A variety of NLP tasks can be formulated as general sequence labeling problem: given a sequence of tokens and a fixed set of labels, assign one of the labels to each token in a sequence. We consider three concrete sequence labeling tasks: Part-of-speech (POS) tagging, Named Entity Recognition (NER), and Chunking (also known as shallow parsing). POS-tagging reduces to assigning a part-of-speech label to each word in a sentence; NER requires detecting (potentially multi-word) named entities, like person or organization names; chunking aims at identifying syntactic constituents within a sentence, like noun- or verb-phrases.
Traditionally, sequence labeling tasks were tackled using linear statistical models, for instance: Hidden Markov ModelsKupiec (1992), Maximum Entropy Markov Models McCallum et al. (2000), and Conditional Random Fields Lafferty et al. (2001)
. In their seminal paper, collobert2011natural have introduced a deep neural network-based solution to the problem, which has spawned immense research in this direction. Multiple works have introduced different neural architectures for universal sequence labeling afterwardsHuang et al. (2015); Lample et al. (2016); Ma and Hovy (2016); Chiu and Nichols (2016); Yang et al. (2016). However, aiming for better results on a particular dataset, these and numerous other works typically employ some form of feature engineering Ando and Zhang (2005); Shen and Sarkar (2005); Collobert et al. (2011); Huang et al. (2015), external data for training Ling et al. (2015); Lample et al. (2016)
or in a form of lexicons and gazetteersRatinov and Roth (2009); Passos et al. (2014); Chiu and Nichols (2016), extensive hyper-parameter search Chiu and Nichols (2016); Ma and Hovy (2016), or multi-task learning Durrett and Klein (2014); Yang et al. (2016).
In this paper, we take an alternative stance by looking at the problem of sequence labeling from a practical perspective. The performance enhancements enumerated above typically require availability of expertise, time, or external resources. Some (or even all) of these may be unavailable to users in a practical situation. Therefore, we deliberately eschew any form of feature engineering, pre-training, external data (with the exception of publicly available word embeddings), and time-consuming hyper-parameter optimization. To this end, we formulate a single general-purpose sequence labeling model and apply it to eight different benchmark datasets to estimate the effectiveness of our approach.
Our model utilizes a bi-directional LSTM Graves and Schmidhuber (2005) to extract morphological information from the bytes of words in a sentence. These, combined with word embeddings bearing semantic cues, are fed to another Bi-LSTM to obtain word-level scores. Ultimately, the word-level scores are passed through a CRF layer Lafferty et al. (2001) to facilitate structured prediction of the labels.
Recurrent Neural Networks Elman (1990) are commonly used for modeling sequences in NLP Mikolov et al. (2010); Cho et al. (2014); Graves et al. (2013). However, because of well-known challenges of capturing long-term dependencies with plain RNNs Pascanu et al. (2013)
, we instead turn to Long Short-Term Memory (LSTM) networksHochreiter and Schmidhuber (1997)
capable of alleviating the vanishing/exploding gradient problem by design. The specific LSTM formulation we are usingZaremba et al. (2014) can be described by the following equations:
denotes the sigmoid activation function;- element-wise (Hadamard) product; - learned network parameters; - input, forget, and output gates; , , and - network input, cell state, and network output at time step respectively.
One issue with ordinary LSTMs is that they capture dependencies between sequence elements only in one direction, whereas it might be beneficial to learn also backward dependencies (e.g., for informed first label prediction). To overcome this limitation, we use bidirectional LSTM (Bi-LSTM) networks Graves and Schmidhuber (2005) comprising two independent LSTM instances (with separate sets of parameters): one processing an input sequence in forward direction and the other in backward direction. The output of Bi-LSTM is formed by concatenating the outputs of the two LSTMs corresponding to each sequence element.
The diagram of the proposed model is shown in Figure 1. The model can be decomposed into three logical components: (1) computing byte embeddings of each word in a sentence by means of the Byte Bi-LSTM; (2) combining byte embeddings with word embeddings and feeding resulting joint word representations to the Word Bi-LSTM to obtain word-level scores; (3) passing word-level scores through the CRF Layer to arrive at joint prediction of labels. We describe each of these components in detail in the following subsections.
