"You might think about slightly revising the title":
Identifying Hedges in Peer-tutoring Interactions

Yann Raphalen, Chloé Clavel, Justine Cassell

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Yann Raphalen, Chloé Clavel, Justine Cassell. "You might think about slightly revising the title": Identifying Hedges in Peer-tutoring Interactions. ACL 2022 - 60th Annual Meeting of the Association for Computational Linguistics, Association for Computational Linguistics, May 2022, Dublin, Ireland. pp.2160-2174, 10.18653/v1/2022.acl-long.153. hal-03990509

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# "You might think about slightly revising the title": Identifying Hedges in Peer-tutoring Interactions

Yann Raphalen¹, Chloé Clavel², Justine Cassell¹,³

¹Inria Paris

²LTCI, Institut Polytechnique de Paris, Telecom-Paris

³Carnegie Mellon University

yann.raphalen.pro@gmail.com, justine@cs.cmu.edu,

chloe.clavel@telecom-paris.fr

## Abstract

Hedges play an important role in the management of conversational interaction. In peer-tutoring, they are notably used by tutors in dyads (pairs of interlocutors) experiencing low rapport to tone down the impact of instructions and negative feedback. Pursuing the objective of building a tutoring agent that manages rapport with students in order to improve learning, we used a multimodal peer-tutoring dataset to construct a computational framework for identifying hedges. We compared approaches relying on pre-trained resources with others that integrate insights from the social science literature. Our best performance involved a hybrid approach that outperforms the existing baseline while being easier to interpret. We employ a model explainability tool to explore the features that characterize hedges in peer-tutoring conversations, and we identify some novel features, and the benefits of such a hybrid model approach.

## 1 Introduction

Rapport, most simply defined as the “... relative harmony and smoothness of relations between people ...” (Spencer-Oatey, 2005), has been shown to play a role in the success of activities as varied as psychotherapy (Leach, 2005) and survey interviewing (Lune and Berg, 2017). In peer-tutoring, rapport, as measured by the annotation of thin slices of video, has been shown to be beneficial for learning outcomes (Zhao et al., 2014; Sinha and Cassell, 2015). The level of rapport rises and falls with conversational strategies deployed by tutors and tutees at appropriate times, and as a function of the content of prior turns. These strategies include self-disclosure, referring to shared experience, and, on the part of tutors, giving instructions in an indirect manner. Some work has attempted to automatically detect these strategies in the service of intelligent tutors (Zhao et al., 2016a), but only a few strategies have been attempted. Other work has con-

centrated on a "social reasoning module" (Romero et al., 2017) to decide which strategies should be generated in a given context, but indirectness was not among the strategies targeted. In this paper, we focus on the automatic classification of one specific strategy that is particularly important for the tutoring domain, and therefore important for intelligent tutors: hedging, a sub-part of indirectness that "softens" what we say. This work is part of a larger research program with the long-term goal of automatically generating indirectness behaviors for a tutoring agent.

<figure><img src="image_2.png" /><figcaption>Figure 1: A mock conversation displaying each type of hedged formulation.</figcaption></figure>

According to Brown and Levinson (1987), hedges are part of the linguistic tools that interlocutors use to produce politeness, by limiting the face threat to the interlocutor (basically by limiting the extent to which the interlocutor might experience embarrassment because of some kind of poor performance). An example is "that's kind of a wrong answer". Hedges are also found when speakers wish to avoid losing face themselves, for example when saying ("I think I might have to add 6."). Madaio et al. (2017) found that in a peer-tutoring task, when rapport between interlocutors is low, tutees attempted more problems and correctly solved more problems when their tutors hedged instruc-
tions, which likewise points towards a "mitigation
of face threat" function. Hedges can also be asso-
ciated with a nonverbal component, for example
averted eye gaze during criticism (Burgoon and
Koper, 1984). Hedges are not, however, always ap-
propriate, as in "I kind of think it's raining today."
when the interlocutors can both see rain (although
it might be taken as humorous). These facts about
hedges motivate a way to automatically detect them
and, ultimately (although not in the current work)
also generate them. In both cases we first have
to be able to characterize them using interpretable
linguistic features, which is what we address in
the current paper. Thus, in the work described
here, based on linguistic descriptions of hedges
(Brown and Levinson, 1987; Fraser, 2010), we built
a rule-based classifier. We show that this classifier
in combination with additional multimodal inter-
pretable context-dependent features significantly
improves the performance of a machine learning
model for hedges, compared to a less interpretable
deep learning baseline from Goel et al. (2019) us-
ing word embeddings. We also relied on a machine
learning model explanation tool (Lundberg and Lee,
2017) to investigate the linguistic features related
to hedges in the context of peer-tutoring, primarily
to see if we could discover surprising features that
the classification model would associate to hedges
in this context, and we describe those below. The
code of the models described in the paper is also
provided. <fnref n="1" />

