski et al. (2018) comprises a total of 115,424 dialogue turns scattered over 7 target domains. It is several times larger than other standard task-based dialogue datasets such as DSTC2 Henderson et al. (2014b) with 23,354 turns, Frames El Asri et al. (2017) with 19,986 turns, or M2M Shah et al. (2018) with 14,796 turns. To illustrate the difference in magnitude, the Reddit corpus used in this work for response selection pretraining comprises 727M dialogue turns.

Recent work on language modelling (LM) pretraining Peters et al. (2018); Howard and Ruder (2018) has shown that task-specific architectures are not necessary in a number of NLP tasks. The best results have been achieved by LM pretraining on large unannotated corpora, followed by supervised fine-tuning on the task at hand Devlin et al. (2019). Given the compelling benefits of large-scale pretraining, our work poses a revamped question for response selection: can we pretrain a general response selection model and then adapt it to a variety of different dialogue domains?

To tackle this problem, we propose a two-step training procedure which: 1) pretrains a response selection model on large conversational corpora (such as Reddit); and then 2) fine-tunes the pretrained model for the target dialogue domain. Throughout the evaluation, we aim to provide answers to the following two questions:

  1. (Q1) How to pretrain? Which encoder structure can best model the Reddit data?

  2. (Q2) How to fine-tune? Which method can efficiently adapt the pretrained model to a spectrum of target dialogue domains?

Regarding the first question, the results support findings from prior work Cer et al. (2018); Yang et al. (2018): the best scores are reported with simple transformer-style architectures Vaswani et al. (2017) for input-response encodings. Most importantly, our results suggest that pretraining plus fine-tuning for response selection is useful across six different target domains.

As for the second question, the most effective training schemes are lightweight: the model is pretrained only once on the large Reddit training corpus, and the target task adaptation does not require expensive retraining on Reddit. We also show that the proposed two-step response selection training regime is more effective than directly applying off-the-shelf state-of-the-art sentence encoders Cer et al. (2018); Devlin et al. (2019).

We hope that this paper will inform future development of response-based task-oriented dialogue. Training and test datasets, described in more detail by Henderson:2019arxiv, are available at: github.com/PolyAI-LDN/conversational-datasets.

2 Methodology

Why Pretrain and Fine-Tune?

By simplifying the conversational learning task to a response selection task, we can relate target domain tasks to general-domain conversational data such as Reddit Al-Rfou et al. (2016). This also means that parameters of response selection models in target domains with scarce training resources can be initialised by a general-domain pretrained model.

The proposed two-step approach, described in §2.1 and §2.2, can be seen as a “lightweight” task adaptation strategy: the expensive Reddit model pretraining is run only once (i.e., training time is typically measured in days), and the model is then fine-tuned on target tasks (i.e., fine-tuning time is in minutes). The alternatives are “heavyweight” data mixing strategies. First, in-domain and Reddit data can be fused into a single training set: besides expensive retraining for each task, the disbalance between in-domain and Reddit data sizes effectively erases the target task signal. An improved data mixing strategy keeps the identities of the origin datasets (Reddit vs. target) as features in training. While this now retains the target signal, our preliminary experiments indicated that the results again fall short of the proposed lightweight fine-tuning strategies. In addition, this strategy still relies on expensive Reddit retraining for each task.

2.1 Step 1: Response Selection Pretraining

Reddit Data.

Our pretraining method is based on the large Reddit dataset compiled and made publicly available recently by Henderson:2019arxiv. This dataset is suitable for response selection pretraining due to multiple reasons as discussed by AlRfou:2016arxiv. First, the dataset offers organic conversational structure and it is large at the same time: all Reddit data from January 2015 to December 2018, available as a BigQuery dataset, span almost 3.7B comments. After preprocessing the dataset to remove both uninformative and long comments222We retain only sentences containing more than 8 and less than 128 word tokens. and pairing all comments with their responses, we obtain more than 727M comment-response pairs which are used for model pretraining. This Reddit dataset is substantially larger than the previous Reddit dataset of AlRfou:2016arxiv, which spans around 2.1B comments and 133M conversational threads, and is not publicly available. Second, Reddit is extremely diverse topically Schrading et al. (2015); Al-Rfou et al. (2016): there are more than 300,000 sub-forums (i.e., subreddits) covering diverse topics of discussion. Finally, compared to message-length-restricted Twitter conversations Ritter et al. (2010), Reddit conversations tend to be more natural. In summary, all these favourable properties hold promise to support a large spectrum of diverse conversational domains.

