prompt_front: You are given a scientific article and a question. Answer the question as concisely as you can, using a single phrase or sentence if possible. If the question cannot be answered based on the information in the article, write "unanswerable". If the question is a yes/no question, answer "yes", "no", or "unanswerable". Do not provide any explanation.

Please answer the questions based on the text provided below. How is the dataset of hashtags sourced?
Context 1: Introduction
A hashtag is a keyphrase represented as a sequence of alphanumeric characters plus underscore, preceded by the # symbol  Hashtags play a central role in online communication by providing a tool to categorize the millions of posts generated daily on Twitter, Instagram, etc  They are useful in search, tracking content about a certain topic BIBREF0 , BIBREF1 , or discovering emerging trends BIBREF2  
Hashtags often carry very important information, such as emotion BIBREF3 , sentiment BIBREF4 , sarcasm BIBREF5 , and named entities BIBREF6 , BIBREF7   However, inferring the semantics of hashtags is non-trivial since many hashtags contain multiple tokens joined together, which frequently leads to multiple potential interpretations (e g , lion head vs  lionhead)  Table TABREF3 shows several examples of single- and multi-token hashtags  While most hashtags represent a mix of standard tokens, named entities and event names are prevalent and pose challenges to both human and automatic comprehension, as these are more likely to be rare tokens  Hashtags also tend to be shorter to allow fast typing, to attract attention or to satisfy length limitations imposed by some social media platforms  Thus, they tend to contain a large number of abbreviations or non-standard spelling variations (e g , #iloveu4eva) BIBREF8 , BIBREF9 , which hinders their understanding 
The goal of our study is to build efficient methods for automatically splitting a hashtag into a meaningful word sequence  Our contributions are:
Our new dataset includes segmentation for 12,594 unique hashtags and their associated tweets annotated in a multi-step process for higher quality than the previous dataset of 1,108 hashtags BIBREF10   We frame the segmentation task as a pairwise ranking problem, given a set of candidate segmentations  We build several neural architectures using this problem formulation which use corpus-based, linguistic and thesaurus based features  We further propose a multi-task learning approach which jointly learns segment ranking and single- vs  multi-token hashtag classification  The latter leads to an error reduction of 24 6% over the current state-of-the-art  Finally, we demonstrate the utility of our method by using hashtag segmentation in the downstream task of sentiment analysis  Feeding the automatically segmented hashtags to a state-of-the-art sentiment analysis method on the SemEval 2017 benchmark dataset results in a 2 6% increase in the official metric for the task 
Background and Preliminaries
Current approaches for hashtag segmentation can be broadly divided into three categories: (a) gazeteer and rule based BIBREF11 , BIBREF12 , BIBREF13 , (b) word boundary detection BIBREF14 , BIBREF15 , and (c) ranking with language model and other features BIBREF16 , BIBREF10 , BIBREF0 , BIBREF17 , BIBREF18   Hashtag segmentation approaches draw upon work on compound splitting for languages such as German or Finnish BIBREF19 and word segmentation BIBREF20 for languages with no spaces between words such as Chinese BIBREF21 , BIBREF22   Similar to our work, BIBREF10 BansalBV15 extract an initial set of candidate segmentations using a sliding window, then rerank them using a linear regression model trained on lexical, bigram and other corpus-based features  The current state-of-the-art approach BIBREF14 , BIBREF15 uses maximum entropy and CRF models with a combination of language model and hand-crafted features to predict if each character in the hashtag is the beginning of a new word 
Generating Candidate Segmentations  Microsoft Word Breaker BIBREF16 is, among the existing methods, a strong baseline for hashtag segmentation, as reported in BIBREF14 and BIBREF10   It employs a beam search algorithm to extract INLINEFORM0 best segmentations as ranked by the n-gram language model probability: INLINEFORM1
where INLINEFORM0 is the word sequence of segmentation INLINEFORM1 and INLINEFORM2 is the window size  More sophisticated ranking strategies, such as Binomial and word length distribution based ranking, did not lead to a further improvement in performance BIBREF16   The original Word Breaker was designed for segmenting URLs using language models trained on web data  In this paper, we reimplemented and tailored this approach to segmenting hashtags by using a language model specifically trained on Twitter data (implementation details in § SECREF26 )  The performance of this method itself is competitive with state-of-the-art methods (evaluation results in § SECREF46 )  Our proposed pairwise ranking method will effectively take the top INLINEFORM3 segmentations generated by this baseline as candidates for reranking 
However, in prior work, the ranking scores of each segmentation were calculated independently, ignoring the relative order among the top INLINEFORM0 candidate segmentations  To address this limitation, we utilize a pairwise ranking strategy for the first time for this task and propose neural architectures to model this 
Multi-task Pairwise Neural Ranking
We propose a multi-task pairwise neural ranking approach to better incorporate and distinguish the relative order between the candidate segmentations of a given hashtag  Our model adapts to address single- and multi-token hashtags differently via a multi-task learning strategy without requiring additional annotations  In this section, we describe the task setup and three variants of pairwise neural ranking models (Figure FIGREF11 ) 
Segmentation as Pairwise Ranking
The goal of hashtag segmentation is to divide a given hashtag INLINEFORM0 into a sequence of meaningful words INLINEFORM1   For a hashtag of INLINEFORM2 characters, there are a total of INLINEFORM3 possible segmentations but only one, or occasionally two, of them ( INLINEFORM4 ) are considered correct (Table TABREF9 ) 
We transform this task into a pairwise ranking problem: given INLINEFORM0 candidate segmentations { INLINEFORM1 }, we rank them by comparing each with the rest in a pairwise manner  More specifically, we train a model to predict a real number INLINEFORM2 for any two candidate segmentations INLINEFORM3 and INLINEFORM4 of hashtag INLINEFORM5 , which indicates INLINEFORM6 is a better segmentation than INLINEFORM7 if positive, and vice versa  To quantify the quality of a segmentation in training, we define a gold scoring function INLINEFORM8 based on the similarities with the ground-truth segmentation INLINEFORM9 : INLINEFORM10
