This paper proposes dynamic chunk reader (DCR), an end-to-end neural reading comprehension (RC) model that is able to extract and rank a set of answer candidates from a given document to answer questions. DCR is able to predict answers of variable lengths, whereas previous neural RC models primarily focused on predicting single tokens or entities. DCR encodes a document and an input question with recurrent neural networks, and then applies a word-by-word attention mechanism to acquire question-aware representations for the document, followed by the generation of chunk representations and a ranking module to propose the topranked chunk as the answer. Experimental results show that DCR could achieve a 66.3% Exact match and 74.7% F1 score on the Stanford Question Answering Dataset (Rajpurkar et al., 2016).
1 INTRODUCTION
Reading comprehension-based question answering (RCQA) is the task of answering a question with a chunk of text taken from related document(s). A variety of neural models have been proposed recently either for extracting a single entity or a single token as an answer from a given text (Hermann et al., 2015; Kadlec et al., 2016; Trischler et al., 2016b; Dhingra et al., 2016; Chen et al., 2016; Sordoni et al., 2016; Cui et al., 2016a); or for selecting the correct answer by ranking a small set of human-provided candidates (Yin et al., 2016; Trischler et al., 2016a). In both cases, an answer boundary is either easy to determine or already given.
Different from the above two assumptions for RCQA, in the real-world QA scenario, people may ask questions about both entities (factoid) and non-entities such as explanations and reasons (nonfactoid) (see Table 1 for examples).
In this regard, RCQA has the potential to complement other QA approaches that leverage structured data (e.g., knowledge bases) for both the above question types. This is because RCQA can exploit the textual evidences to ensure increased answer coverage, which is particularly helpful for nonfactoid answers. However, it is also challenging for RCQA to identify answer in arbitrary position in the passage with arbitrary length, especially for non-factoid answers which might be clauses or sentences.
As a result, apart from a few exceptions (Rajpurkar et al., 2016; Wang & Jiang, 2016), this research direction has not been fully explored yet.
Compared to the relatively easier RC task of predicting single tokens/entities1, predicting answers of arbitrary lengths and positions significantly increase the search space complexity:
the number of possible candidates to consider is in the order of O(n2), where n is the number of passage words. In contrast, for previous works in which answers are single tokens/entities or from candidate lists, the complexity is in O(n) or the size of candidate lists l (usually l ≤5), respectively. To address the above complexity, Rajpurkar et al. (Rajpurkar et al., 2016) used a two-step chunkand-rank approach that employs a rule-based algorithm to extract answer candidates from a passage,
∗Both authors contribute equally 1State-of-the-art RC models have a decent accuracy of ∼70% on the widely used CNN/DailyMail dataset
(Hermann et al., 2015).
followed by a ranking approach with hand-crafted features to select the best answer. The rule-based chunking approach suffered from low coverage (≈ 70% recall of answer chunks) that cannot be improved during training; and candidate ranking performance depends greatly on the quality of the hand-crafted features. More recently, Wang and Jiang (Wang & Jiang, 2016) proposed two end-toend neural network models, one of which chunks a candidate answer by predicting the answer’s two boundary indices and the other classifies each passage word into answer/not-answer. Both models improved significantly over the method proposed by Rajpurkar et al. (Rajpurkar et al., 2016).
Our proposed model, called dynamic chunk reader (DCR), not only significantly differs from both the above systems in the way that answer candidates are generated and ranked, but also shares merits with both works. First, our model uses deep networks to learn better representations for candidate answer chunks, instead of using fixed feature representations as in (Rajpurkar et al., 2016). Second, it represents answer candidates as chunks, as in (Rajpurkar et al., 2016), instead of wordlevel representations (Wang & Jiang, 2016), to make the model aware of the subtle differences among candidates (importantly, overlapping candidates).
The contributions of this paper are three-fold. (1) We propose a novel neural network model for joint candidate answer chunking and ranking, where the candidate answer chunks are dynamically constructed and ranked in an end-to-end manner. (2) we propose a new question-attention mechanism to enhance passage word representation, which is subsequently used to construct chunk representations. (3) We also propose several simple but effective features to strengthen the attention mechanism, which fundamentally improves candidate ranking, with the by-product of higher exact boundary match accuracy.
