Question: What is the key challenge in cyberbullying research?
Evidence: As mentioned earlier, although research on cyberbullying detection is more limited than social studies on the phenomenon, some important advances have been made in recent years. In what follows, we present a brief overview of the most important natural language processing approaches to cyberbullying detection.
Although some studies have investigated the effectiveness of rule-based modelling BIBREF31 , the dominant approach to cyberbullying detection involves machine learning. Most machine learning approaches are based on supervised BIBREF32 , BIBREF33 , BIBREF34 or semi-supervised learning BIBREF35 . The former involves the construction of a classifier based on labeled training data, whereas semi-supervised approaches rely on classifiers that are built from a training corpus containing a small set of labeled and a large set of unlabelled instances (a method that is often used to handle data sparsity). As cyberbullying detection essentially involves the distinction between bullying and non-bullying posts, the problem is generally approached as a binary classification task where the positive class is represented by instances containing (textual) cyberbullying, while the negative class includes instances containing non-cyberbullying or `innocent' text.
A key challenge in cyberbullying research is the availability of suitable data, which is necessary to develop models that characterise cyberbullying. In recent years, only a few datasets have become publicly available for this particular task, such as the training sets provided in the context of the CAW 2.0 workshop and more recently, the Twitter Bullying Traces dataset BIBREF36 . As a result, several studies have worked with the former or have constructed their own corpus from social media websites that are prone to bullying content, such as YouTube BIBREF32 , BIBREF33 , Formspring BIBREF33 , and ASKfm BIBREF37 (the latter two are social networking sites where users can send each other questions or respond to them). Despite the bottleneck of data availability, existing approaches to cyberbullying detection have shown its potential, and the relevance of automatic text analysis techniques to ensure child safety online has been recognised BIBREF38 , BIBREF39 .
Among the first studies on cyberbullying detection are BIBREF34 , BIBREF31 , BIBREF33 , who explored the predictive power of INLINEFORM0 -grams (with and without tf-idf weighting), part-of-speech information (e.g. first and second pronouns), and sentiment information based on profanity lexicons for this task. Similar features were also exploited for the detection of cyberbullying events and fine-grained text categories related to cyberbullying BIBREF37 , BIBREF40 . More recent studies have demonstrated the added value of combining such content-based features with user-based information, such as including users' activities on a social network (i.e., the number of posts), their age, gender, location, number of friends and followers, and so on BIBREF32 , BIBREF35 , BIBREF41 . Moreover, semantic features have been explored to further improve classification performance of the task. To this end, topic model information BIBREF42 , as well as semantic relations between INLINEFORM1 -grams (according to a Word2Vec model BIBREF43 ) have been integrated.
As mentioned earlier, data collection remains a bottleneck in cyberbullying research. Although cyberbullying has been recognised as a serious problem (cf. Section SECREF1 ), real-world examples are often hard to find in public platforms. Naturally, the vast majority of communications do not contain traces of verbal aggression or transgressive behaviour. When constructing a corpus for machine learning purposes, this results in imbalanced datasets, meaning that one class (e.g. cyberbullying posts) is much less represented in the corpus than the other (e.g. non-cyberbullying posts). To tackle this problem, several studies have adopted resampling techniques BIBREF35 , BIBREF41 , BIBREF31 that create synthetic minority class examples or reduce the number of negative class examples (i.e., minority class oversampling and majority class undersampling BIBREF44 ).
Table TABREF9 presents a number of recent studies on cyberbullying detection, providing insight into the state of the art in cyberbullying research and the contribution of the current research to the domain.
The studies discussed in this section have demonstrated the feasibility of automatic cyberbullying detection in social media data by making use of a varied set of features. Most of them have, however, focussed on cyberbullying `attacks', or posts written by a bully. Moreover, it is not entirely clear if different forms of cyberbullying have been taken into account (e.g. sexual intimidation or harassment, or psychological threats), in addition to derogatory language or insults.
In the research described in this paper, cyberbullying is considered a complex phenomenon consisting of different forms of harmful behaviour online, which are described in more detail in our annotation scheme BIBREF30 . Purposing to facilitate manual monitoring efforts on social networks, we develop a system that automatically detects signals of cyberbullying, including attacks from bullies, as well as victim and bystander reactions. Similarly, BIBREF42 investigated bullying traces posted by different author roles (accuser, bully, reporter, victim). However, they collected tweets by using specific keywords (i.e., bully, bullied and bullying). As a result, their corpus contains many reports or testimonials of a cyberbullying incident (example 1), instead of actual signals that cyberbullying is going on. Moreover, their method implies that cyberbullying-related content devoid of such keywords will not be part of the training corpus.
`Some tweens got violent on the n train, the one boy got off after blows 2 the chest... Saw him cryin as he walkd away :( bullying not cool' BIBREF42 
For this research, English and Dutch social media data were annotated for different forms of cyberbullying, based on the actors involved in a cyberbullying incident. After preliminary experiments for Dutch BIBREF37 , BIBREF40 , we currently explore the viability of detecting cyberbullying-related posts in Dutch and English social media. To this end, binary classification experiments are performed exploiting a rich feature set and optimised hyperparameters.
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A key challenge in cyberbullying research is the availability of suitable data, which is necessary to develop models that characterise cyberbullying.
Answer: The key challenge is the availability of suitable data.