Abstract: While social media offer great communication opportunities, they also increase the vulnerability of young people to threatening situations online. Recent studies report that cyberbullying constitutes a growing problem among youngsters. Successful prevention depends on the adequate detection of potentially harmful messages and the information overload on the Web requires intelligent systems to identify potential risks automatically. The focus of this paper is on automatic cyberbullying detection in social media text by modelling posts written by bullies, victims, and bystanders of online bullying. We describe the collection and fine-grained annotation of a training corpus for English and Dutch and perform a series of binary classification experiments to determine the feasibility of automatic cyberbullying detection. We make use of linear support vector machines exploiting a rich feature set and investigate which information sources contribute the most for this particular task. Experiments on a holdout test set reveal promising results for the detection of cyberbullying-related posts. After optimisation of the hyperparameters, the classifier yields an F1-score of 64% and 61% for English and Dutch respectively, and considerably outperforms baseline systems based on keywords and word unigrams.

Introduction: Web 2.0 has had a substantial impact on communication and relationships in today's society. Children and teenagers go online more frequently, at younger ages, and in more diverse ways (e.g. smartphones, laptops and tablets). Although most of teenagers' Internet use is harmless and the benefits of digital communication are evident, the freedom and anonymity experienced online makes young people vulnerable, with cyberbullying being one of the major threats BIBREF0 , BIBREF1 , BIBREF2 . Bullying is not a new phenomenon, and cyberbullying has manifested itself as soon as digital technologies have become primary communication tools. On the positive side, social media like blogs, social networking sites (e.g. Facebook) and instant messaging platforms (e.g. WhatsApp) make it possible to communicate with anyone and at any time. Moreover, they are a place where people engage in social interaction, offering the possibility to establish new relationships and maintain existing friendships BIBREF3 , BIBREF4 . On the negative side however, social media increase the risk of children being confronted with threatening situations including grooming or sexually transgressive behaviour, signals of depression and suicidal thoughts, and cyberbullying. Users are reachable 24/7 and are often able to remain anonymous if desired: this makes social media a convenient way for bullies to target their victims outside the school yard. With regard to cyberbullying, a number of national and international initiatives have been launched over the past few years to increase children's online safety. Examples include KiVa, a Finnish cyberbullying prevention programme, the `Non au harcèlement' campaign in France, Belgian governmental initiatives and helplines (e.g. clicksafe.be, veiligonline.be, mediawijs.be) that provide information about online safety, and so on. In spite of these efforts, a lot of undesirable and hurtful content remains online. BIBREF1 analysed a body of quantitative research on cyberbullying and observed cybervictimisation rates among teenagers between 20% and 40%. BIBREF5 focused on 12 to 17 year olds living in the United States and found that no less than 72% of them had encountered cyberbullying at least once within the year preceding the questionnaire. BIBREF6 surveyed 9 to 26 year olds in the United States, Canada, the United Kingdom and Australia, and found that 29% of the respondents had ever been victimised online. A study among 2,000 Flemish secondary school students (age 12 to 18) revealed that 11% of them had been bullied online at least once in the six months preceding the survey BIBREF7 . Finally, the 2014 large-scale EU Kids Online Report BIBREF8 published that 20% of 11 to 16 year olds had been exposed to hate messages online. In addition, youngsters were 12% more likely to be exposed to cyberbullying as compared to 2010, clearly demonstrating that cyberbullying is a growing problem. The prevalence of cybervictimisation depends on the conceptualisation used in describing cyberbullying, but also on research variables such as location and the number and age span of its participants. Nevertheless, the above-mentioned studies demonstrate that online platforms are increasingly used for bullying, which is a cause for concern given its impact. As shown by BIBREF9 , BIBREF10 , BIBREF11 , cyberbullying can have a negative impact on the victim's self-esteem, academic achievement and emotional well-being. BIBREF12 found that self-reported effects of cyberbullying include negative effects on school grades, feelings like sadness, anger, fear, and depression and in extreme cases, cyberbullying could even lead to self-harm and suicidal thoughts. The above studies demonstrate that cyberbullying is a serious problem the consequences of which can be dramatic. Successful early detection of cyberbullying attempts is therefore of key importance to youngsters' mental well-being. However, the amount of information on the Web makes it practically unfeasible for moderators to monitor all user-generated content manually. To tackle this problem, intelligent systems are required that process this information in a fast way and automatically signal potential threats. This way, moderators can respond quickly and prevent threatening situations from escalating. According to recent research, teenagers are generally in favour of such automatic monitoring, provided that effective follow-up strategies are formulated, and that privacy and autonomy are guaranteed BIBREF13 . Parental control tools (e.g. NetNanny) already block unsuited or undesirable content and some social networks make use of keyword-based moderation tools (i.e., using lists of profane and insulting words to flag harmful content). However, such approaches typically fail to detect implicit or subtle forms of cyberbullying in which no explicit vocabulary is used. There is therefore a need for intelligent and self-learning systems that can go beyond keyword spotting and hence improve recall of cyberbullying detection. The ultimate goal of this sort of research is to develop models which could improve manual monitoring for cyberbullying on social networks. We explore the automatic detection of textual signals of cyberbullying, in which it is approached as a complex phenomenon that can be realised in various ways (see Section SECREF15 for a detailed overview). While a lot of the related research focuses on the detection of cyberbullying `attacks', the present study takes into account a broader range of textual signals of cyberbullying, including posts written by bullies, as well as by victims and bystanders. We propose a machine learning method to cyberbullying detection by making use of a linear SVM classifier BIBREF14 , BIBREF15 exploiting a varied set of features. To the best of our knowledge, this is the first approach to the annotation of fine-grained text categories related to cyberbullying and the detection of signals of cyberbullying events. It is also the first elaborate research on automatic cyberbullying detection on Dutch social media. For the present experiments, we focus on an English and Dutch ASKfm corpus, but the methodology adopted is language and genre independent, provided there is annotated data available. The remainder of this paper is structured as follows: the next section presents a theoretic overview and gives an overview of the state of the art in cyberbullying detection, whereas Section SECREF3 describes the corpus. Next, we present the experimental setup and discuss our experimental results. Finally, Section SECREF6 concludes this paper and provides perspectives for further research.

Question: How long is the training dataset for English?
Answer: 113698