The experiments on the Stanford Question Answering Dataset (SQuAD) (Rajpurkar et al., 2016), which contains a variety of human-generated factoid and non-factoid questions, have shown the effectiveness of above three contributions.
Our paper is organized as follows. We formally define the RCQA problem first. Next, we describe our baseline with a neural network component. We present the end-to-end dynamic chunk reader model next. Finally, we analyze our experimental results and discuss the related work. In appendix, we show formal equations and details of the model.
2 PROBLEM DEFINITION
Table 1 shows an example of our RC setting where the goal is to answer a question Qi, factoid (Q1) or non-factoid (Q2 and Q3), based on a supporting passage Pi, by selecting a continuous sequence of text Ai ⊆ Pi as answer. Qi, Pi, and Ai are all word sequences, where each word is drawn from a vocabulary, V . The i-th instance in the training set is a triple in the form of (Pi, Qi, Ai), where Pi = (pi1, . . . , pi|Pi|), Qi = (qi1, . . . , qi|Qi|), and Ai = (ai1, . . . , ai|Ai|) (pi·, qi·, ai· ∈ V ). Owing to the disagreement among annotators, there could be more than one correct answer for the same question; and the k-th answer to Qi is denoted by Aki = {aki1, . . . , aki|Aki |}. An answer candidate for the i-th training example is defined as cm,ni , a sub-sequence in Pi, that spans from position m to n (1 ≤ m ≤ n ≤ |Pi|). The ground truth answer Ai could be included in the set of all candidates
Ci = {cm,ni |∀m,n ∈ N+, subj(m,n, Pi) and 1 ≤ m ≤ n ≤ |Pi|}, where subj(m,n, Pi) is the constraint put on the candidate chunk for Pi, such as, “c m,n i can have at most 10 tokens”, or “cm,ni must have a pre-defined POS pattern”. To evaluate a system’s performance, its top answer to a question is matched against the corresponding gold standard answer(s).
Remark: Categories of RC Tasks Other simpler variants of the aforementioned RC task were explored in the past. For example, quiz-style datasets (e.g., MCTest (Richardson et al., 2013), MovieQA (Tapaswi et al., 2015)) have multiple-choice questions with answer options. Cloze-style datesets(Hermann et al., 2015; Hill et al., 2015; Onishi et al., 2016), usually automatically generated, have factoid “question”s created by replacing the answer in a sentence from the text with blank. For the answer selection task this paper focuses on, several datasets exist, e.g. TREC-QA for factoid answer extraction from multiple given passages, bAbI (Weston et al., 2014) designed for inference purpose, and the SQuAD dataset (Rajpurkar et al., 2016) used in this paper. To the best of our knowledge, the SQuAD dataset is the only one for both factoid and non-factoid answer extraction with a question distribution more close to real-world applications.
3 BASELINE: CHUNK-AND-RANK PIPELINE WITH NEURAL RC
In this section we modified a state-of-the-art RC system for cloze-style tasks for our answer extraction purpose, to see how much gap we have for the two type of tasks, and to inspire our end-to-end system in the next section. In order to make the cloze-style RC system to make chunk-level decision, we use the RC model to generate features for chunks, which are further used in a feature-based ranker like in (Rajpurkar et al., 2016). As a result, this baseline can be viewed as a deep learning based counterpart of the system in (Rajpurkar et al., 2016). It has two main components: 1) a standalone answer chunker, which is trained to produce overlapping candidate chunks, and 2) a neural RC model, which is used to score each word in a given passage to be used thereafter for generating chunk scores.
Answer Chunking To reduce the errors generated by the rule-based chunker in (Rajpurkar et al., 2016), first, we capture the part-of-speech (POS) pattern of all answer sub-sequences in the training dataset to form a POS pattern trie tree, and then apply the answer POS patterns to passage Pi to acquire a collection of all subsequences (chunk candidates) Ci whose POS patterns can be matched to the POS pattern trie. This is equivalent to putting an constraint subj(m,n, Pi) to candidate answer chunk generation process that only choose the chunk with a POS pattern seen for answers in the training data. Then the sub-sequences Ci are used as answer candidates for Pi. Note that overlapping chunks could be generated for a passage, and we rely on the ranker to choose the best candidate based on features from the cloze-style RC system. Experiments showed that for > 90% of the questions on the development set, the ground truth answer is included in the candidate set constructed in such manner.
