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Proceedings of the Second Italian Conference on Computational Linguistics CLiC-it 2015

 | 
Cristina Bosco
, 
Sara Tonelli
, 
Fabio Massimo Zanzotto

Deep Neural Networks for Named Entity Recognition in Italian

Daniele Bonadiman, Aliaksei Severyn et Alessandro Moschitti

Résumé

In this paper, we introduce a Deep Neural Network (DNN) for engineering Named Entity Recognizers (NERs) in Italian. Our network uses a sliding window of word contexts to predict tags. It relies on a simple word-level log-likelihood as a cost function and uses a new recurrent feedback mechanism to ensure that the dependencies between the output tags are properly modeled. The evaluation on the Evalita 2009 benchmark shows that our DNN performs on par with the best NERs, outperforming the state of the art when gazetteer features are used.

In questo lavoro, si introduce una rete neurale deep (DNN) per progettare estrattori automatici di entità nominate (NER) per la lingua italiana. La rete utilizza una finestra scorrevole di contesti delle parole per predire le loro etichette con associata probabilità, la quale è usata come funzione di costo. Inoltre si utilizza un nuovo meccanismo di retroazione ricorrente per modellare le dipendenze tra le etichette di uscita. La valutazione della DNN sul dataset di Evalita 2009 indica che è alla pari con i migliori NER e migliora lo stato dell’arte quando si aggiungono delle features derivate dai dizionari.

Texte intégral

This work has been partially supported by the EC project CogNet, 671625 (H2020-ICT-2014-2, Research and Innovation action) and by an IBM Faculty Award.

1. Introduction

1Named Entity (NE) recognition is the task of detectings phrases in text, e.g., proper names, which directly refer to real world entities along with their type, e.g., people, organizations, locations, etc. see, e.g., (Nadeau and Sekine, 2007).

2Most NE recognizers (NERs) rely on machine learning models, which require to define a large set of manually engineered features. For example, the state-of-the-art (SOTA) system for English (Ratinov and Roth, 2009) uses a simple averaged perceptron and a large set of local and non-local features. Similarly, the best performing system for Italian (Nguyen et al., 2010) combines two learning systems that heavily rely on both local and global manually engineered features. Some of the latter are generated using basic hand-crafted rules (i.e., suffix, prefix) but most of them require huge dictionaries (gazetteers) and external parsers (POS taggers and chunkers). While designing good features for NERs requires a great deal of expertise and can be labour intensive, it also makes the taggers harder to adapt to new domains and languages since resources and syntactic parsers used to generate the features may not be readily available.

 

3Recently, DNNs have been shown to be very effective for automatic feature engineering, demonstrating SOTA results in many sequence labelling tasks, e.g., (Collobert et al., 2011), also for Italian language (Attardi, 2015).

4In this paper, we target NERs for Italian and propose a novel deep learning model that can match the accuracy of the previous best NERs without using manual feature engineering and only requiring a minimal effort for language adaptation. In particular, our model is inspired by the successful neural network architecture presented by Collobert et al. (2011) to which we propose several innovative and valuable enhancements: (i) a simple recurrent feedback mechanism to model the dependencies between the output tags and (ii) a pre-training process based on two-steps: (a) training the network on a weekly labeled dataset and then (b) refining the weights on the supervised training set. Our final model obtains 82.81 in F1 on the Evalita 2009 Italian dataset (Speranza, 2009), which is an improvement of +0.81 over the Zanoli and Pianta (2009) system that won the competition. Our model only uses the words in the sentence, four morphological features and a gazetteer. Interestingly, if the gazetteer is removed from our network, it achieves an F1 of 81.42, which is still on par with the previous best systems yet it is simple and easy to adapt to new domains and languages.

Figure 1: The architecture of Context Window Network (CWN) of Collobert et al. (2011).

Figure 1: The architecture of Context Window Network (CWN) of Collobert et al. (2011).