We use the Levenshtein distance (minimum number of single-character edits) in this paper, although it is possible to use other similarity measurements as alternatives  We use the top INLINEFORM0 segmentations generated by Microsoft Word Breaker (§ SECREF2 ) as initial candidates 
Pairwise Neural Ranking Model
For an input candidate segmentation pair INLINEFORM0 , we concatenate their feature vectors INLINEFORM1 and INLINEFORM2 , and feed them into a feedforward network which emits a comparison score INLINEFORM3   The feature vector INLINEFORM4 or INLINEFORM5 consists of language model probabilities using Good-Turing BIBREF23 and modified Kneser-Ney smoothing BIBREF24 , BIBREF25 , lexical and linguistic features (more details in § SECREF23 )  For training, we use all the possible pairs INLINEFORM6 of the INLINEFORM7 candidates as the input and their gold scores INLINEFORM8 as the target  The training objective is to minimize the Mean Squared Error (MSE): DISPLAYFORM0
where INLINEFORM0 is the number of training examples 
To aggregate the pairwise comparisons, we follow a greedy algorithm proposed by BIBREF26 cohen1998learning and used for preference ranking BIBREF27   For each segmentation INLINEFORM0 in the candidate set INLINEFORM1 , we calculate a single score INLINEFORM2 , and find the segmentation INLINEFORM3 corresponding to the highest score  We repeat the same procedure after removing INLINEFORM4 from INLINEFORM5 , and continue until INLINEFORM6 reduces to an empty set  Figure FIGREF11 (a) shows the architecture of this model 
Margin Ranking (MR) Loss
As an alternative to the pairwise ranker (§ SECREF15 ), we propose a pairwise model which learns from candidate pairs INLINEFORM0 but ranks each individual candidate directly rather than relatively  We define a new scoring function INLINEFORM1 which assigns a higher score to the better candidate, i e , INLINEFORM2 , if INLINEFORM3 is a better candidate than INLINEFORM4 and vice-versa  Instead of concatenating the features vectors INLINEFORM5 and INLINEFORM6 , we feed them separately into two identical feedforward networks with shared parameters  During testing, we use only one of the networks to rank the candidates based on the INLINEFORM7 scores  For training, we add a ranking layer on top of the networks to measure the violations in the ranking order and minimize the Margin Ranking Loss (MR): DISPLAYFORM0
where INLINEFORM0 is the number of training samples  The architecture of this model is presented in Figure FIGREF11 (b) 
Adaptive Multi-task Learning
Both models in § SECREF15 and § SECREF17 treat all the hashtags uniformly  However, different features address different types of hashtags  By design, the linguistic features capture named entities and multi-word hashtags that exhibit word shape patterns, such as camel case  The ngram probabilities with Good-Turing smoothing gravitate towards multi-word segmentations with known words, as its estimate for unseen ngrams depends on the fraction of ngrams seen once which can be very low BIBREF28   The modified Kneser-Ney smoothing is more likely to favor segmentations that contain rare words, and single-word segmentations in particular  Please refer to § SECREF46 for a more detailed quantitative and qualitative analysis 
To leverage this intuition, we introduce a binary classification task to help the model differentiate single-word from multi-word hashtags  The binary classifier takes hashtag features INLINEFORM0 as the input and outputs INLINEFORM1 , which represents the probability of INLINEFORM2 being a multi-word hashtag  INLINEFORM3 is used as an adaptive gating value in our multi-task learning setup  The gold labels for this task are obtained at no extra cost by simply verifying whether the ground-truth segmentation has multiple words  We train the pairwise segmentation ranker and the binary single- vs  multi-token hashtag classifier jointly, by minimizing INLINEFORM4 for the pairwise ranker and the Binary Cross Entropy Error ( INLINEFORM5 ) for the classifier: DISPLAYFORM0
where INLINEFORM0 is the adaptive gating value, INLINEFORM1 indicates if INLINEFORM2 is actually a multi-word hashtag and INLINEFORM3 is the number of training examples  INLINEFORM4 and INLINEFORM5 are the weights for each loss  For our experiments, we apply equal weights 
More specifically, we divide the segmentation feature vector INLINEFORM0 into two subsets: (a) INLINEFORM1 with modified Kneser-Ney smoothing features, and (b) INLINEFORM2 with Good-Turing smoothing and linguistic features  For an input candidate segmentation pair INLINEFORM3 , we construct two pairwise vectors INLINEFORM4 and INLINEFORM5 by concatenation, then combine them based on the adaptive gating value INLINEFORM6 before feeding them into the feedforward network INLINEFORM7 for pairwise ranking: DISPLAYFORM0
We use summation with padding, as we find this simple ensemble method achieves similar performance in our experiments as the more complex multi-column networks BIBREF29   Figure FIGREF11 (c) shows the architecture of this model  An analogue multi-task formulation can also be used for the Margin Ranking loss as: DISPLAYFORM0
Features
We use a combination of corpus-based and linguistic features to rank the segmentations  For a candidate segmentation INLINEFORM0 , its feature vector INLINEFORM1 includes the number of words in the candidate, the length of each word, the proportion of words in an English dictionary or Urban Dictionary BIBREF30 , ngram counts from Google Web 1TB corpus BIBREF31 , and ngram probabilities from trigram language models trained on the Gigaword corpus BIBREF32 and 1 1 billion English tweets from 2010, respectively  We train two language models on each corpus: one with Good-Turing smoothing using SRILM BIBREF33 and the other with modified Kneser-Ney smoothing using KenLM BIBREF34   We also add boolean features, such as if the candidate is a named-entity present in the list of Wikipedia titles, and if the candidate segmentation INLINEFORM2 and its corresponding hashtag INLINEFORM3 satisfy certain word-shapes (more details in appendix SECREF61 ) 
Similarly, for hashtag INLINEFORM0 , we extract the feature vector INLINEFORM1 consisting of hashtag length, ngram count of the hashtag in Google 1TB corpus BIBREF31 , and boolean features indicating if the hashtag is in an English dictionary or Urban Dictionary, is a named-entity, is in camel case, ends with a number, and has all the letters as consonants  We also include features of the best-ranked candidate by the Word Breaker model 