Input and Response Representation.

We now turn to describing the architecture of the main pretraining model. The actual description focuses on the best-performing architecture shown in Figure 2, but we also provide a comparative analysis of other architectural choices later in §4.1.

Figure 2: Schematic input-response encoder model structure. We show the best-performing architecture for brevity, while we evaluate a variety of other encoder architecture configurations later in §4.1.

First, similar to Henderson:2017arxiv, raw text is converted to unigrams and bigrams, that is, we extract -gram features from each input and its corresponding response from (Reddit) training data. During training we obtain -dimensional feature representations (, see Figure 2) shared between inputs and responses for each unigram and bigram jointly with other neural net parameters. In addition, the model can deal with out-of-vocabulary unigrams and bigrams by assigning a random id from 0 to 50,000 to each, which is then used to look up their embedding. When fine-tuning, this allows the model to learn representations of words that otherwise would be out-of-vocabulary.

Sentence Encoders.

The unigram and bigram embeddings then undergo a series of transformations on both the input and the response side, see Figure 2 again. Following the transformer architecture Vaswani et al. (2017), positional embeddings and self-attention are applied to unigrams and bigrams separately. The representations are then combined as follows (i.e., this refers to the reduction layer in Figure 2

): the unigram and bigram embeddings are each summed and divided by the square root of the word sequence length. The two vectors are then averaged to give a single

-dimensional representation of the text (input or response).

The averaged vector is then passed through a series of fully connected -dim feed-forward hidden layers (; ) with swish as the non-linear activation, defined as: Ramachandran et al. (2017).333We fix as suggested by Ramachandran:2017arxiv. The use of swish is strictly empirically driven: it yielded slightly better results in our preliminary experiments than the alternatives such as tanh

or a family of LU/ReLU-related activations

He et al. (2015); Klambauer et al. (2017). The final layer is linear and maps the text into the final -dimensional () representation: for the input text, and for the accompanying response text. This provides a fast encoding of the text, with some sequential information preserved.444

Experiments with higher-order n-grams, recurrent, and convolutional structures have not provided any substantial gain, and slow down the encoder model considerably.

(a) reddit-direct
(b) ft-direct
(c) ft-mixed
Figure 3: High-level overview of baseline and fine-tuning strategies used in our evaluation. (a) reddit-direct: a pretrained general-domain (Reddit) response selection model is directly applied on each target task, without any target domain fine-tuning; (b) ft-direct: after pretraining the large response selection model on Reddit, the model is fine-tuned for each target task by directly continuing the training on (much smaller) target domain data; (c) ft-mixed: similar to ft-direct, but the crucial difference is in-batch mixing of Reddit input-response pairs with target domain pairs during the target fine-tuning procedure. Another baseline (target-only) trains a response selection model on each target task separately without leveraging general-domain Reddit data (not shown).

Scaled Cosine Similarity Scoring.

The relevance of each response to the given input is then quantified by the score

. It is computed as scaled cosine similarity:

, where is a learned constant, constrained to lie between and . We resort to scaled cosine similarity instead of general dot product as the absolute values are meaningful for the former. In consequence, the scores can be thresholded, and retrained models can rely on the same thresholding.

Training proceeds in batches of (input, response) pairs . The objective tries to distinguish between the true relevant response and irrelevant/random responses for each input sentence . The training objective for a single batch of pairs is as follows:

(1)

Effectively, Eq. (1) maximises the score of pairs that go together in training, while minimising the score of pairing each input with negative examples, that is, responses that are not associated with the input . For simplicity, as in prior work Henderson et al. (2017); Yang et al. (2018), for each input , we treat all other responses in the current batch as negative examples.555Note that the matrix is inexpensive to compute.

As discussed by Henderson:2017arxiv in the context of e-mail reply applications, this design enables efficient response search as it allows for precomputing vectors of candidate responses independently of input queries, and searching for responses with high scaled cosine similarity scores in the precomputed set. It also allows for approximate nearest neighbour search Malkov and Yashunin (2016) which speeds up computations drastically at the modest decrease of retrieval performance.666E.g., experiments on Reddit test data reveal a speed-up using the approximate search method of Malkov:2016arxiv while retaining 95% top-30 recall.