2. Our DNN model for NER

5In this section, we first briefly describe the architecture of the Context Window Network (CWN) from Collobert et al. (2011), pointing out its limitation. We then introduce our Recurrent Context Window Network (RCWN), which extends CWN and aims at solving its drawbacks.

2.1 Context Window Network

  • 1 Barack Obama is the president of the United States of America.
  • 2 In case the target word i is at the beginning/end of a sentence, up to (k −1)/2 placeholders are us (...)

6We adopt a CWN model that has been successfully applied by Collobert et al. (2011) for a wide range of sequence labelling NLP tasks. Its architecture is depicted in Fig. 1. It works as follows: given an input sentence s = [w1,...,wn], e.g., Barack Obama è il presidente degli Stati Uniti D’America1, for each word wi, the sequences of word contexts [wik/2+1,..,wi,..,wi+k/2] of size k around the target word wi (i = 1,..,n) are used as input to the network.2 For example, the Fig. 1 shows a network with k = 5 and the input sequence for the target word è at position i = 3.

7The input words wi from the vocabulary V are mapped to d-dimensional word embedding vectors wi Rd. Embeddings wi for all words in V form an embedding matrix W R|V d, which learned by the network. An embedding vector wi for a word wi is retrieved by a simple lookup operation in W (see lookup frame in Fig. 1). After the lookup, the k embedding vectors of the context window are concatenated into a single vector r1  Rkd, which is passed to the next hidden layer hl. It applies the following linear transformation: hl(r1) = M1.r1+b1, where the matrix of weights M1 and the bias b1 parametrize the linear transformation and are learned by the network. The goal of the hidden layer is to learn feature combinations from the word embeddings of the context window.

8To enable the learning of non-linear discriminative functions, the output of hl is passed through a non-linear transformation also called activation function, i.e., a HardTanh() non-linearity, thus obtaining, r2. Finally, the output classification layer encoded by the matrix M2 R|C|×h and the bias b2 are used to evaluate the vector p = softmax(M2×r2+b2) of class conditional probabilities, i.e., pc = p(c|x),c  C, where C is the set of NE tags, h is the dimension of the hl and x is the input context window.

2.2 Our model

9The CWN model described above has several drawbacks: (i) each tag prediction is made by considering only local information, i.e., no dependencies between the output tags are taken into account; (ii) publicly available annotated datasets for NER are usually too small to train neural networks thus often leading to overfitting. We address both problems by proposing: (i) a novel recurrent context window network (RCWN) architecture; (ii) a network pre-training technique using weakly labeled data; and (iii) we also experiment with a set of recent techniques to improve the generalization of our DNN to avoid overfitting, i.e., we use early stopping (Prechelt, 1998), weight decay (Krogh and Hertz, 1992), and Dropout (Hinton, 2014).

2.2.1 Recurrent Context Window Network

10We propose RCWN for modeling dependencies between labels. It extends CWN by using m previously predicted tags as an additional input, i.e., the previously predicted tags at steps − m,...,i − 1 are used to predict the tag of the word at position i, where m < k/2. Since we proceed from left to right, words in the context window wj with j > i − 1, i.e., at the right of the target word, do not have their predicted tags, thus we simply use the special unknown tag, UNK, for them.

Table 1: Splits of the Evalita 2009 dataset

Dataset

Articles

Sentences

Tokens

Train

525

11,227

212,478

Test

180

4,136

86,419

11Since NNs provide us with the possibility to define and train arbitrary embeddings, we associate each predicted tag type with an embedding vector, which can be trained in the same way as word embeddings (see vectors for tags ti in Fig. 1). More specifically, given k words wi  Rdw in the context window and previously predicted tags ti  Rdt at corresponding positions, we catenate them together along the embedding dimension obtaining new vectors of dimensionality dw + dt. Thus, the output of the first input layer becomes a sequence of k(dw dt) vectors.

  • 3 We can use the same intuitive explanation of error correcting output codes.