2.1 Byte Embeddings
Assuming that input is available in tokenized form, we enable the model to extract morphological information from tokens by analyzing their character-level representations. Following ling2015finding, we apply a Bi-LSTM network for solving this task. However, to be truly neutral with respect to languages and character sets thereof, we consider a sequence of bytes underlying a UTF-8 encoding of each word instead of its characters.
Formally, given a sequence of words in a sentence, we first decompose each word into its characters including dummy start- and end-of-word characters (we assume that the -th word consists of characters including two dummy ones). Now we convert a sequence of characters to a sequence of underlying UTF-8 bytes (for simplicity, here we assume that all characters are single-byte, but this is obviously not necessary). Dummy characters are encoded by special byte values 0x01 (start-of-word) and 0x02 (end-of-word). Next, we compute byte projections of bytes in a sequence by multiplying the byte projection matrix B by a one-of-
coded vector representation of each byte. The matrixis a learned model parameter. Each of the obtained byte projection vectors has a fixed dimensionality , which is a hyper-parameter of the model.
Next, the byte projection sequence is fed to the Byte Bi-LSTM network. The last outputs of its forward and backward LSTMs are concatenated to obtain ”byte embedding” vector of the -th word . These fixed-size vectors are assumed to capture morphological information about corresponding words in a sentence.
2.2 Word-level Representation
Morphological information alone is probably not representative enough to reliably predict target labels in a general sequence labeling setting. For this reason, we would also want to supply our prediction framework with semantic information. We fulfill this requirement by mixing in pre-trained word embeddings of words in a sentence. Then, following huang2015bidirectional, we infer word-level representation using a Bi-LSTM network.
Formally, given a vocabulary of size and fixed (not learned) word embedding matrix , the word embedding vector of the -th word is obtained by multiplying E by a one-of- coded vector representation of the word’s position in the vocabulary ( is the dimensionality of embedding vectors and depends on the choice of word embeddings). For all out-of-vocabulary words we use the same additional ”unknown” word embedding vector.
Next, computed byte embedding of -th word is concatenated with its word embedding to produce a joint embedding . This way we obtain a sequence of joint embeddings corresponding to the words in a sentence. The joint embeddings are assumed to capture both morphological and semantic information about the words. They are fed as inputs to the Word Bi-LSTM network. The outputs of the network at each time step are passed through a linear layer (with no activation function) to yield -dimensional word-level score vectors , where
denotes the number of distinct labels. In essence, word-level score vectors may be interpreted as unnormalized log-probabilities (logits) over the labels at each time step.
Theoretically, we could stop here by applying softmax to each word-level score vector to infer a distribution over possible labels at each time step. However, this approach (albeit bearing a certain degree of context-awareness due to the presence of the upstream Word Bi-LSTM) would treat each word more or less locally, lacking a global view over predicted labels. This is why we turn to a CRF layer as the last step of label inference.
|Dataset||training set||development set||testing set||labels (classes)|
|CoNLL 2000 (English, Chunking)||8,936||211,727||-||-||2,012||47,377||45 (11)|
|CoNLL 2002 (Spanish, NER)||8,323||264,715||1,915||52,923||1,517||51,533||17 (4)|
|CoNLL 2002 (Dutch, NER)||15,806||202,644||2,895||37,687||5,195||68,875||17 (4)|
|CoNLL 2003 (English, NER)||14,041||203,621||3,250||51,362||3,453||46,435||17 (4)|
|CoNLL 2012 (English, NER)||59,924||1,088,503||8,528||147,724||8,262||152,728||73 (18)|
|GermEval 2014 (German, NER)||24,000||452,853||2,200||41,653||5,100||96,499||49 (12)|
|WSJ/PTB (English, POS-tag.)||38,219||912,344||5,527||131,768||5,462||129,654||45 (45)|
|Tiger Corpus (German, POS-tag.)||40,472||719,530||5,000||76,704||5,000||92,004||54 (54)|
2.3 CRF Layer
Oftentimes, labels predicted at different time steps follow certain structural patterns. As an example, the IOB labeling scheme has specific rules constraining label transitions: for example, an I-ORG label can follow only a B-ORG or another I-ORG but no other label. To learn patterns like this one, following lample2016neural, we utilize Conditional Random Fields (CRFs) Lafferty et al. (2001) in the final component of our model. CRFs can capture dependencies between labels predicted at different time steps by modeling probabilities of transitions from one step to the other. Linear chain CRFs model transitions between neighboring pairs of labels in a sequence and allow solving a structured prediction problem in a computationally feasible way.