## 2 Related work

**Hedges:** According to Fraser (2010), hedging is a rhetorical strategy that attenuates the strength of a statement. One way to produce a hedge is by altering the full semantic value of a particular expression through **Propositional hedges** (also called **Approximators** in Prince et al. (1982)), as in "You are *kind of* wrong," that reduce prototypicality (i.e accuracy of the correspondence between the proposition and the reality that the speaker seeks to describe). Propositional hedges are related to fuzzy language (Lakoff, 1975), and therefore to the production of vagueness (Williamson, 2002) and uncertainty (Vincze, 2014).

A second kind are **Relational Hedges** (also called **Shields** in Prince et al. (1982)), such as “*I think that you are wrong.*” or “*The doctor wants you to stop smoking.*”, conveying that the proposition is

considered by the speaker as subjective. In a further
sub-division, **Attribution Shields**, as in "The doc-
tor *wants you* ...", the involvement of the speaker
in the truth value of the proposition is not made
explicit, which allows speakers not to take a stance.
As described above, Madaio et al. (2017) found
that tutors who showed lower rapport with their
tutees used more hedged instructions (they also
employed more positive feedback), however this
was only the case for tutors with a greater belief in
their ability to tutor. Tutees in this context solved
more problems correctly when their tutors hedged
instructions. No effect of hedging was found for
dyads (pairs of interlocutors) with greater social
closeness. However, the authors did not look at the
specific linguistic forms these teenagers used.
Rowland (2007) also describes the role that hedg-
ing plays in this age group, showing that students
use both relational ("*I think that John is smart.*")
and propositional ("John is *kind of smart.*") hedges
for much the same shielding function of demon-
strating uncertainty, to save them from the risk
of embarrassment if they are wrong. The author
observed that teens used few **Adaptors** (*kind of*,
*somewhat*) and preferred to use **Rounders** (*around*,
*close to*). However, this study was performed with
an adult and two children, possibly biasing the re-
sults due to the participation of the adult investiga-
tor. Hedges have been included in virtual tutoring
agents before now. (Howard et al., 2015) integrated
hedges in a tutor agent for undergraduates in CS, as
a way to encourage the student to take the initiative.
Hedges have also been used as a way of integrat-
ing Brown and Levinson's politeness framework
(Wang et al., 2008; Schneider et al., 2015) in vir-
tual tutoring agents. Results were not broken out
by strategy, but politeness in general was shown
to positively influence motivation and learning, in
certain conditions.

**Computational methods for hedge detection:**

A number of studies have targeted the detection
of hedges and uncertainty in text (Medlock and
Briscoe, 2007; Ganter and Strube, 2009; Tang et al.,
2010; Velldal, 2011; Szarvas et al., 2012), partic-
ularly following the CoNLL 2010 dataset release
(Farkas et al., 2010). However, this work is not
as related to hedges in conversation, as it focuses
on a formal and academic language register (Hy-
land, 1998; Varttala, 1999). As noted by Prokofieva
and Hirschberg (2014), the functions of hedges are
domain- and genre-dependent, therefore this bias