Feature Extraction and Ranking For chunk ranking, we (1) use neural RCQA model to annotate each pij in passage Pi to get score sij , then (2) for every chunk c m,n i in passage i, collect scores (sim, . . . , sin) for all the (pim, ..., pin) contained within c m,n i , and (3) extract features on the sequence of scores (sim, . . . , sin) to characterize its scale and distribution information, which serves as the feature representation of cm,ni . In step (1) to acquire sij we train and apply a word-level single-layer Gated Attention Reader 2 (Dhingra et al., 2016), which has state-of-the-art performance on CNN/DailyMail cloze-style RC task. In step (3) for chunk cm,ni , we designed 5 features, including 4 statistics on (sim, . . . , sin): maximum, minimum, average and sum; as well as the count of matched POS pattern within the chunk, which serves as an answer prior. We use these 5 features in a state-of-the-art ranker (Ganjisaffar et al., 2011).
4 DYNAMIC CHUNK READER
The dynamic chunk reader (DCR) model is presented in Figure 1. Inspired by the baseline we built, DCR is deemed to be superior to the baseline for 3 reasons. First, each chunk has a representation constructed dynamically, instead of having a set of pre-defined feature values. Second, each passage
2We tried using more than one layers in Gated Attention Reader, but no improvement was observed.
word’s representation is enhanced by word-by-word attention that evaluates the relevance of the passage word to the question. Third, these components are all within a single, end-to-end model that can be trained in a joint manner.
DCR works in four steps. First, the encoder layer encodes passage and question separately, by using bidirectional recurrent neural networks (RNN).
Second, the attention layer calculates the relevance of each passage word to the question.
Third, the convolution layer generates unigram, bigram and trigram representation for each word. bigram and trigram of a word ends with the same word, and proper padding is applied on the first word to make sure the output is the same length as input to CNN layer.
Fourth, the chunk representation layer dynamically extracts the candidate chunks from the given passage, and create chunk representation that encodes the contextual information of each chunk.
Fifth, the ranker layer scores the relevance between the representations of a chunk and the given question, and ranks all candidate chunks using a softmax layer.
We describe each step below.
Encoder Layer We use bi-directional RNN encoder to encode Pi and Qi of example i, and get hidden state for each word position pij and qik.3 As RNN input, a word is represented by a row vector x ∈ Rn. x can be the concatenation of word embedding and word features (see Fig. 1). The word vector for the t-th word is xt. A word sequence is processed using an RNN encoder with gated recurrent units (GRU) (Cho et al., 2014), which was proved to be effective in RC and neural machine translation tasks (Bahdanau et al., 2015; Kadlec et al., 2016; Dhingra et al., 2016). For each position t, GRU computes ht with input xt and previous state ht−1, as:
3We can have separated parameters for question and passage encoders but a single shared encoder for both works better in the experiments.
rt = σ(Wrxt + Urht−1) (1) ut = σ(Wuxt + Uuht−1) (2) h̄t = tanh(Wxt + U(rt ht−1)) (3) ht = (1− ut) · ht−1 + ut · h̄t (4)
where ht, rt, and ut ∈ Rd are d-dimensional hidden state, reset gate, and update gate, respectively; W{r,u}, W ∈ Rn×d and U{r,u}, U ∈ Rd×d are the parameters of the GRU; σ is the sigmoid function, and denotes element-wise production. For a word at t, we use the hidden state −→h t from the forward RNN as a representation of the preceding context, and the←−h t from a backward RNN that encodes text reversely, to incorporate the context after t. Next, ht = [ −→ ht ; ←− ht ], the bi-directional contextual encoding of xt, is formed. [·; ·] is the concatenation operator. To distinguish hidden states from different sources, we denote the hj of j-th word in P and the hk of k-th word in Q as h p j and hqk respectively.