Implementation Details
We use the PyTorch framework to implement our multi-task pairwise ranking model  The pairwise ranker consists of an input layer, three hidden layers with eight nodes in each layer and hyperbolic tangent ( INLINEFORM0 ) activation, and a single linear output node  The auxiliary classifier consists of an input layer, one hidden layer with eight nodes and one output node with sigmoid activation  We use the Adam algorithm BIBREF35 for optimization and apply a dropout of 0 5 to prevent overfitting  We set the learning rate to 0 01 and 0 05 for the pairwise ranker and auxiliary classifier respectively  For each experiment, we report results obtained after 100 epochs 
For the baseline model used to extract the INLINEFORM0 initial candidates, we reimplementated the Word Breaker BIBREF16 as described in § SECREF2 and adapted it to use a language model trained on 1 1 billion tweets with Good-Turing smoothing using SRILM BIBREF33 to give a better performance in segmenting hashtags (§ SECREF46 )  For all our experiments, we set INLINEFORM1  
Hashtag Segmentation Data
We use two datasets for experiments (Table TABREF29 ): (a) STAN INLINEFORM0 , created by BIBREF10 BansalBV15, which consists of 1,108 unique English hashtags from 1,268 randomly selected tweets in the Stanford Sentiment Analysis Dataset BIBREF36 along with their crowdsourced segmentations and our additional corrections; and (b) STAN INLINEFORM1 , our new expert curated dataset, which includes all 12,594 unique English hashtags and their associated tweets from the same Stanford dataset 
Experiments
In this section, we present experimental results that compare our proposed method with the other state-of-the-art approaches on hashtag segmentation datasets  The next section will show experiments of applying hashtag segmentation to the popular task of sentiment analysis 
Existing Methods
We compare our pairwise neural ranker with the following baseline and state-of-the-art approaches:
The original hashtag as a single token;
A rule-based segmenter, which employs a set of word-shape rules with an English dictionary BIBREF13 ;
A Viterbi model which uses word frequencies from a book corpus BIBREF0 ;
The specially developed GATE Hashtag Tokenizer from the open source toolkit, which combines dictionaries and gazetteers in a Viterbi-like algorithm BIBREF11 ;
A maximum entropy classifier (MaxEnt) trained on the STAN INLINEFORM0 training dataset  It predicts whether a space should be inserted at each position in the hashtag and is the current state-of-the-art BIBREF14 ;
Our reimplementation of the Word Breaker algorithm which uses beam search and a Twitter ngram language model BIBREF16 ;
A pairwise linear ranker which we implemented for comparison purposes with the same features as our neural model, but using perceptron as the underlying classifier BIBREF38 and minimizing the hinge loss between INLINEFORM0 and a scoring function similar to INLINEFORM1   It is trained on the STAN INLINEFORM2 dataset 
Evaluation Metrics
We evaluate the performance by the top INLINEFORM0 ( INLINEFORM1 ) accuracy (A@1, A@2), average token-level F INLINEFORM2 score (F INLINEFORM3 @1), and mean reciprocal rank (MRR)  In particular, the accuracy and MRR are calculated at the segmentation-level, which means that an output segmentation is considered correct if and only if it fully matches the human segmentation  Average token-level F INLINEFORM4 score accounts for partially correct segmentation in the multi-token hashtag cases 
Results
Tables TABREF32 and TABREF33 show the results on the STAN INLINEFORM0 and STAN INLINEFORM1 datasets, respectively  All of our pairwise neural rankers are trained on the 2,518 manually segmented hashtags in the training set of STAN INLINEFORM2 and perform favorably against other state-of-the-art approaches  Our best model (MSE+multitask) that utilizes different features adaptively via a multi-task learning procedure is shown to perform better than simply combining all the features together (MR and MSE)  We highlight the 24 6% error reduction on STAN INLINEFORM3 and 16 5% on STAN INLINEFORM4 of our approach over the previous SOTA BIBREF14 on the Multi-token hashtags, and the importance of having a separate evaluation of multi-word cases as it is trivial to obtain 100% accuracy for Single-token hashtags  While our hashtag segmentation model is achieving a very high accuracy@2, to be practically useful, it remains a challenge to get the top one predication exactly correct  Some hashtags are very difficult to interpret, e g , #BTVbrownSMB refers to the Social Media Breakfast (SMB) in Burlington, Vermont (BTV) 
The improved Word Breaker with our addition of a Twitter-specific language model is a very strong baseline, which echos the findings of the original Word Breaker paper BIBREF16 that having a large in-domain language model is extremely helpful for word segmentation tasks  It is worth noting that the other state-of-the-art system BIBREF14 also utilized a 4-gram language model trained on 476 million tweets from 2009 
Analysis and Discussion
To empirically illustrate the effectiveness of different features on different types of hashtags, we show the results for models using individual feature sets in pairwise ranking models (MSE) in Table TABREF45   Language models with modified Kneser-Ney smoothing perform best on single-token hashtags, while Good-Turing and Linguistic features work best on multi-token hashtags, confirming our intuition about their usefulness in a multi-task learning approach  Table TABREF47 shows a qualitative analysis with the first column ( INLINEFORM0 INLINEFORM1 INLINEFORM2 ) indicating which features lead to correct or wrong segmentations, their count in our data and illustrative examples with human segmentation 
As expected, longer hashtags with more than three tokens pose greater challenges and the segmentation-level accuracy of our best model (MSE+multitask) drops to 82 1%  For many error cases, our model predicts a close-to-correct segmentation, e g , #youbrownknowyoubrownupttoobrownearly, #iseebrownlondoniseebrownfrance, which is also reflected by the higher token-level F INLINEFORM0 scores across hashtags with different lengths (Figure FIGREF51 ) 
Since our approach heavily relies on building a Twitter language model, we experimented with its sizes and show the results in Figure FIGREF52   Our approach can perform well even with access to a smaller amount of tweets  The drop in F INLINEFORM0 score for our pairwise neural ranker is only 1 4% and 3 9% when using the language models trained on 10% and 1% of the total 1 1 billion tweets, respectively 