<footnote n="1">¹https://github.com/AnonymousHedges/HedgeDetection</footnote>
towards formality implies that the existing work
may not adapt well to the detection of hedges in
conversation between teenagers. A consequence is
that the existing work does not consider terms like
"I think," since opinions rarely appear in an aca-
demic writing dataset. Instructions are also almost
absent ("I think you have to add ten to both sides."),
a strong limitation for the study of conversational
hedges since it is in requests (including tutoring in-
structions) that indirect formulations mostly occur
according to Blum-Kulka (1987). Prokofieva and
Hirschberg (2014) also note that it is difficult to
detect hedges because the word patterns associated
with them have other semantic and pragmatic func-
tions: considering "I think that you have to add x
to both sides." vs "I think that you are an idiot.",
it is not clear that the second use of "I think that"
is an hedge marker. They advocate using machine
learning approaches to deal with the ambiguity of
these markers. Working on a conversational dataset,
Ulinski et al. (2018) built a computational system
to assess speaker commitment (i.e. at which point
the speaker seems convinced by the truth value
of a statement), in particular by relying on a rule-
based detection system for hedges. Compared to
that work, our rule-based classification model is
directly detecting hedge classes, and we employ
the predictions of the rule-based model as a feature
for stronger machine learning models, designed to
lessen the impact of the imbalance between classes.
We also consider apologies when they serve a mit-
igation function (we then call them Apologizers),
as was done by the authors of our corpus, and we
also use the term subjectivizers as defined below,
to be able to compare directly with the previous
work carried out on this corpus. As far as we know,
only Goel et al. (2019) have worked with a peer-
tutoring dataset (the same one that we also use),
and they achieved their best classification result by
employing an Attention-CNN model, inspired by
Adel and Schütze (2017).

3 Problem statement

We consider a set D of conversations D =
(c₁, c₂, ..., c|D|), where each conversation is com-
posed of a sequence of independent syntactic
clauses cᵢ = (u₁, u₂, ..., uM), where M is the
number of clauses in the conversation. Note
that two consecutive clauses can be produced
by the same speaker. Each clause is associated
with a unique label corresponding to the differ-

ent hedge classes described in Table 1: $y_i \in C$
= {Propositional Hedges, Apologizers, Subjec-
tivizers, Not hedged}. Finally, an utterance $u_i$
can be represented as a vector of features $X =$
($x_1, x_2, ..., x_N$), where N represents the number of
features we used to describe a clause. Our first
goal is to design a model that correctly predicts the
label $y_i$ associated to $u_i$. It can be understood as
the following research question:

**RQ1:** "Which models and features can be used to automatically characterize hedges in a peer-tutoring interaction?"

Our second goal is to identify, for each hedge class,
the set of features $F_{class} = \{f_k\}, k \in [1, N]$ sorted
by feature importance in the classification of *class*.
It corresponds to the following research question:

**RQ2:** "What are the most important linguistic features that characterize our hedge classes in a peer-tutoring setting?"

4 Methodology

4.1 Corpus

**Data collection:** The dialogue corpus used here was collected as part of a larger study on the effects of rapport-building on reciprocal peer tutoring. 24 American teenagers (mean age = 13.5, min = 12, max = 15), half male and half female, came to a lab where half of the participants were paired with a same-age, same-gender friend, and the other half paired with a stranger. The participants were assigned to a total of 12 dyads in which the participants alternated tutoring one another in linear algebra equation solving for 5 weekly hour-long sessions, for a total corpus of nearly 60 hours of face-to-face interactions. Each session was structured such that the students engaged in brief social chitchat in the beginning, then one of the students was randomly assigned to tutor the other for 20 minutes. They then engaged in another social period, and concluded with a second tutoring period where the other student was assigned the role of tutor. Audio and video data were recorded, transcribed, and segmented for clause-level dialogue annotation, providing nearly 24 000 clauses. Non-speech segments (notably fillers and laughter) were maintained. Because of temporal misalignment for parts of the corpus, many paraverbal phenomena, such as prosody, were unfortunately not available to us. Since our access to the dataset is covered by a Non-Disclosure Agreement, it cannot be released
publicly. However the original experimenters' Institutional Review Board (IRB) approval allows us to view, annotate, and use the data to train models. This also allows us to provide a link to a pixelated video example in the GitHub repository of the project².

