Identifying Well-formed Natural Language Questions

08/28/2018 ∙ by Manaal Faruqui, et al. ∙ 0

Understanding search queries is a hard problem as it involves dealing with "word salad" text ubiquitously issued by users. However, if a query resembles a well-formed question, a natural language processing pipeline is able to perform more accurate interpretation, thus reducing downstream compounding errors. Hence, identifying whether or not a query is well formed can enhance query understanding. Here, we introduce a new task of identifying a well-formed natural language question. We construct and release a dataset of 25,100 publicly available questions classified into well-formed and non-wellformed categories and report an accuracy of 70.7 our classifier can be used to improve the performance of neural sequence-to-sequence models for generating questions for reading comprehension.

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1 Introduction

User issued search queries often do not follow formal grammatical structure, and require specialized language processing Bergsma and Wang (2007); Barr et al. (2008); Manshadi and Li (2009); Mishra et al. (2011). Traditional natural language processing (NLP) tools trained on formal text (e.g. treebanks) often have difficulty analyzing search queries; the lack of regularity in the structure of queries makes it difficult to train models that can optimally process the query to extract information that can help understand the user intent behind the query Baeza-Yates et al. (2006).

One clear direction to improve query processing is to annotate a large number of queries with the desired annotation scheme. However, such an annotation can be prohibitively expensive and models trained on such queries might suffer from freshness issues, as the domain and nature of queries evolve frequently Markatos (2001); Bawa et al. (2003); Roy et al. (2012). Another direction is to obtain a paraphrase of the given query that is a grammatical natural language question, and then analyze that paraphrase to extract the required information Nogueira and Cho (2017); Buck et al. (2018). There are available tools and datasets, such as Quora question paraphrases and the Paralex dataset Fader et al. (2013) – for identifying query paraphrases Wang et al. (2017); Tomar et al. (2017), but these datasets do not contain information about whether a query is a natural language question or not. Identifying well-formed natural language questions can also facilitate a more natural interaction between a user and a machine in personal assistants or chatbots Yang et al. (2014); Mostafazadeh et al. (2016) or while recommending related queries in search-engines.

Identifying a well-formed question should be easy by parsing with a grammar, such as the English resource grammar Copestake and Flickinger (2000)

, but such grammars are highly precise and fail to parse more than half of web queries. Thus, in this paper we present a model to predict whether a given query is a well-formed natural language question. We construct and publicly release a dataset of 25,100 queries annotated with the probability of being a well-formed natural language question (§

2.1

). We then train a feed-forward neural network classifier that uses the lexical and syntactic features extracted from the query on this data (§

2.2). On a test set of 3,850 queries, we report an accuracy of 70.1% on the binary classification task. We also demonstrate that such a query well-formedness classifier can be used to improve the quality of a sequence-to-sequence question generation model Du et al. (2017) by showing an improvement of 0.2 BLEU score in its performance (§3). Our dataset ise available for download at http://goo.gl/language/query-wellformedness.

Query Well-formed? Reasoning
what is the breed of scooby doo? 1 Grammatical and an explicit question
tell me what’s the breed of scooby doo? 0 A command but not a question
headache evenings? 0 Ungrammatical and not a question
what causes headaches during evenings 1 Grammatical and an explicit question
what 12.5 as a fraction? 0 An explicit question but ungrammatical
Table 1: Examples of well-formed and non-wellformed queries according to the annotation guideline.

2 Well-formed Natural Language Question Classifier

In this section we describe the data annotation, and the models used for question well-formedness classification.