Attention Layer Attention mechanism in previous RC tasks (Kadlec et al., 2016; Hermann et al., 2015; Sordoni et al., 2016; Dhingra et al., 2016; Cui et al., 2016a;b) enables question-aware passage representations. We propose a novel attention mechanism inspired by word-by-word style attention methods (Rocktäschel et al., 2015; Wang & Jiang, 2015; Santos et al., 2016). For each pj , a questionattended representation vj is computed as follows (example index i is omitted for simplicity):
αjk = h p j · h q k, (5)
βj = |Q|∑ k=1 αjkh q k (6) vj = [h p j ;βj ] (7)
where hpj and h q k are hidden states from the bi-directional RNN encoders (see Figure 1). An inner product, αjk, is calculated between h p j and every question word h q k. It indicates how well the passage word pj matches with every question word qk. βj is a weighted pooling of |Q| question hidden states, which serves as a pj-aware question representation. The concatenation of h p j and βj leads to a passage-question joint representation, vj ∈ R4d.4 Next, we apply a second bi-GRU layer taking the vjs as inputs, and obtain forward and backward representations −→γj and←−γj ∈ Rd, and in turn their concatenation, γj = [−→γj ;←−γj ]. Convolution Layer Every word is encoded with complete passage context through attention layer RNN. We would like to model more complex representation of the words, by introducing unigram, bigram and trigram representations. There are two benefits for this enhanced representation: 1) each word could be enhanced with local context information to help identify the boundary of the answer chunk. Using previous words has been a common feature used in POS tagging and Named entity recognition; and 2) The information brought in by the ngram into the word representation could enhance the semantic match between the answer chunk internal and the question. Imagine scenario of a three word candidate, where the last word representation includes the two previous words through the convolution layer. Matching to the last word could also lead to the match to the semantics of the internal of the chunk. Specifically, we create for every word position j three representations, by using ngrams ending with the hidden state j:
γ̃j1 = γj ·Wc1 (8) γ̃j2 = [γj−1; γj ] ·Wc2 (9) γ̃j3 = [γj−2; γj−1; γj ] ·Wc3 (10)
4We tried another word-by-word attention methods as in (Santos et al., 2016), which has similar passage representation input to question side. However, this does not lead to improvement due to the confusion caused by long passages in RC. Consequently, we used the proposed simplified version of word-by-word attention on passage side only.
The details shown in equations above. We used three different convolution kernels for different n-grams.
Chunk Representation Layer A candidate answer chunk representation is dynamically created given convolution layer output. We first decide the text boundary for the candidate chunk, and then form a chunk representation using all or part of those γj outputs inside the chunk. To decide a candidate chunk (boundary): we tried two ways: (1) adopt the POS trie-based approach used in our baseline, and (2) enumerate all possible chunks up to a maximum number of tokens. For (2), we create up to N (max chunk length) chunks starting from any position j in Pj . Approach (1) can generate candidates with arbitrary lengths, but fails to recall candidates whose POS pattern is unseen in training set; whereas approach (2) considers all possible candidates within a window and is more flexible, but over-generates invalid candidates.
For a candidate answer chunk cm,n spanning from position m to n inclusively, we construct chunk representation γlm,n ∈ R2d using every γ̃jl within range [m,n], with a function g(·), and l ∈ {1, 2, 3}. Formally,
γlm,n = g(γ̃ml, . . . , γ̃nl)
Each γ̃jl is a convolution output over concatenated forward and backward RNN hidden states from attention layer. So the first half in γ̃jl encodes information in forward RNN hidden states and the second half encodes information in backward RNN hidden states. We experimented with several pooling functions (e.g., max, average) for g(·), and found out that, instead of pooling, the best g(·) function is to concatenate the first half of convolution output of the chunk’s first word and the second half of convolution output of the chunk’s last word. Formally,
γlm,n = g(γ̃ml, . . . , γ̃nl) = [ −→ γ̃ml; ←− γ̃nl] (11)
where −→γ̃ml is half of the hidden state for l-gram word representation corresponding to forward attention RNN output. We hypothesize that the hidden states at that two ends can better represent the chunk’s contexts, which is critical for this task, than the states within the chunk. This observation also agrees with (Kobayashi et al., 2016).