We recall that a sequence of word-level score vectors is inferred by the Word Bi-LSTM. The -th component of the -th score vector - - represents unnormalized log-probability of assigning -th label to the word at the -th position. Joint prediction is modeled by introducing a total score of a sequence of labels given a sequence of words :
where is a matrix of label transition scores ( is a score of transition from label to label ; represents the number of distinct labels) and word-level scores obviously depend on w. The matrix A is another learned model parameter.
The probability of observing a particular sequence of labels given a sequence of words can be computed by applying softmax over total scores of all possible label assignments to a sequence of words w ( denotes the set of all learned model parameters):
And the corresponding log-probability is:
We learn the model parameters by maximizing log-likelihood (2) of given the ground truth labels y corresponding to the input sequence w. Computing the normalizing factor from equation (1), as well as deriving the most likely sequence of labels during test time is performed using dynamic programming Rabiner (1989).
We evaluate the performance of our approach on eight benchmark datasets covering four languages and three sequence labeling tasks. Certain statistics of those are shown in Table 1. The labels of all NER and Chunking datasets are converted to IOBES tagging scheme, as it is reported to increase predictive performance Ratinov and Roth (2009). We briefly discuss each of the datasets in the subsections below.
3.1 CoNLL 2000
The CoNLL 2000 dataset Tjong Kim Sang and Buchholz (2000) was introduced as a part of a shared task on Chunking. Sections 15-18 of the Wall Street Journal part of the Penn Treebank corpus Marcus et al. (1993) are used for training, section 20 for testing. Due to the lack of specifically demarcated development set, we use randomly sampled 10% of the training set for this purposes (see Section 4 for the details of training).
3.2 CoNLL 2002
The CoNLL 2002 dataset Tjong Kim Sang (2002) was used for shared task on language-independent Named Entity Recognition. The data represents news wire covering two languages: Spanish and Dutch. In our experiments we treat Spanish and Dutch data separately, as two different datasets. The dataset is annotated by four entity types: persons (PER), organizations (ORG), locations (LOC), and miscellaneous names (MISC).
3.3 CoNLL 2003
3.4 CoNLL 2012
The CoNLL 2012 dataset Pradhan et al. (2012) was created for a shared task on multilingual unrestricted coreference resolution. The dataset is based on OntoNotes corpus v5.0 Hovy et al. (2006) and, among others, has named entity annotations. It is substantially larger and more diverse than the previously described NER datasets (see Table 1 for detailed comparison). Although some sources Durrett and Klein (2014); Chiu and Nichols (2016) refer to the dataset as ”OntoNotes”, we stick to the name ”CoNLL 2012” as the train/dev/test split that is used by this and other works is not defined in the OntoNotes corpus. Following durrett2014joint, we exclude the New Testament part of the data, as it is lacking gold annotations.
3.5 GermEval 2014
GermEval 2014 Benikova et al. (2014) is a recently organized shared task on German Named Entity Recognition. The corresponding dataset has four main entity types (Location, Person, Organization, and Other) and two sub-types of each type, ”-deriv” and ”-part”, indicating derivation from and inclusion of a named entity respectively. The original dataset has two levels of labeling: outer and inner. However, we use only outer labels in the experiments and compare our results to other works on the ”M3: Outer Chunks” metric.
3.6 Wall Street Journal / Penn Treebank
The Wall Street Journal section of the Penn Treebank corpus Marcus et al. (1993) is commonly used as a benchmark dataset for the English POS-tagging task. We follow this tradition and use a standard split of sections: 0-18 for training, 19-21 for development, and 22-24 for testing Toutanova et al. (2003).