Finally, in this work we rely on a simple strategy based on random negative examples. In future work, we plan to experiment with alternative (non-random) negative sampling strategies. For instance, inspired by prior work on semantic specialisation Mrkšić et al. (2017b) and parallel corpora mining Guo et al. (2018), difficult negative examples might comprise invalid responses that are semantically related to the correct response (measured by e.g. dot-product similarity).

2.2 Step 2: Target Domain Fine-Tuning

The second step concerns the application of the pretrained general Reddit model on target domains. We assume that we have the respective training and test sets of and in-domain input-response pairs for each of the domains, where and are considerably smaller than the number of Reddit training pairs. We test two general fine-tuning strategies, illustrated in Figure 3.

ft-direct directly continues where the Reddit pretraining stopped: it fine-tunes the model parameters by feeding the in-domain (input, response) pairs into the model and by following exactly the same training principle as described in §2.1. The fine-tuned model is then tested in the in-domain response selection task using test pairs, see Figure 2(b).

ft-mixed attempts to prevent the “specialisation” of the Reddit model to a single target domain, that is, it aims to maintain stable performance on the general-domain Reddit data. This way, the model can support multiple target tasks simultaneously. Instead of relying only on in-domain training pairs, we now perform in-batch mixing of Reddit pairs with in-domain pairs: % of the pairs in each batch during fine-tuning are Reddit pairs, while % of the pairs are in-domain pairs, where is a tunable hyper-parameter. With this fine-tuning strategy, outlined in Figure 2(c), each dataset provides negative examples for the other one, enriching the learning signal.

We compare ft-direct and ft-mixed against two straightforward and insightful baselines: the reddit-direct model from Figure 2(a) directly applies the pretrained Reddit model on the target task without any in-domain fine-tuning. Comparisons to this baseline reveal the importance of fine-tuning. On the other hand, the target-only baseline simply trains the response selection model from Figure 2 from scratch directly on the in-domain pairs. Comparisons to this baseline reveal the importance of Reddit pretraining. For all target-only models in all target tasks, we tuned the word embedding sizes and embedding dropout rates on the corresponding training sets.

3 Experimental Setup

Training Setup and Hyper-Parameters.

All input text is lower-cased and tokenised, numbers with 5 or more digits get their digits replaced by a wildcard symbol #, while words longer than 16 characters are replaced by a wildcard token LONGWORD. Sentence boundary tokens <S> and </S> are added to each sentence. The vocabulary consists of the unigrams that occur at least 10 times in a random 1M subset of the Reddit training set –this results in a total of 105K unigrams– plus the 200K most frequent bigrams in the same random subset.

The following training setup refers to the final Reddit model, illustrated in Figure 2, and used in fine-tuning. The model is trained by SGD setting the initial learning rate to 0.03, and then decaying the learning rate by 0.3x every 1M training steps after the first 2.5M steps. Similar to learning rate scaling by the batch size used in prior work Goyal et al. (2017); Codreanu et al. (2017), we scale the unigram and bigram embedding gradients by the batch size. The batch size is 500, and attention projection dimensionality is 64.

We also apply the label smoothing technique Szegedy et al. (2016)

, shown to reduce overfitting by preventing a network to assign full probability to the correct training example

Pereyra et al. (2017). Effectively, this reshapes Eq. (1): each positive training example in each batch gets assigned the probability of 0.8, while the remaining probability mass gets evenly redistributed across in-batch negative examples. Finally, we train the model on 13 GPU nodes with one Tesla K80 each for 18 hours: the model sees around 2B examples and it is sufficient for the model to reach convergence.777Training is relatively cheap compared to other large models: e.g., BERT models Devlin et al. (2019) were pre-trained for 4 days using 4 Cloud TPUs (BERT-SMALL) or 16 Cloud TPUs (BERT-LARGE). Fine-tuning is run by relying on early stopping on in-domain validation data. The ratio of Reddit and in-domain pairs with ft-mixed is set to 3:1 (in favour of Reddit) in all experimental runs.