Ranker Layer A score slm,n for each l-gram chunk representation γlm,n denoting the probability of that chunk to be the true answer is calculated by dot product with question representation. The question representation is the concatenation of the last hidden state in forward RNN and the first hidden state in backward RNN. Formally for the chunk cm,ni we have
sl(cm,ni |Pi, Qi) = γ l m,n · [ −−→ hQi|Qi|; ←−− hQi1 ] (12)
where sl denotes the score generated from l-gram representation. −−→ hQik or ←−− hQik is the k-th hidden state output from question Qi’s forward and backward RNN encoder, respectively.
After that, the final score for cm,ni is evaluated as the linear combination of three scores, followed by a softmax:
s(cm,ni |Pi, Qi) = softmax(W · [s 1; s2; s3]) (13)
where sl is the shorthand notation for sl(cm,ni |Pi, Qi); W ∈ R3. In runtime, the chunk with the highest probability is taken as the answer. In training, the following negative log likelihood is minimized:
L = − N∑ i=1 logP(Ai|Pi, Qi) (14)
Note that the i-th training instance is only used when Ai is included in the corresponding candidate chunk set Ci, i.e. ∃m,nAi = cm,ni . The softmax in the final layer serves as the list-wise ranking module similar in spirit to (Cao et al., 2007).
5 EXPERIMENTS
Dataset We used the Stanford Question Answering Dataset (SQuAD) (Rajpurkar et al., 2016) for the experiment. SQuAD came into our sight because it is a mix of factoid and non-factoid
questions, a real-world data (crowd-sourced), and of large scale (over 100K question-answer pairs collected from 536 Wikipedia articles). Answers range from single words to long, variable-length phrase/clauses. It is a relaxation of assumptions by the cloze-style and quiz-style RC datasets in the Problem Definition section.
Features The input vector representation of each word w to encoder RNNs has six parts including a pre-trained 300-dimensional GloVe embedding (Pennington et al., 2014) and five features (see Figure 1): (1) a one-hot encoding (46 dimensions) for the part-of-speech (POS) tag of w; (2) a one-hot encoding (14 dimensions) for named entity (NE) tag of w; (3) a binary value indicating whether w’s surface form is the same to any word in the quesiton; (4) if the lemma form of w is the same to any word in the question; and (5) if w is caplitalized. Feature (3) and (4) are designed to help the model align the passage text with question. Note that some types of questions (e.g., “who”, “when” questions) have answers that have a specific POS/NE tag pattern. For instance, “who” questions mostly have proper nouns/persons as answers and “when” questions may frequently have numbers/dates (e.g., a year) as answers. Thus, we believe that the model could exploit the co-relation between question types and answer POS/NE patterns easier with POS and NE tag features. Implementation Details We pre-processed the SQuAD dataset using Stanford CoreNLP tool5 (Manning et al., 2014) with its default setting to tokenize the text and obtain the POS and NE annotations. To train our model, we used stochastic gradient descent with the ADAM optimizer (Kingma & Ba, 2014), with an initial learning rate of 0.001. All GRU weights were initialized from a uniform distribution between (-0.01, 0.01). The hidden state size, d, was set to 300 for all GRUs. The question bi-GRU shared parameters with the passage bi-GRU, while the attention-based passage bi-GRU had its own parameters. We shuffled all training examples at the beginning of each epoch and adopted a curriculum learning approach (Bengio et al., 2009), by sorting training instances by length in every 10 batches, to enable the model start learning from relatively easier instances and to harder ones. We also applied dropout of rate 0.2 to the embedding layer of input bi-GRU encoder, and gradient clipping when the norm of gradients exceeded 10. We trained in mini-batch style (mini-batch size is 180) and applied zero-padding to the passage and question inputs in each batch. We also set the maximum passage length to be 300 tokens, and pruned all the tokens after the 300-th token in the training set to save memory and speed up the training process. This step reduced the training set size by about 1.6%. During test, we test on the full length of passage, so that we don’t prune out the potential candidates. We trained the model for at most 30 epochs, and in case the accuracy did not improve for 10 epochs, we stopped training.
