Question: What data cleaning setups were executed?
Evidence: As pre-processing, we applied tokenisation, PoS-tagging and lemmatisation to the data using the LeTs Preprocess Toolkit BIBREF60 . In supervised learning, a machine learning algorithm takes a set of training instances (of which the label is known) and seeks to build a model that generates a desired prediction for an unseen instance. To enable the model construction, all instances are represented as a vector of features (i.e., inherent characteristics of the data) that contain information that is potentially useful to distinguish cyberbullying from non-cyberbullying content.
We experimentally tested whether cyberbullying events can be recognised automatically by lexical markers in a post. To this end, all posts were represented by a number of information sources (or features) including lexical features like bags-of-words, sentiment lexicon features and topic model features, which are described in more detail below. Prior to feature extraction, some data cleaning steps were executed, such as the replacement of hyperlinks and @-replies, removal of superfluous white spaces, and the replacement of abbreviations by their full form (based on an existing mapping dictionary ). Additionally, tokenisation was applied before INLINEFORM0 -gram extraction and sentiment lexicon matching, and stemming was applied prior to extracting topic model features.
After pre-processing of the corpus, the following feature types were extracted:
Word INLINEFORM0 -gram bag-of-words: binary features indicating the presence of word unigrams, bigrams and trigrams.
Character INLINEFORM0 -gram bag-of-words: binary features indicating the presence of character bigrams, trigrams and fourgrams (without crossing word boundaries). Character INLINEFORM1 -grams provide some abstraction from the word level and provide robustness to the spelling variation that characterises social media data.
Term lists: one binary feature derived for each one out of six lists, indicating the presence of an item from the list in a post: proper names, `allness' indicators (e.g. always, everybody), diminishers (e.g. slightly, relatively), intensifiers (e.g. absolutely, amazingly), negation words and aggressive language and profanity words. Person alternation is a binary feature indicating whether the combination of a first and second person pronoun occurs in order to capture interpersonal intent.
Subjectivity lexicon features: positive and negative opinion word ratios, as well as the overall post polarity were calculated using existing sentiment lexicons. For Dutch, we made use of the Duoman BIBREF61 and Pattern BIBREF62 lexicons. For English, we included the Hu and Liu opinion lexicon BIBREF63 , the MPQA lexicon BIBREF64 , General Inquirer Sentiment Lexicon BIBREF65 , AFINN BIBREF66 , and MSOL BIBREF67 . For both languages, we included the relative frequency of all 68 psychometric categories in the Linguistic Inquiry and Word Count (LIWC) dictionary for English BIBREF68 and Dutch BIBREF69 .
Topic model features: by making use of the Gensim topic modelling library BIBREF70 , several LDA BIBREF71 and LSI BIBREF72 topic models with varying granularity ( INLINEFORM0 = 20, 50, 100 and 200) were trained on data corresponding to each fine-grained category of a cyberbullying event (e.g. threats, defamations, insults, defenses). The topic models were based on a background corpus (EN: INLINEFORM1 tokens, NL: INLINEFORM2 tokens) scraped with the BootCAT BIBREF73 web-corpus toolkit. BootCaT collects ASKfm user profiles using lists of manually determined seed words that are characteristic of the cyberbullying categories.
When applied to the training data, this resulted in INLINEFORM0 and INLINEFORM1 features for English and Dutch, respectively.
Prior to feature extraction, some data cleaning steps were executed, such as the replacement of hyperlinks and @-replies, removal of superfluous white spaces, and the replacement of abbreviations by their full form.
Answer: The replacement and @-replies, removal of superfluous white spaces, and the replacement of abbreviations were used.