12RCWN is simple to implement and is computationally more efficient than, for example, NNs computing sentence log-likelihood, which require Viterbi decoding. RCWN may suffer from an error propagation issue as the network can misclassify the word at position − i, propagating an erroneous feature (the wrong label) to the rest of the sequence. However, the learned tag embeddings seem to be robust to noise3. Indeed, the proposed network obtains a significant improvement over the baseline model (see Section 3.2).

3 Experiments

13In these experiments, we compare three different enhancements of our DNNs on the data from the Evalita challenge, namely: (i) our RCWN method, (ii) pre-training on weakly supervised data, and (iii) the use of gazetteers.

3.1 Experimental setup

14Dataset. We evaluate our models on the Evalita 2009 Italian dataset for NERs (Speranza, 2009) summarized in Tab. 1. There are four types of NEs: person (PER), location (LOC), organization (ORG) and geo-political entity (GPE), (see Tab. 2). Data is annotated using the IOB tagging schema, i.e., for inside, outside and beginning of a entity, respectively.

Table 2: Entities distribution in Evalita 2009

Dataset

PER

ORG

LOC

GPE

Train

4,577

3,658

362

2,813

Test

2,378

1,289

156

1,143

15Training and testing the network. We use (i) the Negative Log Likelihood cost function,i.e., −log(pc), where c is the correct label for the target word, (ii) stochastic gradient descent (SGD) to learn the parameters of the network and (iii) the backpropogation algorithm to compute the updates. At test time, the tag c, associated with the highest class conditional probability pc, is selected, i.e., = argmaxcpc.

16Features. In addition to words, all our models also use 4 basic morphological features: all lowercase, all uppercase, capitalized and it contains uppercase character. These can reduce the size of the word embedding dictionary as showed by (Collobert et al., 2011). In our implementation, these 4 binary features are encoded as one discrete feature associated with an embedding vector of size 5, i.e., similarly to the preceding tags in RCWN. Additionally, we use a similar vector to also encode gazetteer features. Gazetteers are collections of names, locations and organizations extracted from different sources such as the Italian phone book, Wikipedia and stock marked websites. Since we use four different dictionaries one for each NE class, we add four feature vectors to the network.

17Word Embeddings. We use a fixed dictionary of size 100K and set the size of the word embeddings to 50, hence, the number parameters to be trained is 5M. Training a model with such a large capacity requires a large amount of labelled data. Unfortunately, the sizes of the supervised datasets available for training NER models are much smaller, thus we mitigate such problem by pre-training the word embeddings on huge unsupervised training datasets. We use word2vec (Mikolov et al., 2013) skip-gram model to pre-train our embeddings on Italian dump of Wikipedia: this only took a few hours.

18Network Hyperparameters. We used = 750 hidden units, a learning rate of 0.05, the word embedding size dw = 50 and a size of 5 for the embeddings of discrete morphological and gazetteer features. Differently, we used a larger embedding, dt = 20 for the NE tags.

19Pre-training DNN with gazetters. Good weight initialization is crucial for training better NN models (Collobert et al., 2011; Bengio, 2009). Over the years different ways of pre-training the network have been designed: layer-wise pre-training (Bengio, 2009), word embeddings (Collobert et al., 2011) or by relying on distant supervised datasets (Severyn and Moschitti, 2015). Here, we propose a pretraining technique using an off-the-shelf NER to generate noisily annotated data, e.g., a sort of distance/weakly supervision or self-training. Our Weakly Labeled Dataset (WLD) is built by automatically annotating articles from the local newspaper ”L’Adige”, which is the same source of the training and test sets of Evalita challenge. We split the articles in sentences and tokenized them. This unlabeled corpus is composed of 20.000 sentences. We automatically tagged it using EntityPro, which is a NER tagger included in the TextPro suite (Pianta et al., 2008).

3.2 Results

  • 4 The official evaluation metric for NER is the F1, which is the harmonic mean between Precision and (...)

20Our models are evaluated on the Evalita 2009 dataset. We applied 10-fold cross-validation to the training set of the challenge4for performing parameter tuning and picking the best models.

Table 3: Results on 10-fold cross-validation

Model

F1

Prec.