For the feature ranking-based system, we used jforest ranker (Ganjisaffar et al., 2011) with LambdaMART-RegressionTree algorithm and the ranking metric was NDCG@10. For the Gated Attention Reader in baseline system, we replicated the method and use the same configurations as in (Dhingra et al., 2016).
Results
Table 2 shows our main results on the SQuAD dataset. Compared to the scores reported in (Wang & Jiang, 2016), our exact match (EM) and F1 on the development set and EM score on the test set are better, and F1 on the test set is comparable. We also studied how each component in our model contributes to the overall performance. Table 3 shows the details as well as the results of the baseline ranker. As the first row of Table 3 shows, our baseline system improves 10% (EM) over Rajpurkar et al. (Rajpurkar et al., 2016) (Table 2, row 1), the feature-based ranking system. However when compared to our DCR model (Table 3, row 2), the baseline (row 1) is more than 12% (EM) behind
5 stanfordnlp.github.io/CoreNLP/
We also did ablation tests on our DCR model. First, replacing the word-by-word attention with Attentive Reader style attention (Hermann et al., 2015) decreases the EM score by about 4.5%, showing the strength of our proposed attention mechanism.
Second, we remove the features in input to see the contribution of each feature. The result shows that POS feature (1) and question-word feature (3) are the two most important features.
Finally, combining the DCR model with the proposed POS-trie constraints yields a score similar to the one obtained using the DCR model with all possible n-gram chunks. The result shows that (1) our chunk representations are powerful enough to differentiate even a huge amount of chunks when no constraints are applied; and (2) the proposed POS-trie reduces the search space at the cost of a small drop in performance.
Analysis To better understand our system, we calculated the accuracy of the attention mechanism of the gated attention reader used in our deep learning-based baseline. We found that it is 72% accurate i.e., 72% of the times a word with the highest attention score is inside the correct answer span. This means that, if we could accurately detect the boundary around the word with the highest attention score to form the answer span, we could achieve an accuracy close to 72%. In addition, we checked the answer recall of our candidate chunking approach. When we use a window size of 10, 92% of the time, the ground truth answer will be included in the extracted Candidate chunk set. Thus the upper bound of the exact match score of our baseline system is around 66% (92% (the answer recall) × 72%). From the results, we see our DCR system’s exact match score is at 62%. This shows that DCR is proficient at differentiating answer spans dynamically.
To further analyze the system’s performance while predicting answers of different lengths, we show the exact match (EM) and F1 scores for answers with lengths up to 10 tokens in Figure 2(a). From the graph, we can see that, with the increase of answer length, both EM and F1 drops, but in different speed. The gap between F1 and exact match also widens as answer length increases. However, the model still yields a decent accuracy when the answer is longer than a single word. Additionally, Figure 2(b) shows that the system is better at “when” and “who” questions, but performs poorly
on “why” questions. The large gap between exact match and F1 on “why” questions means that perfectly identifying the span is harder than locating the core of the answer span.
Since “what”, “which”, and “how” questions contain a broad range of question types, we split them further based on the bigram a question starts with, and Figure 3 shows the breakdown for “what” questions. We can see that “what” questions asking for explanations such as “what happens” and “what happened” have lower EM and F1 scores. In contrast, “what” questions asking for year and numbers have much higher scores and, for these questions, exact match scores are close to F1 scores, which means chunking for these questions are easier for DCR.
6 RELATED WORK
Attentive Reader was the first neural model for factoid RCQA (Hermann et al., 2015). It uses Bidirectional RNN (Cho et al., 2014; Chung et al.,2014) to encode document and query respectively, and use query representation to match with every token from the document. Attention Sum Reader (Kadlec et al., 2016) simplifies the model to just predicting positions of correct answer in the document and the training speed and test accuracy are both greatly improved on the CNN/Daily Mail dataset. (Chen et al., 2016) also simplified Attentive Reader and reported higher accuracy. Windowbased Memory Networks (MemN2N) is introduced along with the CBT dataset (Hill et al., 2015), which does not use RNN encoders, but embeds contexts as memory and matches questions with embedded contexts. Those models’ mechanism is to learn the match between answer context with question/query representation. In contrast, memory enhanced neural networks like Neural Turing Machines (Graves et al., 2014) and its variants (Zhang et al., 2015; Gulcehre et al., 2016; Zaremba & Sutskever, 2015; Chandar et al., 2016; Grefenstette et al., 2015) were also potential candidates for the task, and Gulcehre et al. (Gulcehre et al., 2016) reported results on the bAbI task, which is worse than memory networks. Similarly, sequence-to-sequence models were also used (Yu et al., 2015; Hermann et al., 2015), but they did not yield better results either.
