- Open Access
Emotion classification of social media posts for estimating people’s reactions to communicated alert messages during crises
Security Informatics volume 3, Article number: 7 (2014)
One of the key factors influencing how people react to and behave during a crisis is their digital or non-digital social network, and the information they receive through this network. Publicly available online social media sites make it possible for crisis management organizations to use some of these experiences as input for their decision-making. We describe a methodology for collecting a large number of relevant tweets and annotating them with emotional labels. This methodology has been used for creating a training dataset consisting of manually annotated tweets from the Sandy hurricane. Those tweets have been utilized for building machine learning classifiers able to automatically classify new tweets. Results show that a support vector machine achieves the best results with about 60% accuracy on the multi-classification problem. This classifier has been used as a basis for constructing a decision support tool where emotional trends are visualized. To evaluate the tool, it has been successfully integrated with a pan-European alerting system, and demonstrated as part of a crisis management concept during a public event involving relevant stakeholders.
During crises, enormous amounts of user generated content, including tweets, blog posts, and forum messages, are created, as documented in a number of recent publications –. Undoubtedly, large portions of this user generated content mainly consist of noise with limited or no use to crisis responders, but some of the available information can also be used for detecting that an emergency event has taken place , understanding the scope of a crisis, or to find out details about a crisis . That is, parts of the data can be used for increasing the tactical situational awareness . Unfortunately, the flood of information that is broadcast is infeasible for people to effectively extract information from, organize, make sense of, and act upon without appropriate computer support . For this reason, several researchers and practitioners are interested in developing systems for social media monitoring and analysis to be used in crises. One example is the American Red Cross Digital Operations Center, opened in March 2012 . Another example is the European Union security research project Alert4All, having as its aim to improve the authorities’ effectiveness of alert and communication towards the population during crises –. In order to accomplish this, screening of social media is deemed important for becoming aware of how communicated alert messages are perceived by the citizens . In this paper, we describe our methodology for collecting crisis-related tweets and tagging them manually with the help of a number of annotators. This has been done for tweets sent during the Sandy hurricane, where the annotators have tagged the emotional content as one of the classes positive (e.g., happiness), anger, fear, or other (including non-emotional content as well as emotions not belonging to any of the other classes). The tweets for which we have obtained a good inter-annotator agreement have been utilized in experiments with supervised learning algorithms for creating classifiers being able to classify new tweets as belonging to any of the classes of interest. By comparing the results to those achieved when using a rule-based classifier we show that the used machine learning algorithms have been able to generalize from the training data and can be used for classification of new, previously unseen, crisis tweets. Further, the optimum classifier has been integrated with, and constitutes an important part of, the Alert4All proof-of-concept alerting system. In the presence of relevant stakeholders representing politics, industry, end users, and research communities, this system was successfully demonstrated as a cohesive system during a public event. As part of this event, the classification of social media posts was used to visualize emotional trend statistics for the purpose of demonstrating the idea of using social media input for informing crisis management decisions. Overall, the concept was well received, considered novel, and makes it possible for crisis management organizations to use a new type of input for their decision-making.
The rest of this paper is outlined as follows. In the next section we give an overview of related work. A methodology section then follows, where we describe how crisis-related tweets have been collected, selected using automated processing, and tagged manually by a number of annotators in order to create a training set. We also describe how a separate test set has been constructed. After that, we present experimental results achieved for various classifiers and parameter settings. Details regarding the design and implementation of a decision support tool making use of the developed support vector machine classifier is then elaborated on in a separate section. The results and their implications are then discussed in more detail in a separate section before the paper is concluded in the last section.
The problem of sentiment analysis has attracted much research during the last decade. One reason is probably the growing amounts of opinion-rich text resources made available due to the development of social media, giving researchers and companies access to the opinions of ordinary people . Another important reason for the increased interest in sentiment analysis is the advances that have been made within the fields of natural language processing and machine learning. A survey of various techniques suggested for opinion mining and sentiment analysis is presented in . A seminal work on the use of machine learning for sentiment analysis is the paper by Pang et al. , showing that good performance (approximately 80% accuracy for a well-balanced dataset) can be achieved for the problem of classifying movie reviews as either positive or negative.