Language use in Twitter changes with time BIBREF9   Our pairwise ranker uses language models trained on the tweets from the year 2010  We tested our approach on a set of 500 random English hashtags posted in tweets from the year 2019 and show the results in Table TABREF55   With a segmentation-level accuracy of 94 6% and average token-level F INLINEFORM0 score of 95 6%, our approach performs favorably on 2019 hashtags 
Extrinsic Evaluation: Twitter Sentiment Analysis
We attempt to demonstrate the effectiveness of our hashtag segmentation system by studying its impact on the task of sentiment analysis in Twitter BIBREF39 , BIBREF40 , BIBREF41   We use our best model (MSE+multitask), under the name HashtagMaster, in the following experiments 
Experimental Setup
We compare the performance of the BiLSTM+Lex BIBREF42 sentiment analysis model under three configurations: (a) tweets with hashtags removed, (b) tweets with hashtags as single tokens excluding the # symbol, and (c) tweets with hashtags as segmented by our system, HashtagMaster  BiLSTM+Lex is a state-of-the-art open source system for predicting tweet-level sentiment BIBREF43   It learns a context-sensitive sentiment intensity score by leveraging a Twitter-based sentiment lexicon BIBREF44   We use the same settings as described by BIBREF42 teng-vo-zhang:2016:EMNLP2016 to train the model 
We use the dataset from the Sentiment Analysis in Twitter shared task (subtask A) at SemEval 2017 BIBREF41   Given a tweet, the goal is to predict whether it expresses POSITIVE, NEGATIVE or NEUTRAL sentiment  The training and development sets consist of 49,669 tweets and we use 40,000 for training and the rest for development  There are a total of 12,284 tweets containing 12,128 hashtags in the SemEval 2017 test set, and our hashtag segmenter ended up splitting 6,975 of those hashtags present in 3,384 tweets 
Results and Analysis
In Table TABREF59 , we report the results based on the 3,384 tweets where HashtagMaster predicted a split, as for the rest of tweets in the test set, the hashtag segmenter would neither improve nor worsen the sentiment prediction  Our hashtag segmenter successfully improved the sentiment analysis performance by 2% on average recall and F INLINEFORM0 comparing to having hashtags unsegmented  This improvement is seemingly small but decidedly important for tweets where sentiment-related information is embedded in multi-word hashtags and sentiment prediction would be incorrect based only on the text (see Table TABREF60 for examples)  In fact, 2,605 out of the 3,384 tweets have multi-word hashtags that contain words in the Twitter-based sentiment lexicon BIBREF44 and 125 tweets contain sentiment words only in the hashtags but not in the rest of the tweet  On the entire test set of 12,284 tweets, the increase in the average recall is 0 5% 
Other Related Work
Automatic hashtag segmentation can improve the performance of many applications besides sentiment analysis, such as text classification BIBREF13 , named entity linking BIBREF10 and modeling user interests for recommendations BIBREF45   It can also help in collecting data of higher volume and quality by providing a more nuanced interpretation of its content, as shown for emotion analysis BIBREF46 , sarcasm and irony detection BIBREF11 , BIBREF47   Better semantic analysis of hashtags can also potentially be applied to hashtag annotation BIBREF48 , to improve distant supervision labels in training classifiers for tasks such as sarcasm BIBREF5 , sentiment BIBREF4 , emotions BIBREF3 ; and, more generally, as labels for pre-training representations of words BIBREF49 , sentences BIBREF50 , and images BIBREF51  
Conclusion
We proposed a new pairwise neural ranking model for hashtag segmention and showed significant performance improvements over the state-of-the-art  We also constructed a larger and more curated dataset for analyzing and benchmarking hashtag segmentation methods  We demonstrated that hashtag segmentation helps with downstream tasks such as sentiment analysis  Although we focused on English hashtags, our pairwise ranking approach is language-independent and we intend to extend our toolkit to languages other than English as future work 
Acknowledgments
We thank Ohio Supercomputer Center BIBREF52 for computing resources and the NVIDIA for providing GPU hardware  We thank Alan Ritter, Quanze Chen, Wang Ling, Pravar Mahajan, and Dushyanta Dhyani for valuable discussions  We also thank the annotators: Sarah Flanagan, Kaushik Mani, and Aswathnarayan Radhakrishnan  This material is based in part on research sponsored by the NSF under grants IIS-1822754 and IIS-1755898, DARPA through the ARO under agreement number W911NF-17-C-0095, through a Figure-Eight (CrowdFlower) AI for Everyone Award and a Criteo Faculty Research Award to Wei Xu  The views and conclusions contained in this publication are those of the authors and should not be interpreted as representing official policies or endorsements of the U S  Government 
Word-shape rules
Our model uses the following word shape rules as boolean features  If the candidate segmentation INLINEFORM0 and its corresponding hashtag INLINEFORM1 satisfies a word shape rule, then the boolean feature is set to True 
Context 3: Introduction
Understanding the emotions expressed in a text or message is of high relevance nowadays  Companies are interested in this to get an understanding of the sentiment of their current customers regarding their products and the sentiment of their potential customers to attract new ones  Moreover, changes in a product or a company may also affect the sentiment of a customer  However, the intensity of an emotion is crucial in determining the urgency and importance of that sentiment  If someone is only slightly happy about a product, is a customer willing to buy it again? Conversely, if someone is very angry about customer service, his or her complaint might be given priority over somewhat milder complaints 
BIBREF0 present four tasks in which systems have to automatically determine the intensity of emotions (EI) or the intensity of the sentiment (Valence) of tweets in the languages English, Arabic, and Spanish  The goal is to either predict a continuous regression (reg) value or to do ordinal classification (oc) based on a number of predefined categories  The EI tasks have separate training sets for four different emotions: anger, fear, joy and sadness  Due to the large number of subtasks and the fact that this language does not have many resources readily available, we only focus on the Spanish subtasks  Our work makes the following contributions:
Our submissions ranked second (EI-Reg), second (EI-Oc), fourth (V-Reg) and fifth (V-Oc), demonstrating that the proposed method is accurate in automatically determining the intensity of emotions and sentiment of Spanish tweets  This paper will first focus on the datasets, the data generation procedure, and the techniques and tools used  Then we present the results in detail, after which we perform a small error analysis on the largest mistakes our model made  We conclude with some possible ideas for future work 
Data
For each task, the training data that was made available by the organizers is used, which is a selection of tweets with for each tweet a label describing the intensity of the emotion or sentiment BIBREF1   Links and usernames were replaced by the general tokens URL and @username, after which the tweets were tokenized by using TweetTokenizer  All text was lowercased  In a post-processing step, it was ensured that each emoji is tokenized as a single token 
Word Embeddings
To be able to train word embeddings, Spanish tweets were scraped between November 8, 2017 and January 12, 2018  We chose to create our own embeddings instead of using pre-trained embeddings, because this way the embeddings would resemble the provided data set: both are based on Twitter data  Added to this set was the Affect in Tweets Distant Supervision Corpus (DISC) made available by the organizers BIBREF0 and a set of 4 1 million tweets from 2015, obtained from BIBREF2   After removing duplicate tweets and tweets with fewer than ten tokens, this resulted in a set of 58 7 million tweets, containing 1 1 billion tokens  The tweets were preprocessed using the method described in Section SECREF6   The word embeddings were created using word2vec in the gensim library BIBREF3 , using CBOW, a window size of 40 and a minimum count of 5  The feature vectors for each tweet were then created by using the AffectiveTweets WEKA package BIBREF4  
Translating Lexicons
Most lexical resources for sentiment analysis are in English  To still be able to benefit from these sources, the lexicons in the AffectiveTweets package were translated to Spanish, using the machine translation platform Apertium BIBREF5  
All lexicons from the AffectiveTweets package were translated, except for SentiStrength  Instead of translating this lexicon, the English version was replaced by the Spanish variant made available by BIBREF6  
For each subtask, the optimal combination of lexicons was determined  This was done by first calculating the benefits of adding each lexicon individually, after which only beneficial lexicons were added until the score did not increase anymore (e g  after adding the best four lexicons the fifth one did not help anymore, so only four were added)  The tests were performed using a default SVM model, with the set of word embeddings described in the previous section  Each subtask thus uses a different set of lexicons (see Table TABREF1 for an overview of the lexicons used in our final ensemble)  For each subtask, this resulted in a (modest) increase on the development set, between 0 01 and 0 05 
Translating Data
The training set provided by BIBREF0 is not very large, so it was interesting to find a way to augment the training set  A possible method is to simply translate the datasets into other languages, leaving the labels intact  Since the present study focuses on Spanish tweets, all tweets from the English datasets were translated into Spanish  This new set of “Spanish” data was then added to our original training set  Again, the machine translation platform Apertium BIBREF5 was used for the translation of the datasets 
Algorithms Used
Three types of models were used in our system, a feed-forward neural network, an LSTM network and an SVM regressor  The neural nets were inspired by the work of Prayas BIBREF7 in the previous shared task  Different regression algorithms (e g  AdaBoost, XGBoost) were also tried due to the success of SeerNet BIBREF8 , but our study was not able to reproduce their results for Spanish 
For both the LSTM network and the feed-forward network, a parameter search was done for the number of layers, the number of nodes and dropout used  This was done for each subtask, i e  different tasks can have a different number of layers  All models were implemented using Keras BIBREF9   After the best parameter settings were found, the results of 10 system runs to produce our predictions were averaged (note that this is different from averaging our different type of models in Section SECREF16 )  For the SVM (implemented in scikit-learn BIBREF10 ), the RBF kernel was used and a parameter search was conducted for epsilon  Detailed parameter settings for each subtask are shown in Table TABREF12   Each parameter search was performed using 10-fold cross validation, as to not overfit on the development set 
Semi-supervised Learning
One of the aims of this study was to see if using semi-supervised learning is beneficial for emotion intensity tasks  For this purpose, the DISC BIBREF0 corpus was used  This corpus was created by querying certain emotion-related words, which makes it very suitable as a semi-supervised corpus  However, the specific emotion the tweet belonged to was not made public  Therefore, a method was applied to automatically assign the tweets to an emotion by comparing our scraped tweets to this new data set 
First, in an attempt to obtain the query-terms, we selected the 100 words which occurred most frequently in the DISC corpus, in comparison with their frequencies in our own scraped tweets corpus  Words that were clearly not indicators of emotion were removed  The rest was annotated per emotion or removed if it was unclear to which emotion the word belonged  This allowed us to create silver datasets per emotion, assigning tweets to an emotion if an annotated emotion-word occurred in the tweet

ground_truth: 1,268 randomly selected tweets in the Stanford Sentiment Analysis Dataset BIBREF36, all 12,594 unique English hashtags and their associated tweets from the same Stanford dataset

Stanford Sentiment Analysis Dataset BIBREF36

prompt_front: You are given a scientific article and a question. Answer the question as concisely as you can, using a single phrase or sentence if possible. If the question cannot be answered based on the information in the article, write "unanswerable". If the question is a yes/no question, answer "yes", "no", or "unanswerable". Do not provide any explanation.

Please answer the questions based on the text provided below. which languages are explored?