Recently, several models have been proposed to enable more complex inference for RC task. For instance, gated attention model (Dhingra et al., 2016) employs a multi-layer architecture, where each layer encodes the same document, but the attention is updated from layer to layer. EpiReader (Trischler et al., 2016b) adopted a joint training model for answer extractor and reasoner, where the extractor proposes top candidates, and the reasoner weighs each candidate by examining entailment relationship between question-answer representation and the document. An iterative alternating attention mechanism and gating strategies were proposed in (Sordoni et al., 2016) to optimize the attention through several hops. In contrast, Cui et al. (Cui et al., 2016a;b) introduced fine-grained document attention from each question word and then aggregated those attentions from each question token by summation with or without weights. This system achieved the state-of-the-art score on the CNN dataset. Those different variations all result in roughly 3-5% improvement over attention sum reader, but none of those could achieve higher than that. Other methods include using dynamic entity representation with max-pooling (Kobayashi et al., 2016) that aims to change entity representation with context, and Weissenborn’s (Weissenborn, 2016) system, which tries to separate entity from the context and then matches the question to context, scoring an accuracy around 70% on the CNN dataset.
However, all of those models assume that the answers are single tokens. This limits the type of questions the models can answer. Wang and Jiang (Wang & Jiang, 2016) proposed a match-lstm and achieved good results on SQuAD. However, this approach predicts a chunk boundary or whether a word is part of a chunk or not. In contrast, our approach explicitly constructs the chunk representations and similar chunks are compared directly to determine correct answer boundaries.
7 CONCLUSION
In this paper we proposed a novel neural reading comprehension model for question answering. Different from the previously proposed models for factoid RCQA, the proposed model, dynamic chunk reader, is not restricted to predicting a single named entity as an answer or selecting an answer from a small, pre-defined candidate list. Instead, it is capable of answering both factoid and nonfactoid questions as it learns to select answer chunks that are suitable for an input question. DCR achieves this goal with a joint deep learning model enhanced with a novel attention mechanism and five simple yet effective features. Error analysis shows that the DCR model achieves good performance, but still needs to improve on predicting longer answers, which are usually non-factoid in nature.
SUMMARY.
The paper propose a reading-comprehension question answering system for the recent QA task where answers of a question can be either single tokens or spans in the given text passage.
The model first encodes the passage and the query using a recurrent neural network.
With an attention mechanism the model calculates the importance of each word on the passage with respect to each word in the question.
The encoded words in the passage are concatenated with the attention; the resulting vector is re-encoded with a further RNN.
Three convolutional neural networks with different filter size (1,2,3-gram) are used to further capture local features.
Candidate answers are selected either matching POS patterns of answers in the training set or choosing all possible text span until a certain length.
Each candidate answer has three representations, one for each n-gram representation. The compatibility of these representation with the question representation is then calculated.
The scores are combined linearly and used for calculating the probability of the candidate answer being the right answer for the question.
The method is tested on the SQUAD dataset and outperforms the proposed baselines.
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OVERALL JUDGMENT
The method presented in this paper is interesting but not very motivated in some points.
For example, it is not explained why in the attention mechanism it is beneficial to concatenate the original passage encoding with the attention-weighted ones.
The contributions of the paper are moderately novel proposing mainly the attention mechanism and the convolutional re-encoding.
In fact, combining questions and passages and score their compatibility has became a fairly standard procedure in all QA models.
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DETAILED COMMENTS
Equation (13) i should be s, not s^l.
I still do not understand the sentence " the best function is to concatenate the hidden stat of the fist word in a chunk in forward RNN and that of the last word in backward RNN". The RNN is over what all the words in the chunk? in the passage? 
The answer the authors gave in the response does not clarify this point.