**Data annotation:** The dataset was previously annotated by Madaio et al. (2017), following an annotation manual that used hedge classes derived from Rowland (2007) (see Table 1). Only the task periods of the interactions were annotated. Comparing the annotations with the classes mentioned in the related work section, **Subjectivizers** correspond to **Relational hedges** (Fraser, 2010), **Propositional hedges** and **Extenders** correspond to **Approximators** (Prince et al., 1982) with the addition of some discourse markers such as *just*. **Apologizers** are mentioned as linguistic tools related to negative politeness in Brown and Levinson (1987). Krippendorff's alpha obtained for this corpus annotated by four coders was over 0.7 for all classes (denoting an acceptable inter-coder reliability according to Krippendorff (2004)). The dataset is widely imbalanced, with more than 90% of the utterances belonging to the **Not hedged** class.

In reviewing the corpus and the annotation manual, however, we noticed two issues. First, the annotation of the **Extenders** class was inconsistent, leading to the **Extenders** and **Propositional hedges** classes carrying similar semantic functions. We therefore merged the two classes and grouped utterances labeled as **Extenders** and those labeled as **Propositional hedges** under the heading of **Propositional hedges**. Second, the annotation of clauses containing the tokens "just" and "would" (two terms occurring frequently in the dataset that are key components of **Propositional Hedges** and **Subjectivizers** but that are not in fact hedges in all cases) was also inconsistent, leading to virtually all clauses with those two tokens being considered hedges. We therefore re-considered all the clauses associated with any of the hedge classes, as well as all the clauses in the "Not hedged" class that contained "just" or "would". The re-annotation was carried out by two annotators who achieved a Krippendorff's alpha inter-rater reliability of .9 or better for **Apologizers**, **Subjectivizers**, and **Propositional hedges** before independently re-annotating the relevant clauses. An example of a re-annotation was removing "I *would* kill you!" from the hedge

classes.

## 4.2 Features

**Label from rule-based classifier (Label RB):** We use the class label predicted by the rule-based classifier described in Section 4.3 as a feature. Our hypothesis is that the machine learning model can use this information to counterbalance the class imbalance. To take into account the fact that some rules are more efficient than others, we weighted the class label resulting from the rule-based model by the precision of the rule that generated it.

**Unigram and bigram:** We count the number of occurrences of unigrams and bigrams of the corpus in each clause. We used the lemma of the words for unigrams and bigrams using the nltk lemmatizer (Loper, 2002) and selected unigrams and bigrams that occurred in the training dataset at least fifty times. The goal was to investigate, with a bottom-up approach, to what extent the use of certain words characterizes hedge classes in tutoring. In Section 5 we examine the overlap between these words and those a *priori* identified by the rules.

**Part-of-speech (POS):** Hedge classes seem to be associated with different syntactic patterns: for example, subjectivizers most often contain a personal pronoun followed by a verb, as in "I guess", "I believe", "I think". We therefore considered the number of occurrences of POS-Tag n-grams (n=1, 2, 3) as features. We used the spaCy POS-tagger and considered POS unigrams, bigrams and trigrams that occur at least 10 times in the training dataset.

**LIWC:** Linguistic Inquiry and Word Count (LIWC) (Pennebaker et al., 2015) is standard software for extracting the count of words belonging to specific psycho-social categories (e.g., emotions, religion). It has been successfully used in the detection of conversational strategies (Zhao et al., 2016a). We therefore count the number of occurrences of all the 73 categories from LIWC.

**Tutoring moves (TM):** Intelligent tutoring systems rely on specific tutoring moves to successfully convey content (as do human tutors). We therefore looked at the link between the tutoring moves, as annotated in Madaio et al. (2017), and hedges. For tutors, these moves are (1) instructional directives and suggestions, (2) feedback, and (3) affirmations, mostly explicit reflections on their partners' comprehension, while for tutees, they are (1) questions, (2) feedbacks, and (3) affirmations,

²https://github.com/AnonymousHedges/HedgeDetection
<table><caption>Table 1: Definition of the classes</caption><thead><tr><th>Class</th><th>Definition</th><th>Example</th></tr></thead><tbody><tr><td>Subjectivizers</td><td>Words that reduce intensity or certainty</td><td>"So then I would divide by two."</td></tr><tr><td>Apologizers</td><td>Apologies used to soften direct speech acts</td><td>"Oh sorry six b."</td></tr><tr><td>Propositional hedges</td><td>Qualifying words to reduce intensity or certainty of utterances</td><td>"It's actually eight."</td></tr><tr><td>Extenders</td><td>Words used to indicate uncertainty by referring to vague categories</td><td>"It'll be the number x or whatever variable you have."</td></tr></tbody></table>