Dataset Reference Domain Training Size Test Size
reddit Henderson et al. (2019) discussions on various topics 654,396,778 72,616,937
OpenSub Lison and Tiedemann (2016) movies, TV shows 283,651,561 33,240,156
amazonQA Wan and McAuley (2016) e-commerce, retail 3,316,905 373,007
ubuntu Lowe et al. (2017) computers, technical chats 3,954,134 72,763
banking New e-banking applications, banking FAQ 10,395 1,485
semeval15 Nakov et al. (2015) lifestyle, tourist and residential info 9,680 1,158
Table 1: Summary of all target domains and data. Data sizes: a total number of unique (input, response) pairs. Note that some datasets contain many-to-one pairings (i.e., multiple inputs are followed by the same response; banking) and one-to-many pairings (i.e., one input generates more than one plausible response; semeval15).

Test Domains and Datasets.

We conduct experiments on six target domains with different properties and varying corpora sizes. The diversity of evaluation probes the robustness of the proposed pretraining and fine-tuning regime. The summary of target domains and the corresponding data is provided in Table 1. All datasets are in the form of (input, response) pairs. For ubuntu888https://github.com/rkadlec/, semeval15999http://alt.qcri.org/semeval2015/task3/, and amazonQA101010http://jmcauley.ucsd.edu/data/amazon/qa/ we use standard data splits into training, dev, and test portions following the original work Lowe et al. (2017); Nakov et al. (2015); Wan and McAuley (2016). For the OpenSubtitles dataset (OpenSub) Lison and Tiedemann (2016), we rely on the data splits introduced by Henderson:2019arxiv. We evaluate pretrained Reddit models on the reddit held-out data: 50K randomly sampled (input, response) pairs are used for testing.

We have also created a new FAQ-style dataset in the e-banking domain which includes question-answer pairs divided into 77 unique categories with well-defined semantics (e.g., “card activation”, “closing account”, “refund request”). Such FAQ information can be found in various e-banking customer support pages, but the answers are highly hierarchical and often difficult to locate. Our goal is to test the fine-tuned encoder’s ability to select the relevant answers to the posed question. To this end, for each question we have collected 10 paraphrases that map to the same answer. All unique (question, answer) pairs are added to the final dataset, which is then divided into training (70%), validation (20%) and test portions (10%), see Table 1.

Baseline Models.

Besides the direct encoder model training on each target domain without pretraining (target-only), we also evaluate two standard IR baselines based on keyword matching: 1) a simple tf-idf query-response scoring Manning et al. (2008), and 2) Okapi bm25 Robertson and Zaragoza (2009).

Furthermore, we also analyse how pretraining plus fine-tuning for response selection compares to a representative sample of publicly available neural network embedding models which embed inputs and responses into a vector space. We include the following embedding models, all of which are readily available online.

111111https://www.tensorflow.org/hub (1) Universal Sentence Encoder of Cer:2018arxiv is trained using a transformer-style architecture Vaswani et al. (2017) on a variety of web sources such as Wikipedia, web news, discussion forums as well as on the Reddit data. We experiment with the base use model and its larger variant (use-large). (2) We run fixed mean-pooling of ELMo contextualised embeddings Peters et al. (2018) pretrained on the bidirectional LM task using the LM 1B words benchmark Chelba et al. (2013): elmo. (3) We also compare to two variants of the bidirectional transformer model of Devlin:2018arxiv (bert-small and bert-large).121212Note that the encoder architectures similar to the ones used by use can also be used in the Reddit pretraining phase in lieu of the architecture shown in Figure 2. However, the main goal is to establish the importance of target response selection fine-tuning by comparing it to direct application of state-of-the-art pretrained encoders, used to encode both input and responses in the target domain.

We compare to two model variants for each of the above vector-based baseline models. First, the sim method ranks responses according to their cosine similarity with the context vector: it relies solely on pretrained models without any further fine-tuning or adaptation, that is, it does not use the training set at all. The map variant learns a linear mapping on top of the response vector. The final score of a response with vector for an input with vector is the cosine similarity of the context vector with the mapped response vector:

(2)

are parameters learned on a random sample of 10,000 examples from the training set using the same dot product loss from Eq. (1), and

is the identity matrix. Vectors are

-normalised before being fed to the map method. For all baseline models, learning rate and regularization parameters are tuned using a held-out development set.