Although interesting, the classification of movie reviews as positive or negative has limited impact on the security domain. However, the monitoring of social media to spot emerging trends and to assess public opinion is also of importance to intelligence and security analysts, as demonstrated in . Microblogs such as Twitter pose a particular challenge for sentiment analysis techniques since messages are short (the maximum size of a tweet is 140 characters) and may contain sarcasm and slang. The utilization of machine learning techniques on Twitter data to discriminate between positive and negative tweets is evaluated in ,, suggesting that classification accuracies of 60–80% can be obtained. Social media monitoring techniques for collecting large amounts of tweets during crises and classifying them with machine learning algorithms has become a popular topic within the crisis response and management domain. The use of natural language processing and machine learning techniques to extract situation awareness from Twitter messages is suggested in  (automatic identification of tweets containing information about infrastructure status),  (classification of tweets as positive or negative), and  (classification of tweets as contributing to situational awareness or not).
The main difference between our work and the papers mentioned above is that most of the previous work focus on sentiment analysis (classifying crisis tweets as positive or negative), whilst we focus on affect analysis or emotion recognition , i.e., classifying crisis tweets as belonging to an emotional state. This problem is even more challenging since it is a multinomial classification problem rather than a binary classification problem. We are not aware of any previous attempts to use machine learning for emotion recognition of crisis-related tweets. The use of affect analysis techniques for the security domain has, however, been proposed previously, such as the affect analysis of extremist web forums and blogs presented in ,.
The work presented in this article is the result of a long-term research effort where related studies have been presented along the way. A first visionary paper  discusses and presents the concept of using social media monitoring for coming into dialogue with the population. The overall idea is for emergency management organizations to follow what people publish and adjust their information strategies in a way that matches the expectations and needs of the public. A systematic literature review and a parallel interview study were then undertaken , where the possibility to use social media analysis for informing crisis communication was deemed promising and important design issues to take into account were highlighted. Based on this insight, we outlined a more detailed design concept for how a screening tool could potentially be used for the purpose of increasing situational awareness during crises . This paper identifies data acquisition and data analysis to be two important parts of such a tool. Then, in parallel to presenting the initial results with regard to tweet classification , crisis management stakeholders were involved in a series of user-centered activities in order to understand the user needs and further inform the design of a social media screening tool to be used for crisis management . It became clear that within crisis management it is more important to be able to distinguish between negative emotions such as fear and anger than to be able to differentiate between different positive emotions. Also, a further understanding of crisis management working procedures was obtained, which made it clear that a social media screening tool needs to be focused on trend analysis since, in crisis management, relevant actions are to be undertaken for the purpose of improving some kind of crisis state in order to bring the situation into a better state.
Within the research project Alert4All we have discovered the need for automatically finding out whether a tweet (or other kinds of user generated content) is to be classified as containing emotional content . Through a series of user-centered activities involving crisis management stakeholders , the classes of interest for command and control have been identified as positive, anger, fear, and other, where the first class contains positive emotions such as happiness, and the last class contains emotions other than the ones already mentioned, as well as neutral or non-subjective classifications. In the following, we describe the methodology used for collecting crisis-related tweets, selecting a relevant subset of those, and letting human annotators tag them in order to be used for machine learning purposes.
The first step in our methodology was to collect a large set of crisis-related tweets. For this purpose we have used the Python package tweetstream to retrieve tweets related to the Sandy hurricane, hitting large parts of the Caribbean and the Mid-Atlantic and Northeastern United States during October 2012. The tweetstream package fetches tweets from Twitter’s streaming API in real-time. It should be noted that the streaming API only gives access to a random sample of the total volume of tweets sent at any given moment, but still this allowed us to collect approximately six million tweets related to Sandy during October 29 to November 1, using the search terms sandy, hurricane, and # s a n d y. After automatic removal of non-English tweets, retweets, and duplicated tweets, approximately 2.3 million tweets remained, as exemplified in Table 1. An average tweet in the dataset contained 14.7 words in total and 0.0786 “emotional words” according to the lists of identified keywords as will be described in the next subsection.
After an initial manual review of the remaining collected posts, we quickly discovered that a large proportion of the tweets not unexpectedly belong to the category other. Since the objective was to create a classifier being able to discriminate between the different classes, we needed a balanced training dataset, or at least a large number of samples for each class. This caused a problem since random sampling of the collected tweets most likely would result in almost only those belonging to the class other. Although this in theory could be solved by sampling a large enough set of tweets to annotate, there is a limit to how many tweets that can be tagged manually in a reasonable time (after all, this is the main motivation for learning such classifiers in the first place). To overcome this problem, we decided to use manual inspection to identify a small set of keywords which were likely to indicate emotional content belonging to any of the emotional classes positive, fear, or angera. The list of identified keywords looks as follows:
anger: anger, angry, bitch, fuck, furious, hate, mad,
fear: afraid, fear, scared,
positive: :), :-), =), :D, :-D, =D, glad, happy, positive, relieved.
