In recent years, with the growing popularity of social media applications, there has been an exponential increase in the amount of user-generated text including microblog posts, status updates and comments posted on the web. The power of communicating freely with large number of users has resulted in not only sharing news and exchanging content but has also led to a problem of large number of harmful, offensive and aggressive interactions online Duggan (2017). Previous work on identifying abusive language has tackled this problem by training computational methods that are capable of automatically recognizing offensive content for text on MySpace Yin et al. (2009), Twitter Waseem and Hovy (2016); Davidson et al. (2017), Wikipedia comments Wulczyn et al. (2017) and Facebook posts Vigna et al. (2017); Kumar et al. (2018).
Recent advances in unsupervised pre-training of language models have led to strong improvements on various general natural language processing and understanding tasks such as question answering, sentiment and natural language inference Peters et al. (2018); Devlin et al. (2018). However, it is unclear how such models trained on standard text would transfer information when fine-tuned on noisy user generated text. In additional to studying word, subword and character-based model performances on abusive language detection we also combine these with pre-trained embeddings and fine-tuning these pre-trained language models and understand their efficiency and robustness in identifying abusive text.
Specifically, in this work, we address the effectiveness of character-based models, subword or Byte Pair Encoding (BPE) based models and word features based models along with pre-trained word embeddings and fine tuning pretrained language models for detecting abusive language in text. Precisely we make following contributions:
We compare the effectiveness of end-to-end character based models, with word + character embedding models, byte pair encoding and subword models, to show which of the techniques perform better than pure word based models.
We demonstrate how fine-tuning large pre-trained language models, the latest breakthrough in NLP, enhance state of the art on few of the abusive language datasets, and show that the domain shift isn’t considerable when applied to abusive language datasets.
We also examine how preprocessing documents with byte pair encoding model pretrained on a large corpus, boost the performance of several word embedding based models massively.
2 Related Work
Identifying abusive context on the web is one of the widely studied topics on social media text. This problem has been studied for Hate Speech detection Kwok and Wang (2013); Waseem and Hovy (2016); Waseem (2016); Ross et al. (2016); Saleem et al. (2017); Warner and Hirschberg (2012), Harassment Yin et al. (2009); Cheng et al. (2015), Cyberbullying Willard (2007); Tokunaga (2010); Schrock and Boyd (2011), Abusive language detection Sahlgren et al. (2018); Nobata et al. (2016), aggression identification Kumar et al. (2018); Aroyehun and Gelbukh (2018); Modha et al. (2018), identifying toxic comments on forums Wulczyn et al. (2017) and offensive language identification Wiegand et al. (2018); Zampieri et al. (2019). While most of the work in identifying abusive on social media is predominantly studied for English social media posts some of the latest work include study on German Wiegand et al. (2018), Italian Bosco et al. (2018) and Mexican Spanish Álvarez-Carmona et al. (2018).
Some of the early methods on identifying abusive text used word n-gram, part-of-speech (POS) tagging (syntactic features), manually created profanity lexicons or stereotypical words, TF-IDF features along with sentiment and contextual features and trained supervised classifiers such as support vector machines.Yin et al. (2009); Warner and Hirschberg (2012). Waseem (2016) studied character n-grams, skipgrams, brown clusters and POS tag based features for identifying hatespeech. Waseem and Hovy (2016) studied usefulness of various socio-linguistic features such as gender, location, word-length distribution, Author Historical Salient Terms (AHST) features in identifying hatespeech.
Pavlopoulos et al. (2017)
used deep-learning based models specifically they employed RNN with a novel classification-specific attention mechanism and achieve state-of-the-art results on identifying attack and toxic content in Wikipedia comments.Badjatiya et al. (2017)
investigated three different neural networks for hatespeech detection: (i) Convolutional neural network (inspired by CNN’s for sentiment classification byKim (2014)
) (ii) Long short-term memory networks (LSTM) to capture long range dependencies and (iii) FastText classification model that represents document by averaging word vectors that can be fine-tuned for the hatespeech task.
While Badjatiya et al. (2017) analyzed various architectures to encode text for hatespeech detection, we are not aware of any work that studied various word decomposition models for identifying abusive language in text. Recent work on identifying offensive language in text include fine-tuning large pretrained languege model BERT which use subword units to encode text Zampieri et al. (2019); Zhu et al. (2019). For the SEMEVAL-2019 task of offensive language identification 7 out of top 10 submissions used BERT finet tuning. Zampieri et al. (2019) highlighted that 8% of 104 systems participated in the shared task used BERT based fine-tuning.