The combination of the two model variants with the vector-based models results in a total of 10 baseline methods, as listed in Table 3.

Evaluation Protocol.

We rely on a standard IR evaluation measure used in prior work on retrieval-based dialogue Lowe et al. (2017); Zhou et al. (2018); Chaudhuri et al. (2018): Recall@k. Given a set of responses to the given input/query, where only one response is relevant, it indicates whether the relevant response occurs in the top ranked candidate responses. We refer to this evaluation measure as , and set : . This effectively means that for each query, we indicate if the correct response is the top ranked response between 100 candidates. The final score is the average across all queries.

4 Results and Discussion

Full Reddit Model 61.3
- Wider hidden layers; , 24h training 61.1
- Narrower hidden layers; , 18h training 60.8
- Narrower hidden layers; 59.8
- Batch size 50 (before 500) 57.4
- (before 56.9
- tanh activation (before swish) 56.1
- no label smoothing 55.3
- no self-attention 48.7
- remove bigrams 35.5
Table 2: The results of different encoder configurations on the Reddit test data ( scores ). Starting from the full model (top row), each subsequent row shows a configuration with one component removed or edited from the configuration from the previous row.
Reddit OpenSub amazonQA ubuntu banking semeval15
tf-idf 26.7 10.9 51.8 27.5 27.3 38.0
bm25 27.6 10.9 52.3 19.6 23.4 35.5
use-sim 36.6 13.6 47.6 11.5 18.2 36.0
use-map 40.8 15.8 54.4 17.2 79.2 45.5
use-large-sim 41.4 14.9 51.3 13.6 27.3 44.0
use-large-map 47.7 18.0 61.9 18.5 81.8 56.5
elmo-sim 12.5 9.5 16.0 3.5 6.5 19.5
elmo-map 19.3 12.3 33.0 6.2 87.0 34.5
bert-small-sim 17.1 13.8 27.8 4.1 13.0 13.0
bert-small-map 24.5 17.5 45.8 9.0 77.9 37.5
bert-large-sim 14.8 12.2 25.9 3.6 10.4 10.0
bert-large-map 24.0 16.8 44.1 8.0 68.8 34.5
reddit-direct 61.3 19.1 61.4 9.6 27.3 46.0
target-only - 29.0 (18.2) 83.3 (11.6) 6.2 (2.3) 88.3 (1.2) 7.5 (1.1)
ft-direct - 30.6 (40.0) 84.2 (30.8) 38.7 (51.9) 94.8 (55.3) 52.5 (55.2)
ft-mixed - 25.5 (60.0) 77.0 (59.6) 38.1 (59.4) 90.9 (59.8) 56.5 (59.4)
Table 3: Summary of the results ( scores ) with fine-tuning on all six target domains. Datasets are ordered left to right based on their size. The scores in the parentheses in the target-only, ft-direct and ft-mixed rows give the performance on the general-domain reddit test data. The scores are computed with de-duplicated inputs for semeval15 (i.e., the initial dataset links more responses to the same input), and de-duplicated answers for banking.

This section aims to provide answers to the two main questions posed in §1: which encoder architectures are more suitable for pretraining (Q1; §4.1), and how to adapt/fine-tune the pretrained model to target tasks (Q2; §4.2).

4.1 Reddit Pretraining

The full encoder model is described in §2.1 and visualised in Figure 2. In what follows, we also analyse performance of other encoder configurations, which can be seen as ablated or varied versions of the full model. The results on the reddit response selection task are summarised in Table 2.

Results and Discussion.

The scores suggest that the final model gets contribution from its multiple components: e.g., replacing tanh with the recently proposed swish activation Ramachandran et al. (2017) is useful, and label smoothing also helps. Despite contradictory findings from prior work related to the batch size (e.g., compare Smith et al. (2017) and Masters and Luschi (2018)), we obtain better results with larger batches. This is intuitive given the model design: increasing the batch size in fact means learning from a larger number of negative examples. The results also suggest that the model saturates when provided with a sufficient number of parameters, as wider hidden layers and longer training times did not yield any substantial gains.