3.7 Tiger Corpus
Tiger Corpus Brants et al. (2002) is an extensive collection of German newspaper texts. The dataset has several different types of annotations. We use part-of-speech annotations for setting up a German POS tagging task. Following fraser2013knowledge, we use the first 40,472 of the originally ordered sentences for training, the next 5,000 for development, and the last 5,000 for testing.
The training procedure described in this section is used for every experiment on every dataset mentioned in this paper. The model is trained end-to-end, accepting tokenized sentences as input and predicting per-token labels as output.
The model is trained using an Adam optimizer Kingma and Ba (2014). Following ma2016end, we apply staircase learning rate decay:
where is the learning rate used throughout
-th epoch (starts at ), is the initial learning rate, and is the learning rate decay factor. In our experiments we use and .
Motivated by the initial experiments, the dimension of the byte projections is set to , the size of Byte and Word Bi-LSTM to and respectively. Training lasts for epochs with a batch size of sentences. Early stopping Caruana et al. (2001) is used: at the end of every epoch the model is evaluated against the development set; eventually, the parameter values performing best on the development set are declared the final values.
Due to the high level of expressive power of the proposed model, we use dropout Srivastava et al. (2014), with the rate of , to reduce the possibility of overfitting. Dropout is applied to word embeddings and byte projections, as well as the outputs of Byte Bi-LSTM and Word Bi-LSTM. As per zaremba2014recurrent, we don’t apply dropout to state transitions of the LSTM networks.
Publicly available word embeddings are used for every language in the experiments. For English datasets we use -dimensional uncased GloVe embeddings Pennington et al. (2014) trained on English Wikipedia and Gigaword 5 corpora and comprising K unique word forms. For other languages we use -dimensional cased Polyglot embeddings Al-Rfou et al. (2013) trained on a respective Wikipedia corpus and comprising K unique (case-sensitive) word forms per language. Maintaining our commitment to the practical approach, we freeze the word embeddings during training not allowing them to train (except for the ”unknown” word embedding, which is trained).
To achieve higher efficiency, we compute the joint embedding of every unique word in a batch only once Ling et al. (2015). Unique joint embeddings are scattered according to the word positions in the input sentences, before being fed to the Word Bi-LSTM. The gradient with respect to each unique byte embedding is accumulated before being back-propagated once through the Byte Bi-LSTM. Albeit marginal during training (in small batches), performance improvement becomes considerable during inference (in larger batches).
|Dataset||word||word + crf||byte||byte + crf||byte + word||byte + word + crf|
|CoNLL 2000 (English, Chunking, F)||91.39||92.70||92.75||93.41||93.93||94.74|
|CoNLL 2002 (Spanish, NER, F)||77.55||80.31||77.29||79.62||82.04||84.36|
|CoNLL 2002 (Dutch, NER, F)||74.95||76.91||75.14||78.06||83.26||85.61|
|CoNLL 2003 (English, NER, F)||86.35||87.58||81.80||82.85||89.70||91.11|
|CoNLL 2012 (English, NER, F)||79.00||82.95||79.93||83.70||84.69||87.84|
|GermEval 2014 (German, NER, F)||68.86||71.90||65.79||69.30||76.74||79.21|
|WSJ/PTB (English, POS-tag., %)||95.22||95.30||97.11||97.14||97.45||97.43|
|Tiger Corpus (German, POS-tag., %)||95.60||95.76||97.68||97.78||98.39||98.40|
We present the results of our experiments in two different contexts. Table 2 shows the performance of different model configurations gauged in ablation studies. Tables 3, 4, 5, 6, 7, 8, 9, and 10 juxtapose our results on each dataset with those of other works reporting their results on the same dataset. When citing results of other works, we indicate the best performance reported in the corresponding publication (independent of the methodology used). Each of our scores reported in Tables 2-10 was achieved by a trained model on the dataset’s official test set. The scores were verified using CoNLL 2000 evaluation script222https://www.clips.uantwerpen.be/conll2000/chunking/conlleval.
|ratinov2009design||83.45333The result is taken from durrett2014joint.|
|ling2015finding||98.08444The result is not directly comparable to ours, as the authors used different train/dev/test split of Tiger Corpus.|
The ablation studies examined different partial configurations of the full model described in Section 2 evaluated on each of the eight datasets. The configurations were obtained by altering the contents of joint embeddings and omitting the CRF layer. Joint embeddings were set to only word, only byte, or both word and byte embeddings. For each of these three settings, the CRF layer was included or excluded, amounting to six configurations in total.