In this work, we analyze the effectiveness of different ways of learning representations with character-based models, subword or BPE based models and word features based models. We also combine these with well known pre-trained word embeddings and very large pretrained language models for fine-tuning and detecting abusive language in text. In Section 3 we describe the datasets that we study in this work for hatespeech and abusive detection.
We experiment with three datasets: Twitter dataset Waseem and Hovy (2016), Personal Attack and Toxicity datasets from Wikipedia Talk dataset Wulczyn et al. (2017) that covers sexism/racism, personal attack and toxicity aspects of abuse in user generated text online.
3.1 Twitter Dataset
We use the hatespeech Twitter dataset (Hatespeech) provided by Waseem and Hovy (2016). This dataset was created from a corpus of 136k tweets collected from Twitter by searching for commonly used racist and sexist slurs on various ethnic, gender and religious minorities over a two-month period. The original data had 16,907 tweets corresponding to sexist, racist and neither labels (3378, 1970 and 11559 respectively). However, we could only retrieve 11170 of the tweets (2914: sexism, 17: racism and 8239: neither) with python’s Tweepy library. Similar issue of missing tweets has been reported by Mishra et al. (2018). However, the percent of tweets we lost are much higher than theirs and most of the tweets lost are for the racism label. We have lost majority of the tweets corresponding to sexism label. Since we lost large chunk of tweets we conduct our experiments on cross validation of 5 splits and report scores on all of the 5 splits.
3.2 Wikipedia talk page
We use the personal attacks (W-ATT) and toxicity (W-TOX) datasets that were randomly sampled from 63 Million talk page comments from the public dump of English Wikipedia by Wulczyn et al. (2017)
. Each comment in both the datasets were annotated by at least 10 workers and we use the majority label as its gold label. Overall, we have 115.8k comments in W-ATT dataset (69.5k, 23.1k and 23.1k in train, dev and test splits respectively) and 159.6k comments in W-TOX dataset (95.6k, 32.1k and 31.8k in train, dev and test splits). Similar to hatespeech dataset both the W-ATT and W-TOX datasets also have skewed distribution of labels having 13.5% and 15.3% of them labeled as abusive.
In this section, we present various word decomposition methods and modeling architectures we analysed for studying Twitter and Wiki Talk page W-ATT and W-TOX comment datasets.
4.1 Word-based Model
As a baseline, we adpot the fastText Grave et al. (2017) classification algorithm. The fastText algorithm performs mean pooling on top of the word embeddings
to obtain a document representation. This document representation is passed through a Softmax layer to obtain classification scores. The embeddings can either be learned or can be initialized with pre-trained embeddings and fine-tuned during training. We run multiple variants of fastText in our experiments.
4.2 Subword-based Model
Subwords are formed by concatenating all the characters of a particular length within a word boundary. Addition of subwords gives the model ability to learn words which are misspelled such as emnlp and emnnlp are similar. A pure word based model would consider emnnlp as out-of-vocabulary (OOV) word, if not seen in training set, but a subword model would decompose emnnlp into “emn” and “nlp”, and train subword embeddings for each of these subwords. We take subword variant of fastText model to incorporate subword context into the model. The algorithm considers all subwords of varying lengths within the boundary of a word.
4.3 Joint Word and Character Embedding Model
Let be the input word and be its character representation, where is the number of characters in the word. We transform representation by passing through a character embedding layer, which is a n-gram Character-CNN similar to Peters et al. (2019). The output of the n-gram CharacterCNN is concatenated with the word’s corresponding pretrained embedding to obtain as described in 1 Character-level features are concatenated with , the word embedding of word , to form , the full set of word-level input features:
We randomly replace singleton words with special [UNK] (unknown) tokens for obtaining its , and also apply dropout Srivastava et al. (2014) on . The input word embeddings , in a sentence with tokens and convolutional window size , is transformed through a convolution filter :
is a bias term and f is a non-linear function (ReLU). This produces a feature map, on-top of which we apply a global max-over time pooling.
This process for one feature is repeated to obtain filters with different window sizes . The resulting filters are concatenated to form TextCNN document representation. The document representation is passed through Softmax layer to obtain classification predictions. We also experiment with original version of TextCNN, which is a pure word based model, without the character embedding variant.
4.4 End-to-end Character Embedding Model
To understand the potential of end to end character based models in dealing noisy text, we use Very Deep Convolutional Neural Network (VDCNN) architecture proposed by Conneau et al. (2017) that operates at character level by stacking multiple convolutional and pooling layers that sequentially extract a hierarchical representation of the text. This representation is fed into a fully connceted layer which is trained to maximize the classification accuracy on training data.