Context 2: Introduction
Part-of-speech tagging is now a classic task in natural language processing, for which many systems have been developed or adapted for a large variety of languages  Its aim is to associate each “word” with a morphosyntactic tag, whose granularity can range from a simple morphosyntactic category, or part-of-speech (hereafter PoS), to finer categories enriched with morphological features (gender, number, case, tense, mood, etc ) 
The use of machine learning algorithms trained on manually annotated corpora has long become the standard way to develop PoS taggers  A large variety of algorithms have been used, such as (in approximative chronological order) bigram and trigram hidden Markov models BIBREF0 , BIBREF1 , BIBREF2 , decision trees BIBREF3 , BIBREF4 , maximum entropy Markov models (MEMMs) BIBREF5 and Conditional Random Fields (CRFs) BIBREF6 , BIBREF7   With such machine learning algorithms, it is possible to build PoS taggers for any language, provided adequate training data is available 
As a complement to annotated corpora, it has previously been shown that external lexicons are valuable sources of information, in particular morphosyntactic lexicons, which provide a large inventory of (word, PoS) pairs  Such lexical information can be used in the form of constraints at tagging time BIBREF8 , BIBREF9 or during the training process as additional features combined with standard features extracted from the training corpus BIBREF10 , BIBREF11 , BIBREF12  
In recent years, a different approach to modelling lexical information and integrating it into natural language processing systems has emerged, namely the use of vector representations for words or word sequences BIBREF13 , BIBREF14 , BIBREF15 , BIBREF16 , BIBREF17 , BIBREF18   Such representations, which are generally extracted from large amounts of raw text, have proved very useful for numerous tasks including PoS tagging, in particular when used in recurrent neural networks (RNNs) and more specifically in mono- or bi-directional, word-level and/or character-level long short-term memory networks (LSTMs) BIBREF19 , BIBREF16 , BIBREF17 , BIBREF20  
Both approaches to representing lexical properties and to integrating them into a PoS tagger improve tagging results  Yet they rely on resources of different natures  The main advantage of word vectors is that they are built in an unsupervised way, only requiring large amounts of raw textual data  They also encode finer-grained information than usual morphosyntactic lexicons, most of which do not include any quantitative data, not even simple frequency information  Conversely, lexical resources often provide information about scarcely attested words, for which corpus-based approaches such as word vector representations are of limited relevance  Moreover, morphological or morphosyntactic lexicons already exist for a number of languages, including less-resourced langauges for which it might be difficult to obtain the large amounts of raw data necessary to extract word vector representations 
Our main goal is therefore to compare the respective impact of external lexicons and word vector representations on the accuracy of PoS models  This question has already been investigated for 6 languages by BIBREF18 using the state-of-the-art CRF-based tagging system MarMoT  The authors found that their best-performing word-vector-based PoS tagging models outperform their models that rely on morphosyntactic resources (lexicons or morphological analysers)  In this paper, we report on larger comparison, carried out in a larger multilingual setting and comparing different tagging models  Using different 16 datasets, we compare the performances of two feature-based models enriched with external lexicons and of two LSTM-based models enriched with word vector representations  A secondary goal of our work is to compare the relative improvements linked to the use of external lexical information in the two feature-based models, which use different models (MEMM vs  CRF) and feature sets 
More specifically, our starting point is the MElt system BIBREF12 , an MEMM tagging system  We first briefly describe this system and the way we adapted it by integrating our own set of corpus-based and lexical features  We then introduce the tagging models we have trained for 16 different languages using our adapted version of MElt  These models are trained on the Universal Dependencies (v1 2) corpus set BIBREF21 , complemented by morphosyntactic lexicons  We compare the accuracy of our models with the scores obtained by the CRF-based system MarMoT BIBREF22 , BIBREF18 , retrained on the same corpora and the same external morphosyntactic lexicons  We also compare our results to those obtained by the best bidirectional LSTM models described by BIBREF20 , which both make use of Polyglot word vector representations published by BIBREF23   We will show that an optimised enrichment of feature-based models with morphosyntactic lexicon results in significant accuracy gains  The macro-averaged accuracy of our enriched MElt models is above that of enriched MarMoT models and virtually identical to that of LSTMs enriched with word vector representations  More precisely, per-language results indicate that lexicons provide more useful information for languages with a high lexical variability (such as morphologically rich languages), whereas word vectors are more informative for languages with a lower lexical variability (such as English) 
MElt
MElt BIBREF12 is a tagging system based on maximum entropy Markov models (MEMM) BIBREF5 , a class of discriminative models that are suitable for sequence labelling BIBREF5   The basic set of features used by MElt is given in BIBREF12   It is a superset of the feature sets used by BIBREF5 and BIBREF24 and includes both local standard features (for example the current word itself and its prefixes and suffixes of length 1 to 4) and contextual standard features (for example the tag just assigned to the preceding word)  In particular, with respect to Ratnaparkhi's feature set, MElt's basic feature set lifts the restriction that local standard features used to analyse the internal composition of the current word should only apply to rare words 
One of the advantages of feature-based models such as MEMMs and CRFs is that complementary information can be easily added in the form of additional features  This was investigated for instance by BIBREF25 , whose best-performing model for PoS tagging dialogues was obtained with a version of MElt extended with dialogue-specific features  Yet the motivation of MElt's developers was first and foremost to investigate the best way to integrate lexical information extracted from large-scale morphosyntactic lexical resources into their models, on top of the training data BIBREF12   They showed that performances are better when this external lexical information is integrated in the form of additional lexical features than when the external lexicon is used as constraints at tagging time  These lexical features can also be divided into local lexical features (for example the list of possible tags known to the external lexicon for the current word) and contextual lexical features (for example the list of possible tags known to the external lexicon for surrounding words)  In particular, lexical contextual features provide a means to model the right context of the current word, made of words that have not yet been tagged by the system but for which the lexicon often provides a list of possible tags  Moreover, tagging accuracy for out-of-vocabulary (OOV) words is improved, as a result of the fact that words unknown to the training corpus might be known to the external lexicon 
Despite a few experiments published with MElt on languages other than French BIBREF12 , BIBREF40 , BIBREF41 , the original feature set used by MElt (standard and lexical features) was designed and tested mostly on this language, by building and evaluating tagging models on a variant of the French TreeBank  Since our goal was to carry out experiments in a multilingual setting, we have decided to design our own set of features, using the standard MElt features as a starting point  With respect to the original MElt feature set, we have added new ones, such as prefixes and suffixes of the following word, as well as a hybrid contextual feature obtained by concatenating the tag predicted for the preceding word and the tag(s) provided by the external lexicon for the following word 
In order to select the best performing feature set, we carried out a series of experiments using the multilingual dataset provided during the SPMRL parsing shared task BIBREF42   This included discarding useless or harmful features and selecting the maximal length of the prefixes and suffixes to be used as features, both for the current word and for the following word 
We incorporated in MElt the best performing feature set, described in Table TABREF1   All models discussed in this paper are based on this feature set 
Corpora