The results of the ablation studies in Table 2 show that, on every dataset, word and byte embeddings used jointly substantially outperformed any one of them used individually. This emphasizes the importance of using both embedding types, bringing in both semantic and morphological information about the words, for solving the general sequence labeling task. The role of the CRF layer proved to be crucial for all NER and Chunking, but not POS-tagging tasks. Configurations with and without CRF layer, given the same word representation, demonstrated very similar results on both English and German POS-tagging datasets. This supports the conjecture that a CRF layer can provide substantial incremental benefit, when used for solving a structured prediction task (e.g., NER and Chunking). It is also worth mentioning that on five out of eight datasets, using byte embeddings alone yielded better results than using word embeddings alone, both with and without CRF layer. This may be caused by the fact that word embeddings are external (and not necessarily related) to the data, while byte embeddings are always learned from the data itself.
Comparison of our results with those of other works, shown in Tables 3 through 10, manifests that our model has achieved state-of-the-art performance on four out of eight datasets: namely, 85.61 F on CoNLL 2002 (Dutch NER), 87.48 F on CoNLL 2012 (English NER), 79.21 F on GermEval 2014 (German NER), and 98.40% on Tiger Corpus (German POS-tagging). We consider the scores obtained on the remaining four datasets to be competitive.
6 Related Work
Arguably, the modern era of deep neural network-based NLP, in particular that of sequence labeling, has emerged with the work of collobert2011natural. Using pre-trained word and feature embeddings as word-level inputs, the authors have applied a CNN with ”max over time” pooling and downstream MLP. As an objective, they maximized ”sentence-level log-likelihood”, similar to the CRF-based approach from Section 2.3.
Since then, many more sophisticated neural models were proposed. Combining a Bi-LSTM network with a CRF layer to model sequence of words in a sentence was first introduced by huang2015bidirectional. The authors have employed substantial amount of engineered features, as well as external gazetteers. ling2015finding have proposed extracting morphological information from words using a character-level Bi-LSTM. The authors have combined the character-level embeddings with proprietary word embeddings (trained in-house) to obtain current stat-of-the-art result on the WSJ/PTB English POS-tagging dataset.
dos2015boosting have augmented the model of collobert2011natural with a CNN-based character-level feature extractor network (”CharWNN”) and applied the resulting model for NER. chiu2016named have combined CharWNN with a Bi-LSTM on word level. A similar approach was proposed by ma2016end. However, ma2016end did not use any feature engineering or external lexicons, as opposed to chiu2016named, and still obtained respectably high performance.
One of the two models proposed by lample2016neural is quite similar to ours, except that the authors have assumed a fixed set of characters for each language, whereas we turn to bytes as a universal medium of encoding character-level information. Also, lample2016neural have pre-trained their own word embeddings, which turned out to have a crucial impact on their results.
An interesting approach of applying cross-lingual multi-task learning to sequence labeling problem was introduced in the work of yang2016multi. The authors have used hierarchical bi-directional GRU (on character and word levels) and optimized a modified version of a CRF objective function. Their model, in conjunction with the applied multi-task learning framework, has allowed the authors to obtain state-of-the-art results on multiple datasets.
We evaluate the performance of a single general-purpose neural sequence labeling model, assuming unavailability of domain expertise and scarcity of informational and computational resources. The work explores the frontiers of what may be achieved with a generic and resource-efficient sequence labeling framework applied to a diverse set of NLP tasks and languages.
For this purpose, we’ve applied the model (Section 2) and the end-to-end training methodology (Section 4) to eight benchmark datasets (Section 3), covering four languages and three tasks. The obtained results have convinced us that, with a model of sufficient learning capacity, it is indeed possible to achieve competitive sequence labeling performance without the burden of delving into specificities of each particular task and language, and summoning additional resources.
For the future work, we envision the integration of multi-task learning techniques (e.g., those used by yang2016multi) into the proposed approach. We suppose that this may improve the current results without compromising our general applicability and practicality assumptions.
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