Rec.

Baseline

78.32

79.45

77.23

RCWN

81.39

82.63

80.23

RCWN+Gazz

83.59

84.85

82.40

RCWN+WLD

81.74

82.93

80.63

RCWN+WLD+Gazz

83.80

85.03

82.64

21Table 3 reports performance of our models averaged over 10-folds. We note that (i) modeling the output dependencies with RCWN leads to a considerable improvement in F1 over the CWN model of Collobert et al. (2011) (our baseline); (ii) adding the gazetteer features leads to an improvement both in Precision and Recall, and therefore in F1; and (iii) pre-training the network on the weakly labeled training set produces improvement (although small), which is due to a better initialization of the network weights.

22Table 4 shows the comparative results between our models and the current state of the art for Italian NER on the Evalita 2009 official test set. We used the best parameter values derived when computing the experiments of Table 3. Our model using both gazetteer and pre-training outperforms all the systems participating to theEvalita 2009 (Zanoli and Pianta, 2009; Gesmundo, 2009). It should be noted that Nguyen et al. (2010) obtained better results using a CRF classifier followed by a reranker (RR) based on tree kernels. However, our approach only uses one learning algorithm, which is simpler than models applying multiple learning approaches, such as those in (Nguyen et al., 2010) and (Zanoli and Pianta, 2009). Moreover, our model outperforms the Nguyen et al. (2010) CRF baseline (which is given in input to the tree-kernel based reranker) by  2.5 points in F1. Thus it is likely that applying their reranker on top of our model’s output might produce a further improvement over SOTA.

Table 4: Comparison with the best NER systems for Italian. Models below the double line were computed after the Evalita challenge.

Models

F1

Prec.

Rec.

Gesmundo (2009)

81.46

86.06

77.33

Zanoli and Pianta (2009)

82.00

84.07

80.02

Nguyen et al. (2010) (CRF)

80.34

83.43

77.48

Nguyen et al. (2010) + RR

84.33

85.99

82.73

RCWN

79.59

81.39

77.87

RCWN+WLD

81.42

82.74

80.14

RCWN+Gazz

81.47

83.48

79.56

RCWN+WLD+Gazz

82.81

85.69

80.10

23Finally, it is important to note that our model obtains an F1 comparable to the best system in Evalita 2009 without using any extra features (we only use words and 4 morphological features). In fact, when we remove the gazetteer features, our method still obtains the very high F1 of 81.42.

4. Conclusion

24In this paper, we propose a new DNN for designing NERs in Italian. Its main characteristics are: (i) the RCWN feedback method, which can model dependencies of the output label sequence and (ii) a pre-training technique involving a weakly supervised dataset. Our system is rather simple and efficient as it involves only one model at test time. Additionally, it does not require time-consuming feature engineering or extensive data processing for their extraction.

25In the future, we would like to apply rerankers to our methods and explore combinations of DNNs with structural kernels.

Bibliographie

Giuseppe Attardi. 2015. Deepnl: a deep learning nlp pipeline. In Proceedings of the 1st Workshop on Vector Space Modeling for Natural Language Processing, pages 109–115, Denver, Colorado, June. Association for Computational Linguistics.

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Notes

1 Barack Obama is the president of the United States of America.

2 In case the target word i is at the beginning/end of a sentence, up to (k −1)/2 placeholders are used in place of the empty input words.

3 We can use the same intuitive explanation of error correcting output codes.

4 The official evaluation metric for NER is the F1, which is the harmonic mean between Precision and Recall.

Table des illustrations

Titre Figure 1: The architecture of Context Window Network (CWN) of Collobert et al. (2011).
URL http://books.openedition.org/aaccademia/docannexe/image/1317/img-1.jpg
Fichier image/jpeg, 104k

Auteurs

DISI - University of Trento, Italy - bonadiman.daniele@gmail.com

Qatar Computing Research Institute, HBKU, Qatar - DISI - University of Trento, Italy amoschitti@gmail.com

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