We carried out our experiments on the Universal Dependencies v1 2 treebanks BIBREF21 , hereafter UD1 2, from which morphosyntactically annotated corpora can be trivially extracted  All UD1 2 corpora use a common tag set, the 17 universal PoS tags, which is an extension of the tagset proposed by BIBREF43  
As our goal is to study the impact of lexical information for PoS tagging, we have restricted our experiments to UD1 2 corpora that cover languages for which we have morphosyntactic lexicons at our disposal, and for which BIBREF20 provide results  We considered UD1 2 corpora for the following 16 languages: Bulgarian, Croatian, Czech, Danish, English, French, German, Indonesian, Italian, Norwegian, Persian, Polish, Portuguese, Slovenian, Spanish and Swedish  Although this language list contains only one non-Indo-European (Indonesian), four major Indo-European sub-families are represented (Germanic, Romance, Slavic, Indo-Iranian)  Overall, the 16 languages considered in our experiments are typologically, morphologically and syntactically fairly diverse 
Lexicons
We generate our external lexicons using the set of source lexicons listed in Table TABREF3   Since external lexical information is exploited via features, there is no need for the external lexicons and the annotated corpora to use the same PoS inventory  Therefore, for each language, we simply extracted from the corresponding lexicon the PoS of each word based on its morphological tags, by removing all information provided except for its coarsest-level category  We also added entries for punctuations when the source lexicons did not contain any 
We also performed experiments in which we retained the full original tags provided by the lexicons, with all morphological features included  On average, results were slightly better than those presented in the paper, although not statistically significantly  Moreover, the granularity of tag inventories in the lexicons is diverse, which makes it difficult to draw general conclusions about results based on full tags  This is why we only report results based on (coarse) PoS extracted from the original lexicons 
Baseline models
In order to assess the respective contributions of external lexicons and word vector representations, we first compared the results of the three above-mentioned systems when trained without such additional lexical information  Table TABREF11 provides the results of MElt and MarMoT retrained on UD1 2 corpora, together with the results publised on the same corpora by BIBREF20 , using their best model not enhanced by external word vector representations —i e  the model they call INLINEFORM0 , which is a bidirectional LSTM that combines both word and character embeddings 
These results show that Plank et al 's (2016) bi-LSTM performs extremely well, surpassed by MarMoT on only 3 out of 16 datasets (Czech, French and Italian), and by MElt only once (Indonesian) 
Models enriched with external lexical information
Table TABREF13 provides the results of four systems enriched with lexical information  The feature-based systems MElt and MarMoT, respectively based on MEMMs and CRFs, are extended with the lexical information provided by our morphosyntactic lexicons  This extension takes the form of additional features, as described in Section SECREF2 for MElt  The results reported by BIBREF20 for their bidirectional LSTM when initialised with Polyglot embeddings trained on full wikipedias are also included, together with their new system FREQBIN, also initialised with Polyglot embeddings  FREQBIN trains bi-LSTMs to predict for each input word both a PoS and a label that represents its log frequency in the training data  As they word it, “the idea behind this model is to make the representation predictive for frequency, which encourages the model not to share representations between common and rare words, thus benefiting the handling of rare tokens ”
The results, which are also displayed in Figures FIGREF14 and FIGREF15 , show that all systems reach very similar results on average, although discrepancies can be observed from one dataset to another, on which we shall comment shortly  The best performing system in terms of macro-average is MElt (96 60%)  Both bi-LSTM systems reach the same score (96 58%), the difference with MElt's results being non significant, whereas MarMoT is only 0 14% behind (96 46%)  Given the better baseline scores of the neural approaches, these results show that the benefit of using external lexicons in the feature-based models MElt and MarMoT are much higher than those using Polyglot word vector representations as initialisations for bi-LSTMs 
Yet these very similar overall results reflect a different picture when focusing on OOV tagging accuracy  The best models for OOV tagging accuracy are, by far, FREQBIN models, which are beaten by MarMoT and by MElt only once each (on English and Danish respectively)  The comparison on OOV tagging between MElt and MarMoT shows that MElt performs better on average than MarMoT, despite the fact that MarMoT's baseline results were better than those reached by MElt  This shows that the information provided by external morphosyntactic lexicons is better exploited by MElt's lexical features than by those used by MarMoT  On the other hand, the comparison of both bi-LSTM-based approaches confirm that the FREQBIN models is better by over 10% absolute on OOV tagging accuracy (94 28% vs  83 59%), with 65% lower error rate 
One of the important differences between the lexical information provided by an external lexicon and word vectors built from raw corpora, apart from the very nature of the lexical information provided, is the coverage and accuracy of this lexical information on rare words  All words in a morphosyntactic lexicon are associated with information of a same granularity and quality, which is not the case with word representations such as provided by Polyglot  Models that take advantage of external lexicons should therefore perform comparatively better on datasets containing a higher proportion of rarer words, provided the lexicons' coverage is high  In order to confirm this intuition, we have used a lexical richness metric based on the type/token ratio  Since this ratio is well-known for being sensitive to corpus length, we normalised it by computing it over the 60,000 first tokens of each training set  When this normalised type/token ratio is plotted against the difference between the results of MElt and both bi-LSTM-based models, the expected correlation is clearly visible (see Figure FIGREF16 )  This explains why MElt obtains better results on the morphologically richer Slavic datasets (average normalised type/token ratio: 0 28, average accuracy difference: 0 32 compared to both bi-LSTM+Polyglot and FREQBIN+Polyglot) and, at the other end of the spectrum, significantly worse results on the English dataset (normalised type/token ratio: 0 15, average accuracy difference: -0 56 compared to bi-LSTM+Polyglot, -0 57 compared to FREQBIN+Polyglot) 
Conclusion
Two main conclusions can be drawn from our comparative results  First, feature-based tagging models adequately enriched with external morphosyntactic lexicons perform, on average, as well as bi-LSTMs enriched with word embeddings  Per-language results show that the best accuracy levels are reached by feature-based models, and in particular by our improved version of the MEMM-based system MElt, on datasets with high lexical variability (in short, for morphologically rich languages), whereas neural-based results perform better on datatsets with lower lexical variability (e g  for English) 
We have only compared the contribution of morphosyntactic lexicons to feature-based models (MEMMs, CRFs) and that of word vector representations to bi-LSTM-based models as reported by BIBREF20   As mentioned above, work on the contribution of word vector representations to feature-based approaches has been carried out by BIBREF18   However, the exploitation of existing morphosyntactic or morphological lexicons in neural models is a less studied question  Improvements over the state of the art might be achieved by integrating lexical information both from an external lexicon and from word vector representations into tagging models 
In that regard, further work will be required to understand which class of models perform the best  An option would be to integrate feature-based models such as a CRF with an LSTM-based layer, following recent proposals such as the one proposed by BIBREF45 for named entity recognition 

ground_truth: Bulgarian, Croatian, Czech, Danish, English, French, German, Indonesian, Italian, Norwegian, Persian, Polish, Portuguese, Slovenian, Spanish and Swedish

Bulgarian, Croatian, Czech, Danish, English, French, German, Indonesian, Italian, Norwegian, Persian, Polish, Portuguese, Slovenian, Spanish , Swedish

prompt_front: You are given a scientific article and a question. Answer the question as concisely as you can, using a single phrase or sentence if possible. If the question cannot be answered based on the information in the article, write "unanswerable". If the question is a yes/no question, answer "yes", "no", or "unanswerable". Do not provide any explanation.