4.5 Byte Pair Encoding + Word + Char embedding models
We train a Byte Pair Encoding(BPE) based model introduced by Sennrich et al. (2016) on the given training corpus. We use this trained BPE model on training data to tokenize/encode our documents in training, validation and test data and use each BPE unit as a word to learn embeddings. We perform merge operations on each training dataset to learn subword or BPE units.
4.6 Pretrained Language Models
Recent liteature have shown that transferring knowledge from large pre-trained language models could benefit various NLP tasks either by adding a task specific architecture or by fine-tuning the language model for the end task Peters et al. (2018); Devlin et al. (2018); Peters et al. (2019). In this work, we use model and we fine-tune the model for each of our train datasets.
|fastText + BERT tokentization||N||Y||Y||71.2||83.0||83.0||95.2||83.4||94.5||96.1|
|fastText + Custom BPE||N||Y||Y||66.3||72.0||74.8||73.2||72.4||81.5||84.6|
|fastText + subword||N||Y||N||64.3||71.2||75.9||92.2||93.1||95.8|
|fastText + subword
+ BERT tok
|fastText + subword + + BERT tokentization + preE||Y||Y||N||71.5||76.9||87.9||93.2||75.7||93.4|
|TextCNN Kim (2014)||N||N||N||69.8||76.9||85.3||95.7||85.9||92.8||95.6|
|TextCNN + Character n-grams||N||N||N||70.6||78.1||87.1||96.3||85.9||93.2||95.9|
|TextCNN + BERT tokenization||N||N||Y||71.6||76.8||84.2||96.6||85.2||94.1||96.2|
|VDCNN (9 layers)||N||N||N||65.3||71.6||80.7||89.3||85.9||91.6||93.9|
|BERT (dropout = 0.2)||N||N||N||72.2||80.1||85.2||97.0||78.2||95.7||96.8|
We use multiple variants of fastText model. Our fastText uses embeddings learned for each unigram. We treat this as our baseline model without any preprocessing of the text. Our fastText model also uses bigrams along with unigrams as independent tokens to learn embeddings. All pairs of bigrams are chosen wtihout ant frequency threshold. Our fastText + subword also uses all subwords within a word boundary within the range of . All our models are trained with learning rate of and for epochs.
TextCNN Kim (2014):
We run the TextCNN for classification in non-static mode, with learning rate of , dropout of for epochs. We have used default kernel window sizes with filters. We initialize the embeddings layer with word2vec pretrained embeddings111https://code.google.com/archive/p/word2vec/ publicly available from google. We used the non-static variant of TextCNN, with pretrained embedding initialization for word embedding layer.
TextCNN + char n-grams:
The word embedding layer is constructed for this approach as mentioned in 1. The kernel window sizes for character tokens are with filters respectively. Increasing the number of filters further to match those of parameters in Peters et al. (2018) for character tokens led to overfitting on our datasets, and hence we reduced the parameters. All the layers are allowed to be tuned while training. The character embeddings CNN layer is initialized randomly with Xavier initialization Glorot and Bengio (2010). We set the character embedding layer output to , upon concatenation the word embedding length would be . This model is trained in exactly similar settings as the above mentioned word based TextCNN model.
Fully Character Embeddings Model:
For our BERT experiments we use the (uncased) model. model consists of 12 Transformer layers with 12 self-attention heads with 768 hidden dimensions and consists of 110 M total parameters. This model is trained in BookCorpus and English Wikipedia corpus. We attach a linear layer on top of model and the [CLS] token representation is fine-tuned on the training set. We use a binary cross-entropy loss to fine-tune BERT for our datasets. The fine tuned model is evaluated on the test set. We experimented with dropout values set at between the transformer encoder layers. We achieved best results at dropout of , which we report in our experiments.
5.1 BERT Wordpiece Tokenizer Model with Word models
We use Wordpiece (BPE) model of BERT Devlin et al. (2018) pretrained on BooksCorpus and English Wikipedia, produced using 30000 merge operations. BERT uses this model as precursor before encoding the text through transformer. We try to examine the benefit of the wordpiece text encoding vs the benefit we obtain from fine-tuning the pretrained LM. We hypothesize that pretrained BPE model splits a word into most frequent subwords found in the wikipedia corpus, which can help in mining the informative subwords. The informative subwords might prove very beneficial in noisy settings where we observe missing spaces and typos. In order to achieve this, we use this pretrained BPE model for encoding the document text before inputing to our word based models, TextCNN and fastText word variant. This is demonstrated in Figure 2. We have tried following variants with BERT Wordpiece tokenization as preprocessing step.