2.1 Dataset Construction

We use the Paralex corpus Fader et al. (2013) that contains pairs of noisy paraphrase questions. These questions were issued by users in WikiAnswers (a Question-Answer forum) and consist of both web-search query like constructs (“5 parts of chloroplast?”) and well-formed questions (“What is the punishment for grand theft?”), and thus is a good resource for constructing the question well-formedness dataset. We select 25,100 queries from the unique list of queries extracted from the corpus such that no two queries in the selected set are paraphrases. The queries are then annotated into well-formed or non-wellformed questions. We define a query to be a well-formed natural language question if it satisfies the following:

  1. Query is grammatical.

  2. Query is an explicit question.

  3. Query does not contain spelling errors.

Table 1 shows some examples that were shown to the annotators to illustrate each of the above conditions. Every query was labeled by five different crowdworkers with a binary label indicating whether a query is well-formed or not. We average the ratings of the five annotators to get the probability of a query being well-formed. Table 2 shows some queries with obtained human annotation. Humans are pretty good at identifying an implicit query (“Population of owls…”) or a simple well-formed question (“What is released…”), but may miss out on subtle spelling mistakes like “disscovered” or disagree on whether the determiner “the” is needed before the word “genocide” (“What countries have genocide happened in?”). Similar to other NLP tasks like entailment Dagan et al. (2006); Bowman et al. (2015), paraphrasing Wieting et al. (2015) etc. we rely on the wisdom of the crowd to get such annotations in order to make the data collection scalable and language-independent.

Query ()
population of owls just in north america? 0.0
who disscoverd rihanna? 0.2
what countries have genocide happened in? 0.6
what is released when an ion is formed? 1.0
Table 2: Examples of human annotations on query well-formedness.
Figure 1: The distribution of the annotated questions according to well-formedness probability.

Figure 1 is the histogram of query well-formedness probability across the dataset. Interestingly, the number of queries where at least 4 or more annotators agree111At least 4 annotators label the query with 0 or 1. on well-formedness is large: queries. These constitute of all queries in the dataset. The Fleiss’ kappa Fleiss (1971) for measuring agreement among multiple annotators is computed to be which shows moderate agreement Landis and Koch (1977). We then randomly divided the dataset in approx. , , ratio into training, development and test sets containing , , and queries respectively. While testing, we consider every query well-formed where at least 4 out of 5 annotators () marked it as well-formed.222We randomly selected queries and manually determined if each of those queries were well-formed. We found to be the value above which all queries were well-formed.

2.2 Model

We use a feed-forward neural network with 2 hidden layers with ReLU activations

Glorot et al. (2011) on each layer and a softmax at the output layer predicting or . We extract a variety of features from the query which can be helpful in the classification. We extract character--grams and word--grams as they can be helpful in capturing spelling errors. In addition to lexical features, we also extract syntactic features that can inform the model on any anomaly in the structure of the query. Specifically, we annotate the query with POS-tags using SyntaxNet POS tagger Alberti et al. (2015) and extract POS--grams.333The use of dependency labels as features and use of pre-trained Glove embeddings did not show improvement and hence omitted here for space constraints. Every feature in the network is represented as a real-valued embedding. All the -grams embeddings of every feature type are summed together and concatenated to form the input layer as shown in Figure 2

. The model is trained using cross-entropy loss against the gold labels for each query. The hyperparameters are tuned to maximize accuracy on the dev set and results are reported on the test set.

Hyperparameters.

We fix the size of the first and second hidden layers to be and

respectively. The character n-gram embeddings were of length

and all other feature embeddings were of length

. We use stochastic gradient descent with momentum for optimization with learning rate tuned over

, a batch size of and training steps.

Figure 2: A feed-forward neural network for query well-formedness classification.

2.3 Experiments

Model Accuracy ()
majority class baseline
word bi-LSTM baseline
question word baseline
word-
word-
word- char-
word- POS-
word- char- POS-
Approx. human upper bound
Table 3: Performance of well-formedness query classifier on the test set.

Baselines.

The majority class baseline is which corresponds to all queries being classified non-wellformed. The question word baseline that classifies any query starting with a question word as a well-formed question gets .444List of question words: https://en.wikipedia.org/wiki/Interrogative_word Also, we used a single-layer word-level biLSTM encoder with hidden layer of length

to encode the question and then use this representation in the softmax layer to predict the label

Lee and Dernoncourt (2016). This classifier achieved .

Results.

The best performance obtained is while using word--grams and POS--grams as features. Using POS -grams gave a strong boost of points over word unigrams and bigrams. Although character--grams gave improvement over word unigrams and bigrams, the performance did not sustain when combined with POS tags.555We assumed character -grams to help identify spelling mistakes, but our dataset has relatively few misspelled words–only 6 in 100 random queries. A random sample of 1000 queries from the test set were annotated by one of the authors of the paper with proficiency in English, which matched the gold label with accuracy providing an approximate upper-bound for model performance.