The scores also show the benefits of self-attention and positional embeddings instead of deep feed-forward averaging of the input unigram and bigram embeddings Iyyer et al. (2015). This is in line with prior work on sentence encoders Cer et al. (2018); Yang et al. (2018), which reports similar gains on several classification tasks. Finally, we observe a large gap with the unigram-only model variant, confirming the importance of implicitly representing underlying sequences with -grams Henderson et al. (2017); Mrkšić et al. (2017a). Following the results, we fix the pretraining model in all follow-up experiments (top row in Table 2).

(a) elmo-sim
(b) use-map
(c) reddit-direct (no fine-tuning)
(d) ft-mixed (with fine-tuning)
Figure 4: t-SNE plots van der Maaten and Hinton (2012) of encoded questions/inputs for a selection of 10 categories from the banking test set. The most coherent clusters for each category with well-defined semantics are observed with the ft-mixed fine-tuning model applied on top of Reddit response selection pretraining.

4.2 Target-Domain Fine-Tuning

Results and Discussion. The main results on all target tasks after fine-tuning are summarised in Table 3. First, the benefits of Reddit pretraining and fine-tuning are observed in all tasks regardless of the in-domain data size. We report large gains over the target-only model (which trains a domain-specific response selection encoder from scratch) especially for tasks with smaller training datasets (e.g., banking, semeval15). The low scores of target-only with smaller training data suggest overfitting: the encoder architecture cannot see enough training examples to learn to generalise. The gains are also present even when target-only gets to see much more in-domain input-response training data: e.g., we see slight improvements on OpenSub and amazonQA, and large gains on ubuntu when relying on the ft-direct fine-tuning variant.

What is more, a comparison to reddit-direct further suggests that fine-tuning even with a small amount of in-domain data can lead to large improvements: e.g., the gains over reddit-direct are +67.5% on banking, +32.5% on ubuntu, +22.8% on amazonQA, and +11.5% on OpenSub. These results lead to the following crucial conclusion: while in-domain data are insufficient to train response selection models from scratch for many target domains, such data are invaluable for adapting a pretrained general-domain model to the target domain. In other words, the results indicate that the synergy between the abundant response selection Reddit data and scarce in-domain data is effectively achieved through the proposed training regime, and both components are crucial for the final improved performance in each target domain. In simple words, this finding confirms the importance of fine-tuning for the response selection task.

Comparison to Baselines.

The results of tf-idf and bm25 reveal that lexical evidence from the preceding input can partially help in the response selection task and it achieves reasonable performance across the target tasks. For instance, on some tasks (e.g., amazonQA, banking), such keyword matching baselines even outperform some of the vector-based baseline models, and are comparable to the reddit-direct model variant. They are particularly strong for amazonQA and ubuntu, possibly because rare and technical words (e.g., the product name) are very informative in these domains. However, these baselines are substantially outperformed by the proposed fine-tuning approach across the board.

A comparison to other pretrained sentence encoders in Table 3 further stresses the importance of training for the response selection task in particular. Using off-the-shelf sentence encoders such as use or bert directly on in-domain sentences without distinguishing the input and the response space leads to degraded performance compared even to tf-idf, or the reddit-direct baseline without in-domain fine-tuning. The importance of learning the mapping from input to response versus simply relying on similarity is also exemplified by the comparison between the map method and the simple sim method: regardless of the actual absolute performance, map leads to substantial gains over sim for all vector-based baseline models. However, even the map method cannot match the performance of our two-step training regime: we report substantial gains with our ft-direct and ft-mixed fine-tuning on top of Reddit pretraining for all target domains but one (semeval15).

Further Discussion.

The comparison of two fine-tuning strategies suggests that the simpler ft-direct fine-tuning has an edge over ft-mixed, and it seems that the gap between ft-direct and ft-mixed is larger on bigger datasets. However, as expected, ft-direct adapts to the target task more aggressively: this leads to its degraded performance on the general-domain Reddit response selection task, see the scores in parentheses in Table 3. With more in-domain training data ft-direct becomes worse on the reddit test set. On the other hand, ft-mixed manages to maintain its high performance on reddit due to the in-batch mixing used in the fine-tuning process.131313Varying the parameter in ft-mixed from the ratio 3:1 to 1:3 leads only to slight variations in the final results.

Qualitative Analysis.