Those lists were automatically extended by finding synonyms to the words using WordNet . Some of the resulting words were then removed from the lists as they were considered poor indicators of emotions during a hurricane. An example of a word that was removed is “stormy”, which was more likely to describe hurricane Sandy than expressing anger. By using the words in the created lists as search terms, we sampled 1000 tweets which according to our simple rules were likely to correspond to “positive” emotions. The same was done for “anger” and “fear”, while a random sampling strategy was used to select the 1000 tweets for “other”. In this way we constructed four data files containing 1000 tweets each. The way we selected the tweets may have an impact on the end results since there is a risk that such a biased selection process will lead to classifiers that are only able to learn the rules used to select the tweets in the first place. We were aware of such a potential risk, but could not identify any other way to come up with enough tweets corresponding to the “positive”, “anger”, and “fear” tags. In order to check the generalizability of the resulting classifiers, we have in the experiments compared the results to a baseline, implemented as a rule-based algorithm based on the keywords used to select the appropriate tweets. The experiments are further described in the next section.
Once the files containing tweets had been constructed, each file was sent by e-mail to three independent annotators, i.e., all annotators were given one file (containing 1000 tweets) each. All annotators were previously familiar with the Alert4All project (either through active work within the project or through acting as advisory board members) and received the instructions which can be found in the appendix. It should be noted that far from all the tweets in a file were tagged as belonging to the corresponding emotion by the annotators. In fact, a majority of the tweets were tagged as other also in the “anger”, “fear”, and “positive” files. In order to get a feeling for the inter-annotator agreement, we have calculated the percentages of tweets for which a majority of the annotators have classified a tweet in the same way (majority agreement) and where all agree (full agreement) as shown in Table 2. As can be seen, the majority agreement is consistently reasonably high. On the other hand, it is seldom that all three annotators agree on the same classification. For a tweet to become part of the resulting training set, we require that there has been a majority agreement regarding how it should be tagged. Now, ignoring which class a tweet was “supposed” to end up in given the used keywords (i.e., the used categories) and instead looking at the emotion classes tweets actually ended up in after the annotation, we received the distribution shown in Table 3. Since we wanted to have a training dataset with equally many samples for each class, we decided to balance the classes, resulting in 461 training samples for each class.
Creating a separate test dataset
While it is popular in the machine learning community to make use of n-fold cross validation to allow for training as well as testing on all the available data, we have decided to create a separate test set in this study. The reason for this is the way training data has been generated. If the used strategy to select tweets based on keywords would impact the annotated data and thereby also the learned classifiers too much, this could result in classifiers that perform well using the annotated data, but generalizes poorly to “real” data without the bias. Hence, our test data has been generated by letting a human annotator (not part of the first annotation phase) tag tweets from the originally collected Twitter dataset until sufficiently many tweets had been discovered for each emotion. Since it, as a rule of thumb, is common to use 90% of the available data for training and 10% for testing, we continued the tagging until we got 54 tweets in each class (after balancing the set), corresponding to approximately 10% of the total amount of data used for training and testing.
There exists many parameters related to affect analysis that influence the feature set. This section describes the parameters that have been varied during the experiments, and discusses how the parameters affected the achieved experimental results.
We have experimented with two standard machine learning algorithms for classification: Naïve Bayes (NB) and Support Vector Machine (SVM) classifiers. Available in Weka , the multinomial NB classifier  was used for the NB experiments, and the sequential minimal optimization algorithm  was used for training a linear kernel SVM. Although many additional features such as part-of-speech could have been used, we have limited the experiments to a simple bag-of-words representation. Initial experimentation showed that feature presence gave better results than feature frequency, wherefore only feature presence has been utilized. Before the training data was used, the tweets were transformed into lower case. Many different parameters have been varied throughout the experiments:
n-gram size: 1 (unigram)/2 (unigram + bigram),
stop words: yes/no,
minimum number of occurrences: 2/3/4,
information gain (in %): 25/50/75/100,
negation impact (number of words): 0/1/2,
threshold τ: 0.5/0.6/0.7.