A major source of error is our model’s inability to understand deep semantics and syntax. For example, “What is the history of dirk bikes?” is labeled as a non-wellformed question with by annotators because of the misspelled word “dirk” (the correct word is “dirt”). However, the POS tagger identifies “dirk” as a noun and as “NN NNS” is a frequent POS-bigram, our model tags it as a well-formed question with , unable to identify that the word does not fit in the context of the question. Another source of error is the inability to capture long term grammatical dependencies. For example, in “What sort of work did Edvard Munch made ?” the verb “made” is incorrectly in the past tense instead of present tense. Our model is unable to capture the relationship between “did” and “made” and thus marks this as a well-formed question.

3 Improving Question Generation

Automatic question generation is the task of generating questions that ask about the information or facts present in either a given sentence or paragraph Vanderwende (2008); Heilman and Smith (2010). du2017learning present a state-of-the-art neural sequence-to-sequence model to generate questions from a given sentence/paragraph. The model used is an attention-based encoder-decoder network Bahdanau et al. (2015), where the encoder reads in a given text and the decoder is an LSTM RNN that produces the question by predicting one word at a time.

du2017learning use the SQuAD question-answering dataset Rajpurkar et al. (2016) to develop a question generation dataset by pairing sentences from the text with the corresponding questions. The question generation dataset contains approx k, k, and k training, development and test examples. Their current best model selects the top ranked question from the -best list produced by the decoder as the output. We augment their system by training a discriminative reranker Collins and Koo (2005) with the model score of the question generation model and the well-formedness probability of our classifier as features to optimize BLEU score Papineni et al. (2002) between the selected question from the -best list and the reference question on the development set. We then use this reranker to select the best question from the -best list of the test set.

Model BLEU-1 BLEU-4
Baseline
Reranked
Table 4: Reranking the -best output of a neural seq2seq question generation model using well-formedness probability.

We use the evaluation package released by Chen2015MicrosoftCC to compute BLEU-1 and BLEU-4 scores.666BLEU-x uses precision computed over -grams. Table 4 shows that the reranked question selected using our query well-formedness classifier improves the BLEU-4 score of a seq-to-seq question generation model from to . The oracle improvement, by selecting the sentence from the list that maximizes the BLEU-4 score is . However, its worth noting that an increase in well-formedness doesn’t guarantee an improved BLEU score, as the oracle sentence maximizing the BLEU score might be fairly non-wellformed Callison-Burch et al. (2006). For example, “who was elected the president of notre dame in?” has a higher BLEU score to the reference “who was the president of notre dame in 1934?” than our well-formed question “who was elected the president of notre dame?”. Figure 3 shows a question generation example with the output of du2017learning as the baseline result and the reranked question using the wellformed probability.

Sentence: montana is home to the rocky mountain elk foundation and has a historic big game hunting tradition.
Gold question: what is the name of the big game hunting foundation in montana?
seq2seq: what is a historic big game hunting tradition? ()
Reranked: what is the name of the historic big game tradition? ()
Figure 3: Example showing question selection from the -best list using our reranking model.

4 Related Work

We have referenced much of the related work throughout the paper. We now review another orthogonally related field of work. Grammatical error correction (GEC) is the task of correcting the grammatical errors (if any) in a piece of text Ng et al. (2014). As GEC includes not just identification of ungrammatical text but also correcting the text to produce grammatical text, its a more complex task. However, grammatical error prediction Schmaltz et al. (2016); Daudaravicius et al. (2016) is the task of classifying whether or not a sentence is grammatical, which is more closely related to our task as classifying a question as well-formed requires making judgement on both the style and grammar of the text.

5 Conclusion

We proposed a new task of well-formed natural language question identification and established a strong baseline on a new dataset that can be downloaded at: http://goo.gl/language/query-wellformedness. We also showed that question well-formedness information can be a helpful signal in improving state-of-the-art question generation systems.

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