BERT Tokenizer with fastText & TextCNN Word model:
We preprocess the given dataset text using pretrained BPE model, and run a fastText bigram classification model on the preprocessed output. We also evaluate the TextCNN word model with the preprocessed text as input.
BERT Tokenizer with fastText subword:
The preprocessed dataset with BERT trained BPE for training fastText subword model as described in Section 5.
Custom BPE model on the dataset:
We also tried to examine if we would get a similar performance boost we obtained from BERT Wordpiece model by encoding text via a custom wordpiece model trained on the text. This helps us differentiate if the gains are from training a wordpiece model on a large text or if the gains are from using subword splitting. We used 30,000 number of merge operations for the custom BPE model, which is the same as in BERT BPE to aim for a meaningful comparison. We have also tried other values of merge operations from the custom BPE model, but none have yielded substantially better performance.
|Original||a complaint about your disruptive behavior here
: https : / / en . wikipedia . org / wiki / wikipedia : administrators
% 27_noticeboard / incidents # disruptive_users_vandalizing_article_about_spiro_koleka
|Custom BPE||complain about your disruptive behavior here
: https : / / en . wikipedia . org / wi@@ ki / wikipedia : administrators
% 27_noticeboard / incidents # disrup@@ ti@@ ve_@@ user@@ s_@@
vandali@@ z@@ ing_@@ article_@@ about_@@ spi@@ ro@@ _@@ ko@@ le@@ ka
|BERT tokentization||complain about your disrupt @@ive behavior here
: https : / / en . wikipedia . org / wiki / wikipedia : administrators
% 27 _ notice @@board / incidents # disrupt @@ive _ users
_ van @@dal @@izing @@_ article @@_ about @@_ sp @@iro @@_ ko @@le @@ka
6 Results and Analysis
Table 1 presents the Weighted F1 score based on the support of each of the classes in the test set for our classification task. For a classification problem with samples in the test set and classes, Weighted F1 score 222we use sklearn library for computing macro and weighted f1 scores in the paper https://scikit-learn.org/stable/modules/generated/sklearn.metrics.f1_score.html is defined as
where denotes the number of samples in class
. We have reported weighted F1 as the twitter data we obtained had only 17 samples for racism, with stratified CV split having only 4 samples on average. As the results on this label could be very random and prone to lot of variance due to very little number of samples in the train and test set, we choose to use weightedover macro . We also have observed very high variance among performance in different CV splits, hence report the numbers separately on each of them.
|WS Mishra et al. (2018)||84.4||85.4|
|CONTEXT HS + CNG Mishra et al. (2018)||87.4||89.3|
|fastText(ngrams=2, BERT BPE)||85.9||88.6|
|fastText(ngrams=2, BERT BPE, PreE)||86.8||88.6|
|Kim2014 (BERT BPE)||83.4||89.3|
|BERT (dropout = 0.2)||89.5||90.6|
Table 1 also mentions if each of the experiment involves using word splitting via BPE, either by pretrained BERT Wordpiece tokenization model, or by training a custom BPE model on our given dataset. We have also highlighted the individual best performance from a modeling architecture with a .
Table 2 presents the Macro score on W-ATT and W-TOX datasets. Macro score is defined as :
|Original||believe that he was the greatest mother-fucker in the world|
|BPE||believe that he was the greatest mother## -## fuck## er in the world|
|Original||many thanks for your leaving all edits alone in future with such idiotic diatribes|
|BPE||many thanks for your leaving all edit## s alone in future with such idiot## ic## dia## tri## bes|
We have picked the best performing models from 1 for macro comparison. We have also compared to previous approaches that have achieved best performance on these datasets. Mishra et al. (2018) reported Macro F1 on both validation and test data together. From their work it is unclear if the model is tuned on validation, and same data was used along with test to report numbers. Hence, we only use their number as reference. The main conclusions of these experiments are fourfold:
1. Pretrained BPE models transfer well:
Pretraining a Wordpiece model on a large general corpus like wikipedia, and using this for encoding input text by splitting words has shown significant improvements for all the word based models. The fastText word model with bigrams (row 3 in table 1) trained with BERT tokenization achieves the best performance on 1st split of the hatespeech data, and also shows improvement over the native fastText bigrams model on Wiki-ATT dataset. The same observation can be made with TextCNN word model with preprocessing by pretrained BERT Wordpiece tokenization model(row 11 in Table 1). However, we have either noticed a slight degradation or an insignificant improvement by applying BPE encoding with fastText subword based model. This is expected as breaking the informative subwords from BERT into much smaller units might result in lot of noisy updates.