If a unigram representation is used, individual words are utilized as features, whereas if bigrams are used, pairs of words are utilized as features. Stemming refers to the process in which inflected or derived words are reduced to their base form (e.g., fishing → fish). As stop words we have used a list of commonly occurring function words, so if a word in the tweet matches such a stop word it is removed (and is hence not used as a feature). The minimum number of occurrences refers to how many times a term has to occur in the training data in order to be used as a feature. Information gain refers to a method used for feature selection, where the basic idea is to select features that reveal the most information about the classes. When, e.g., setting the information gain parameter to 50, the fifty percent “most informative features” are kept, reducing the size of the resulting model. Negation impact refers to the situation when a negation (such as “not”) is detected, and the used algorithm replaces the words following the negation by adding the prefix “NOT_” to them. The specified negation impact determines how many words after a negation that should be affected by the negation (where 0 means that no negation is used). Finally, the threshold τ has been used for discriminating between emotional content versus other content, as described below.
In the learning phase we used the tweets tagged as positive, anger, and fear as training data, which resulted in classifiers that learned to discriminate between these three classes. For the actual classification of new tweets we then let the machine learning classifiers estimate the probabilities P(a n g e r| f1,…, f n ), P(f e a r| f1,…, f n ), and P(p o s i t i v e| f1,…, f n ), where f1,…, f n refers to the used feature vector extracted from the tweet we want to classify. If the estimated probability for the most probable class is greater than a pre-specified threshold τ, we return the label of the most probable class as the output from the classifier. Otherwise other is returned as the output from the classifier. The rationale behind this is that the content of tweets to be classified as other cannot be learned in advance (due to the spread of what this class should contain). Instead, we learn what is considered to be representative for the other classes and interpret low posterior probabilities for anger, fear, and positive as other being the most likely class.
The best results achieved when evaluating the learned classifiers on the used test set are shown in Figure 1, with the used parameter settings shown in Table 4. The results are also compared to two baseline algorithms: 1) a naïve algorithm that picks a class at random (since all the classes are equally likely in a balanced dataset, this corresponds to a simple majority classifier), and 2) a somewhat more complex rule-based classifier constructed from the heuristics (keywords) used when selecting the tweets to be annotated manually in the training data generation phase. The results suggest that both the NB and SVM classifiers outperform the baseline algorithms, and that the SVM (59.7%) performs somewhat better than the NB classifier (56.5%). For a more detailed accuracy assessment, see Tables 5 and 6 where the confusion matrices show how the respective classifiers perform. The use of stemming, stop words, minimum number of occurrences, and information gain according to Table 4 have consistently been providing better results, while the best choices of n-gram size, negation impact, and threshold τ have varied more in the experiments.
For comparison, Table 7 contains the confusion matrix for the baseline classifier, i.e., the rule-based classifier which chooses its class based on possible emotion words found within a tweet. As can be seen in Table 7, the classifications of emotions (i.e., “anger”, “fear”, or “positive”) are often correct, but a large amount of the tweets tend to erroneously fall into the other category. Now looking back at the machine learning confusion matrices according to Tables 5 and 6, we see that these classifiers do not exhibit the same behavior as the rule-based classifier with regard to the other category, but instead shows more evenly distributed errors. Hence, we can see that the machine learning classifiers have indeed learnt about emotional patterns that cannot be distinguished by simply applying rules based on a predefined list of emotion words.
In addition to evaluating the classifiers’ accuracy on the original test set, we have also tested what happens if the task is simplified so that the classifiers only have to distinguish between the emotional classes positive, fear, and anger (i.e., it is assumed that the other class is not relevant). This latter task can be of interest in a system where a classifier distinguishing between emotional and non-emotional or subjective and non-subjective content has already been applied. As can be seen in Figure 1, the SVM gets it right in three out of four classifications (75.9%) on this task, while the accuracy of the NB classifier reaches 69.1%. See Tables 8 and 9 for the corresponding confusion matrices.
Design and implementation of a tool for visualizing emotional trends
Based on a series of stakeholder workshops , the developed emotion classifier has been used as a basis for the design and implementation of a decision support system entitled the “screening of new media” tool where emotional trends are visualized. To evaluate the tool, it has been integrated with the Alert4All system, which is an implemented prototype of a future pan-European public alert concept. As shown during the final demonstration of the Alert4All system and through the collocated user-centered activities, the social media analysis component of Alert4All provides additional benefit for command and control personnel in terms of providing immediate feedback regarding the development of a crisis in general and regarding the reception of crisis alerts in particular.