Please answer the questions based on the text provided below. How was the training data translated?
Context 3: 
Our semi-supervised approach is quite straightforward: first a model is trained on the training set and then this model is used to predict the labels of the silver data  This silver data is then simply added to our training set, after which the model is retrained  However, an extra step is applied to ensure that the silver data is of reasonable quality  Instead of training a single model initially, ten different models were trained which predict the labels of the silver instances  If the highest and lowest prediction do not differ more than a certain threshold the silver instance is maintained, otherwise it is discarded 
This results in two parameters that could be optimized: the threshold and the number of silver instances that would be added  This method can be applied to both the LSTM and feed-forward networks that were used  An overview of the characteristics of our data set with the final parameter settings is shown in Table TABREF14   Usually, only a small subset of data was added to our training set, meaning that most of the silver data is not used in the experiments  Note that since only the emotions were annotated, this method is only applicable to the EI tasks 
Ensembling
To boost performance, the SVM, LSTM, and feed-forward models were combined into an ensemble  For both the LSTM and feed-forward approach, three different models were trained  The first model was trained on the training data (regular), the second model was trained on both the training and translated training data (translated) and the third one was trained on both the training data and the semi-supervised data (silver)  Due to the nature of the SVM algorithm, semi-supervised learning does not help, so only the regular and translated model were trained in this case  This results in 8 different models per subtask  Note that for the valence tasks no silver training data was obtained, meaning that for those tasks the semi-supervised models could not be used 
Per task, the LSTM and feed-forward model's predictions were averaged over 10 prediction runs  Subsequently, the predictions of all individual models were combined into an average  Finally, models were removed from the ensemble in a stepwise manner if the removal increased the average score  This was done based on their original scores, i e  starting out by trying to remove the worst individual model and working our way up to the best model  We only consider it an increase in score if the difference is larger than 0 002 (i e  the difference between 0 716 and 0 718)  If at some point the score does not increase and we are therefore unable to remove a model, the process is stopped and our best ensemble of models has been found  This process uses the scores on the development set of different combinations of models  Note that this means that the ensembles for different subtasks can contain different sets of models  The final model selections can be found in Table TABREF17  
Results and Discussion
Table TABREF18 shows the results on the development set of all individuals models, distinguishing the three types of training: regular (r), translated (t) and semi-supervised (s)  In Tables TABREF17 and TABREF18 , the letter behind each model (e g  SVM-r, LSTM-r) corresponds to the type of training used  Comparing the regular and translated columns for the three algorithms, it shows that in 22 out of 30 cases, using translated instances as extra training data resulted in an improvement  For the semi-supervised learning approach, an improvement is found in 15 out of 16 cases  Moreover, our best individual model for each subtask (bolded scores in Table TABREF18 ) is always either a translated or semi-supervised model  Table TABREF18 also shows that, in general, our feed-forward network obtained the best results, having the highest F-score for 8 out of 10 subtasks 
However, Table TABREF19 shows that these scores can still be improved by averaging or ensembling the individual models  On the dev set, averaging our 8 individual models results in a better score for 8 out of 10 subtasks, while creating an ensemble beats all of the individual models as well as the average for each subtask  On the test set, however, only a small increase in score (if any) is found for stepwise ensembling, compared to averaging  Even though the results do not get worse, we cannot conclude that stepwise ensembling is a better method than simply averaging 
Our official scores (column Ens Test in Table TABREF19 ) have placed us second (EI-Reg, EI-Oc), fourth (V-Reg) and fifth (V-Oc) on the SemEval AIT-2018 leaderboard  However, it is evident that the results obtained on the test set are not always in line with those achieved on the development set  Especially on the anger subtask for both EI-Reg and EI-Oc, the scores are considerably lower on the test set in comparison with the results on the development set  Therefore, a small error analysis was performed on the instances where our final model made the largest errors 
Error Analysis
Due to some large differences between our results on the dev and test set of this task, we performed a small error analysis in order to see what caused these differences  For EI-Reg-anger, the gold labels were compared to our own predictions, and we manually checked 50 instances for which our system made the largest errors 
Some examples that were indicative of the shortcomings of our system are shown in Table TABREF20   First of all, our system did not take into account capitalization  The implications of this are shown in the first sentence, where capitalization intensifies the emotion used in the sentence  In the second sentence, the name Imperator Furiosa is not understood  Since our texts were lowercased, our system was unable to capture the named entity and thought the sentence was about an angry emperor instead  In the third sentence, our system fails to capture that when you are so angry that it makes you laugh, it results in a reduced intensity of the angriness  Finally, in the fourth sentence, it is the figurative language me infla la vena (it inflates my vein) that the system is not able to understand 
The first two error-categories might be solved by including smart features regarding capitalization and named entity recognition  However, the last two categories are problems of natural language understanding and will be very difficult to fix 
Conclusion
To conclude, the present study described our submission for the Semeval 2018 Shared Task on Affect in Tweets  We participated in four Spanish subtasks and our submissions ranked second, second, fourth and fifth place  Our study aimed to investigate whether the automatic generation of additional training data through translation and semi-supervised learning, as well as the creation of stepwise ensembles, increase the performance of our Spanish-language models  Strong support was found for the translation and semi-supervised learning approaches; our best models for all subtasks use either one of these approaches  These results suggest that both of these additional data resources are beneficial when determining emotion intensity (for Spanish)  However, the creation of a stepwise ensemble from the best models did not result in better performance compared to simply averaging the models  In addition, some signs of overfitting on the dev set were found  In future work, we would like to apply the methods (translation and semi-supervised learning) used on Spanish on other low-resource languages and potentially also on other tasks 

ground_truth: using the machine translation platform Apertium 

machine translation platform Apertium BIBREF5