2. Fine tuning pretrained language models:
We observe that fine-tuning large pretrained language models achieve best performance on toxicity dataset. BERT with dropout=0.2 achieves the best performance on most of the datasets and splits. It achieves better or at par performance over any word based model. Only fastText subwords and textCNN/fastText word based model trained on BERT Wordpiece tokenization preprocessing achieve higher performance compared to BERT finetuning. The gains from BERT Wordpiece tokenization model encoding to fastText word model outperforms performance of BERT model itself. We leave it as future work to further investigate the contribution from BPE Wordpiece tokenization to other classification tasks.
3. End to End Char models arent as effective as subword or word + char models:
Adding character based embedding to aid word embedding based models, and subword models enhance the performance over their pure word based modeling baselines. This proves the hyptohesis of modeling at subword level definitely is beneficial for detecting abusive language. Interestingly, end to end character models arent as effective, which demonstrates the basic fact – knowledge of word leads to a powerful representation, and word boundary information is still informative in noisy settings.
4. State-of-the-art performance on W-TOX and W-ATT with BERT finetuning:
Table 3 shows the results for Macro score of our models in comparison to previous approaches that have achieved best performance on these datasets. Mishra et al. (2018) reported Macro F1 on both validation and test data together. From their work it is unclear if the model is tuned on validation, and same data was used along with test to report numbers. Hence, we only use their number as reference. We have also observed better numbers with their approach. We have achieved state of the art macro score on W-ATT and W-TOX datasets with BERT finetuning. We have also added performance of BERT Wordpiece tokenized text with word based models for comparison, with their numbers running really close to those of BERT.
5. Effect of custom BPE model trained on the dataset:
We have noticed significant performance degaradation as reported in Table 1, by tokenizing the text with custom BPE model trained on the W-ATT and W-TOX corpus, in comparison to using the original text or the BERT BPE encoded text. It’s interesting to notice the text tokenized by BERT yields very informative subwords, that can help the word based model in comparison to subwords yielded by custom BPE model, even though the vocabulary size of both the models is very similar. Table 4 presents a qualitative example on how the BERT BPE mines informative subwords compared to the custom BPE model. One can note that BERT BPE model clearly splits the text on underscores & extracts stem of the word in few cases.
7 Qualitative Analysis
Table 4 represents couple of examples from W-ATT dataset, where the pure word based model has failed to detect abusive language, but the model trained and tested on BERT Wordpiece tokenized text is able to detect the . As we can see, Wordpiece model trained on large wikipedia text with 30k operations(BERT) doesnt merge or create relatively uncommon word like from and . This helps the model to just learn about clearly from training set, and later use this for clear demarcation.
8 Conclusion and Future Work
Existing literature has shown the importance of using finer units such as character or subword units to learn better models and robust representations for identifying abusive language in social media. In this work, we explore various combinations of such word decomposition techniques and present experiments that bring new insights and/or confirm previous findings. Additionally, we study the effectiveness of large pretrained language models trained on standard text in understanding noisy user generated text. We further investigate the effectiveness of subword units (“wordpieces”) learned for unsupervised language modeling can improve the performance of bag-of-words based text classification models such as fastText. We evaluate our models on Twitter hatespeeech, Wikipedia toxicity and attack datasets.
Our experiments demonstrate that encoding noisy text via BERT wordpiece tokenization model before passing it through word-based models (fastText and TextCNN) can boost the performance of word-based models and achieve state-of-the-art performance. Based on our experiments, we conclude that subword models perform competitively with character-based models and occasionally outperform them. We observe that adding character embeddings to TextCNN model can slightly boost the performance compared to word-CNN models.
Our experiments on fine-tuning BERT show improvements on both Wikipedia toxicity and attack datasets. We observe that BERT can effectively transfer pretrained information to classifying tweets and user comments despite the domain shift of pre-training on BookCorpus, Wikipedia Text . Future work in this direction could include pretraining BERT on huge collection of social media text, which might further enhance the performance of identifying abusive language on social media text. Recent work by Wiegand et al. (2019) highlights that most of the datasets that study abusive language are prone to data sampling bias and abusive language identification on realistic scenario is much harder with higher percentage of implicit content. A potential future direction would be to explore how pretrained models on generic text could incorporate or handle implicit abuse.
We would like to thank Faisal Ladhak for reviewing our work, and all anonymous reviewers for their valuable feedback and comments.
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