At the bottom of the tool, the user has the possibility to drill down into the underlying dataset and see the actual posts in the database. From a command and control perspective, it is important to remember that these individual messages cannot and should not be used for inference regarding the whole dataset, but should be used solely for generating new hypotheses that need to be tested further by, e.g., experimenting with the filters in order to obtain sound statistical measures. Also to be noted, the posts are color coded so that it is easy to see which emotion a certain post has been classified as. However, the classification is not always correct, and therefore the user has the possibility to manually reclassify a post and, at a later stage, use the manually classified post as a basis for improving the classifier.
The GUI provides a number of ways to apply filters to the underlying dataset and thereby choose the social media posts to be visualized. The different visualizations are always kept consistent with these filters and with all other settings, i.e., the different parts of the graphical user interface provide different means to visualize one and the same dataset. As can be seen in Figure 2, there exists three main components for applying the filters: a time-line for filtering the time interval to be used, a tag cloud for filtering based on keywords, and the grey box located to the upper left that provides means to filter based on keywords, emotion classes, and data sources.
An important part of the GUI, and a result of the earlier-mentioned design workshops, is the possibility to shift between the absolute probability distribution according to Figure 2 vis-à-vis the relative probability distribution as depicted in Figure 3. Most often, it will be important to visualize both the relative graph and the absolute graph since it will be easier to visualize the trend using the relative graph whilst the absolute graph is still needed in order to visualize, e.g., trends regarding how the total volume of posts vary.
The obtained results show that the machine learning classifiers perform significantly better than chance and the rule-based algorithm that has been used as a baseline. Especially, the comparison to the rule-based algorithm is of interest, since the difference in accuracy indicates that the NB and SVM algorithms have been able to learn something more than just the keywords used to select the tweets to include in the annotation phase. In other words, even though the use of keywords may bias what tweets to include in the training data, this bias is not large enough to stop the machine learning classifiers from learning useful patterns in the data. In this sense the obtained results are successful. The confusion matrices also indicate that even better accuracy could have been achieved using a simple ensemble combining the output from the rule-based and machine learning-based algorithms.
Although the results are promising it can be questioned whether the obtained classification accuracy is good enough to be used in real-world social media analysis systems for crisis management. We believe that the results are good enough to be used on an aggregate level (“the citizens’ fear levels are increasing after the last alert message”), but are not necessarily precise enough to be used to correctly assess the emotions in a specific tweet. Nevertheless, this is a first attempt to classify emotions in crisis-related tweets, and by improving the used feature set and combining the machine learning paradigm with more non-domain specific solutions such as the affective lexicon WordNet-Affect , better accuracy can most likely be achieved. More training data would probably also improve the accuracy, but the high cost in terms of manpower needed for the creation of even larger training datasets needs to be taken into account. Additionally, the learned classifiers ought to be evaluated on other datasets in order to test the generalizability of the obtained results.
Some of the classification errors were a result of the annotators receiving instructions to classify tweets containing any of the emotions fear, anger, or positive as other if the tweets relate to a “historical” state or if the expressed emotion related to someone else than the author of the tweet. Such a distinction can be important if the used classifications should be part of a social media analysis system (since we do not want to take action on emotions that are not present anymore), but no features have been used to explicitly take care of spatio-temporal constraints in the current experiments. If such features were added (e.g., using part-of-speech tags and extraction of terms that contain temporal information), some of the classification errors could probably have been avoided.
Although we in this article have focused on crisis management, there are obviously other potential areas within the intelligence and security domain to which the suggested methodology and algorithms can be applied. As an example, it can be of interest to determine what kind of emotions that are expressed toward particular topics or groups in extremist discussion forums (cf. ,). In the same manner, it can be used to assess the emotions expressed by, e.g., bloggers, in order to try to identify signs of emergent conflicts before they actually take place (cf. ,). Similarly, the tool and algorithms described in this article could also be adapted to be used for evaluating the effects of information campaigns or psychological operations during military missions .
