Current NLP tools tend to work poorly on social media texts, because these texts are informal, not carefully edited, and they contain grammatical errors, misspellings, new types of abbreviations, emoticons, etc. They are very different than the types of texts used for training the NLP tools. Therefore, the tools need to be adapted in order to achieve reasonable levels of performance on social media texts.
Table 2.1 shows three examples of Twitter messages, taken from Ritter et al. [2011], just to illustrate how noisy the texts can be.
Table 2.1: Three examples of Twitter texts
There are two ways to adapt NLP tools to social media texts. The first one is to perform text normalization so that the informal language becomes closer to the type of texts on which the tools were trained. The second one is to re-train the models inside the tool on annotated social media texts. Depending on the goal of the NLP application, a combination of the two techniques could be used, since both have their own limitations, as discussed below (see Eisenstein [2013b] for a more detailed discussion).
2.2.1 TEXT NORMALIZATION
Text normalization is a possible solution for overcoming or reducing linguistic noise. The task can be approached in two stages: first, the identification of orthographic errors in an input text, and second, the correction of these errors. Normalization approaches typically include a dictionary of known correctly spelled terms, and detects in-vocabulary and out-of-vocabulary (OOV) terms with respect to this dictionary. The normalization can be basic or more advanced. Basic normalization deals with the errors detected at the POS tagging stage, such as unknown words, misspelled words, etc. Advanced normalization is more flexible, taking a lightly supervised automatic approach trained on an external dataset (annotated with short forms vs. their equivalent long or corrected forms).
For social media texts, the normalization that can be done is rather shallow. Because of its informal and conversational nature, social media text cannot become carefully edited English. Similar issues appear in SMS text messages on phones, where short forms and phonetic abbreviations are often used to save the typing time. According to Derczynski et al. [2013b], text normalization in Twitter messages did not help too much in the named entity recognition task.
Twitter text normalization into traditional written English [Han and Baldwin, 2011] is not only difficult, but it can be viewed as a “lossy” translation task. For example, many of Twitter’s unique linguistic phenomena are due not only to its informal nature, but also to a set of authors that is heavily skewed toward younger ages and minorities, with heavy usage of dialects that are different than standard English [Eisenstein, 2013a, Eisenstein et al., 2011].
Demir [2016] describes a method of context-tailored text normalization. The method considers contextual and lexical similarities between standard and non-standard words, in order to reduce noise. The non-standard words in the input context in a given sentence are tailored into a direct match, if there are possible shared contexts. A morphological parser is used to analyze all the words in each sentence. Turkish social media texts were used to evaluate the performance of the system. The dataset contains tweets (~11 GB) and clean Turkish texts (~6 GB). The system achieved state-of-the-art results on the 715 Turkish tweets.
Akhtar et al. [2015] proposed a hybrid approach for text normalization for tweets. Their methodology proceeds in two phases: the first one detects noisy text, and the second one uses various heuristic-based rules for normalization. The researchers trained a supervised learning model, using 3-fold cross validation to determine the best feature set. Figure 2.1 depicts a schematic diagram of the proposed approach. Their system yielded precision, recall, and F-measure values of 0.90, 0.72, and 0.80, respectively, for their test dataset.
Most practical applications leverage the simpler approach of replacing non-standard words with their standard counterparts as a “one size fits all” task. Baldwin and Li [2015] devised a method that uses a taxonomy of normalization edits. The researchers evaluated this method on three different downstream applications: dependency parsing, named entity recognition, and text-to-speech synthesis. The taxonomy of normalization edits is shown in Figure 2.2. The method categorizes edits at three levels of granularity and its results demonstrate that the targeted application of the taxonomy is an efficient approach to normalization.
Figure 2.1: Methodology for tweet normalization. The dotted horizontal line separates the two steps (detecting the text to be normalized and applying normalization rules) [Akhtar et al., 2015].
Figure 2.2: Taxonomy of normalization edits [Baldwin and Li, 2015].
2.2.2 RE-TRAINING NLP TOOLS FOR SOCIAL MEDIA TEXTS
Re-training NLP tools for social media texts is relatively easy if annotated training data are available. In general, adapting a tool to a specific domain or a specific type of text requires producing annotated training data for that kind of text. It is easy to collect text of the required kind, but to annotate it can be a difficult and time-consuming process.
Currently, some annotated social media data have become available, but the volume is not high enough. Several NLP tools have been re-trained on newly annotated data, sometimes by also keeping the original annotated training data for newspaper texts, in order to have a large enough training set. Another approach is to use some unannotated social media text in an unsupervised manner in addition to the small amounts of annotated social media text.
Another question is what kinds of social media texts to use for training. It seems that Twitter messages are more difficult to process than blog posts or messages from forums. Because of the limitation of Twitter messages to 140 characters, more abbreviations and shortened forms of words are used, and more simplified syntax. Therefore, training data should include several kinds of social media texts (unless somebody is building a tool designed for a particular kind of social media text).
We define the tasks accomplished by each kind of tool and we discuss techniques for adapting them to social media texts.
2.3 TOKENIZERS
The first step in processing a text is to separate the words from punctuation and other symbols. A tool that does this is called a tokenizer. White space is a good indicator of words separation (except in some languages, e.g., Chinese), but even white space is not sufficient. The question of what is a word is not trivial. When doing corpus analysis, there are strings of characters that are clearly words, but there are strings for which this is not clear. Most of the time, punctuation needs to be separated from words, but some abbreviations might contain punctuation characters as part of the word. Take, for example, the sentence: “We bought apples, oranges, etc.” The commas clearly need to be separated from the word “apples” and from the word “oranges,” but the dot is part of the abbreviation “etc.” In this case, the dot also indicates the end of the sentence (two dots were reduced to one). Other examples among the many issues that appear are: how to treat numbers (if they contain commas or dots, these characters should not be separated), or what to do with contractions such as “don’t” (perhaps to expand them into two words “do” and “not”).
While tokenization usually consists of two subtasks (sentence boundary detection and token boundary detection), the EmpiriST shared task1 provided sentence boundaries and the participating teams only had to detect token boundaries. Missing whitespace characters presents a major challenge to the task of tokenization. Table 2.2 shows a few examples with their correct tokenization.
Methods