The effect of fine-tuning is also exemplified by t-SNE plots for the banking domain shown in Figure 4.141414For clarity, we show the plots with 10 (out of 77) selected categories, while the full plots with all 77 categories are available in the supplemental material. Recall that in our banking FAQ dataset several questions map to the same response, and ideally such questions should be clustered together in the semantic space. While we do not see such patterns at all with ELMo-encoded questions without mapping (elmo-sim, Figure 3(a)), such clusters can already be noticed with use-map (Figure 3(b)) and with the model pretrained on Reddit without fine-tuning (Figure 3(c)). However, fine-tuning yields the most coherent clusters by far: it attracts encodings of all similar questions related to the same category closer to each other in the semantic space. This is in line with the results reported in Table 3.

5 Related Work

Retrieval-Based Dialogue Systems. Retrieval-based systems (Yan et al., 2016; Bartl and Spanakis, 2017; Wu et al., 2017; Song et al., 2018; Weston et al., 2018, inter alia) provide less variable output than generative dialogue systems Wen et al. (2015, 2017a); Vinyals and Le (2015), but they offer a crucial advantage of producing more informative, semantically relevant, controllable, and grammatically correct responses Ji et al. (2014). Unlike modular and end-to-end task-oriented systems Young (2010); Wen et al. (2017b); Mrkšić and Vulić (2018); Li et al. (2018), they do not require expensive curated domain ontologies, and bypass the modelling of complex domain-specific decision-making policy modules Gašić et al. (2015); Chen et al. (2017). Despite these desirable properties, their potential has not been fully exploited in task-oriented dialogue.

Their fundamental building block is response selection Banchs and Li (2012); Wang et al. (2013); Al-Rfou et al. (2016); Baudis and Sedivý (2016). We have witnessed a recent rise of interest in neural architectures for modelling response selection Wu et al. (2017); Chaudhuri et al. (2018); Zhou et al. (2018); Tao et al. (2019), but the progress is still hindered by insufficient domain-specific training data El Asri et al. (2017); Budzianowski et al. (2018). While previous work typically focused on a single domain (e.g., Ubuntu technical chats Lowe et al. (2015, 2017)), in this work we show that much larger general-domain Reddit data can be leveraged to pretrain response selection models that support more specialised target dialogue domains.

To the best of our knowledge, the work of Henderson:2017arxiv and Yang:2018repl is closest to our response selection pretraining introduced in §2.1. However, Henderson:2017arxiv optimise their model for one single task: replying to e-mails with short messages Kannan et al. (2016). They use a simpler feed-forward encoder architecture and do not consider wide portability of a single general-domain response selection model to diverse target domains through fine-tuning. Yang:2018repl use Reddit conversational context to simply probe semantic similarity of sentences Agirre et al. (2012, 2013); Nakov et al. (2016), but they also do not investigate response selection fine-tuning across diverse target domains.

Pretraining and Fine-Tuning.

Task-specific fine-tuning of language models (LMs) pretrained on large unsupervised corpora Peters et al. (2018); Devlin et al. (2019); Howard and Ruder (2018); Radford et al. (2018); Lample and Conneau (2019); Liu et al. (2019) has taken NLP by storm. Such LM-based pretrained models support a variety of NLP tasks, ranging from syntactic parsing to natural language inference Peters et al. (2018); Devlin et al. (2019), as well as machine reading comprehension Nishida et al. (2018); Xu et al. (2019) and information retrieval tasks Nogueira and Cho (2019); Yang et al. (2019). In this work, instead of the LM-based pretraining, we put focus on the response selection pretraining in particular, and show that such models coupled with target task fine-tuning Howard and Ruder (2018) lead to improved modelling of conversational data in various domains.

6 Conclusion and Future Work

We have presented a novel method for training neural response selection models for task-oriented dialogue systems. The proposed training procedure overcomes the low-data regime of task-oriented dialogue by pretraining the response selection model using general-domain conversational Reddit data and efficiently adapting this model to individual dialogue domains using in-domain data. Our evaluation demonstrates the compelling benefits of such pretraining, with the proposed training procedure achieving strong performance across each of the five different dialogue domains. In future work, we will port this approach to additional target domains, other languages, and investigate more sophisticated encoder architectures and fine-tuning strategies.

References