Conclusions and future work
We have described a methodology for collecting large amounts of crisis-related tweets and tagging relevant tweets using human annotators. The methodology has been used for annotating large quantities of tweets sent during the Sandy hurricane. The resulting dataset has been utilized when constructing classifiers able to automatically distinguish between the emotional classes positive, fear, anger, and other. Evaluation results suggest that a SVM classifier performs better than a NB classifier and a simple rule-based system. The classification task is difficult as suggested by the quite low reported inter-annotator agreement results. Seen in this light and considering that it is a multi-classification problem, the obtained accuracy for the SVM classifier (59.7%) seems promising. The classifications are not good enough to be trusted on the level of individual postings, but on a more aggregate level the citizens’ emotions and attitudes toward the crisis can be estimated using the suggested algorithms. Results obtained when ignoring the non-specific category other (reaching accuracies over 75% for the SVM) also suggest that combining the learned classifiers with algorithms for subjectivity recognition can be a fruitful way forward.
As future work we see a need for combining machine learning classifiers learned from crisis domain data with more general affective lexicons. In this way we think that better classification performance can be achieved than using the methods individually. Moreover, we suggest extending the used feature set with extracted part-of-speech tags since such information most likely will help determine if it is the author of a tweet who is having a certain emotion, or if it is someone else. Other areas to look into is how to deal with the use of sarcasm and slang in the user generated content.
From a crisis management perspective, it will also be necessary to investigate to what extent the used methodology and the developed classifiers are capable of coping with more generic situations. That is, we hope to have developed classifiers that to at least some significant extent classify based on hurricane and crises behavior in general, rather than solely being able to classifying Sandy-specific data. Investigating this requires that one retrieves and tags new datasets to test the classifiers on. Doing this for several different crisis types and then applying the same classifiers, should make it possible to quantify how capable the developed classifiers are when it comes to classifying tweets from 1) other hurricanes, 2) other types of natural disasters, and 3) crises in general.
a We use class to refer to the class a tweet actually belongs to (given the annotation), and “class” to refer to the class suggested by the used keywords.
Appendix: instructions given to annotators
You have been given 1000 tweets and a category. The tweets were written when hurricane Sandy hit the US in 2012. Hopefully most of the tweets you’ve been given are associated with your emotion. Your task is to go through these tweets, and for each tweet confirm whether this tweet is associated with the emotion you have been given, and if not, associate it with the correct emotion. To help make sure that the tagging is as consistent as possible between all annotators, you will be given some guidelines to make sure that everyone tags the tweets in a similar way:
“Fear” is the category containing tweets from people who are scared, afraid or worried.
“Anger” contains tweets from people that are upset or angry. It’s not always obvious whether someone is angry or sad, but if you think they are angry, tag it as “anger”. It is acceptable if the person feels sadness as well.
“Positive” contains tweets from people that are happy or at least feel positive.
“Other” represents the tweets that don’t belong to any of the other three categories. Tweets with none of the three emotions or mixed emotions where one of them isn’t dominating belong to this category.
The emotion should relate to the author of the tweet, not other people mentioned by the author. For example, the tweet “Maggie seems real concerned about Hurricane Sandy…” should not be tagged as “fear”, since it’s not the author of the tweet that is being concerned. Instead it should be tagged with “other”.
The tag should be based on the author’s mood when the tweet was written. For example, the tweet “I was really scared yesterday!” should not be tagged as “fear”, since it relates to past events, while we want to know how people were feeling when the tweets were posted. Exceptions can be made to events that happened very recently, for example: “I just fell because sandy scared me”, which can be tagged as “fear”.
Obvious sarcasm and irony should be tagged as “Other”. If you can’t decide whether the author is being sarcastic or not, assume that he is not being sarcastic or ironic.
A couple of the tweets might not be in English. Non-English tweets belong to “Other” regardless of content.
A few of the tweets are not related to the hurricane. Treat them in the same way as the rest of the tweets.
If a tweet contains conflicting emotions, and one of them doesn’t clearly dominate the other, it belongs to “Other”.
Some of the tweets will be difficult to tag. Even so, don’t leave a text untagged, please choose the alternative you believe is the most correct.
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We would like to thank Roberta Campo, Luísa Coelho, Patrick Drews, Montserrat Ferrer Julià, Sébastien Grazzini, Paul Hirst, Thomas Ladoire, Håkan Marcusson, Miguel Mendes, María Luisa Moreo, Javier Mulero Chaves, Cristina Párraga Niebla, Joaquín Ramírez, and Leopoldo Santos Santos for their effort during the tagging process.
This work has been supported by the European Union Seventh Framework Programme through the Alert4All research project (contract no 261732), and by the research and development program of the Swedish Armed Forces.
The authors declare that they have no competing interests.
All authors drafted, read and approved the final manuscript.