<table><caption>Table 2: Distribution of the classes</caption><thead><tr><th>Prop. hedges</th><th>Apologizers</th><th>Subjectivizers</th><th>Not hedged</th><th>Total</th></tr></thead><tbody><tr><td>1210</td><td>128</td><td>626</td><td>21192</td><td>23156</td></tr></tbody></table>

<table><caption>Table 3: List of automatically extracted and manually annotated features with their size.</caption><thead><tr><th>Features name</th><th>Automatic extraction</th><th>Vector size</th></tr></thead><tbody><tr><td>Rule-based label</td><td>Yes</td><td>4</td></tr><tr><td>Unigram</td><td>Yes</td><td>~250</td></tr><tr><td>Bigram</td><td>Yes</td><td>~250</td></tr><tr><td>POS</td><td>Yes</td><td>~1200</td></tr><tr><td>LIWC</td><td>Yes</td><td>73</td></tr><tr><td>Nonverbal</td><td>No</td><td>24</td></tr><tr><td>Tutoring moves</td><td>No</td><td>6</td></tr><tr><td>Total</td><td></td><td>~1800</td></tr></tbody></table>

mostly tentative answers.

**Nonverbal and paraverbal behaviors:** As in Goel et al. (2019), we included the nonverbal and paraverbal behaviors that are related to hedges. Specifically, we consider laughter and smiles, that have been shown to be effective methods of mitigation (Warner-Garcia, 2014), cut-offs indicating self-repairs, fillers like "Um", gaze shifts (annotated as 'Gaze at Partner', 'Gaze at the Math Worksheet', and 'Gaze elsewhere'), and head nods. Each feature was present twice in the feature vector, one time for each interlocutor. Inter-rater reliability for nonverbal behavior was 0.89 (as measured by Krippendorff's alpha) for eye gaze, 0.75 for smile count, 0.64 for smile duration and 0.99 for head nod. Laughter is also reported in the transcript at the word level. We separate the tutor's behaviors from those of the tutee. The collection process for these behaviors is detailed further in Zhao et al. (2016b).

The clause-level feature vector was normalized by the length of the clause (except for the rule-based label). This length was also added as a feature. Table 3 presents an overview of the final feature vector.

## 4.3 Classification models

The classification models used are presented here according to their level of integration of external linguistic knowledge.

**Rule-based model:** On the basis of the annotation manual used to construct the dataset from Madaio et al. (2017), and with descriptions of hedges from Rowland (2007), Fraser (2010) and Brown and Levinson (1987), we constructed a rule-based classifier that matches regular expressions indicative of hedges. The rules are detailed in Table 7 in the Appendix.

**LGBM:** Since hedges are often characterized by explicit lexical markers, we tested the assumption that a machine learning model with a knowledge-driven representation for clauses could compete with a BERT model in performance, while being much more interpretable. We relied on LightGBM, an ensemble of decision trees trained with gradient boosting (Ke et al., 2017). This model was selected because of its performance with small training datasets and because it can ignore uninformative features, but also for its training speed compared to alternative implementations of gradient boosting methods.

**Multi-layer perceptron (MLP):** As a simple baseline, we built a multi-layer perceptron using three sets of features: a pre-trained contextual representation of the clause (SentBERT; Reimers and Gurevych (2019)); the concatenation of this contextual representation of the clause and a rule-based label (not relying on the previous clauses); and finally the concatenation of all the features mentioned in section 4.2, without the contextualized representation.

**LSTM over a sequence of clauses:** Since we are working with conversational data, we also wanted to test whether taking into account the previous clauses helps to detect the type of hedge class in the next clause. Formally, we want to infer $y_i$ using $y_i = \max_{y \in \text{Classes}} P(y|X(u_i), X(u_{i-1}), \dots, X(u_{i-K}))$, where K is the number of previous clauses that the model will take into account. The
MLP model presented above infers $y_i$ using
$y_i = \max_{y \in \text{Classes}} P(y|X(u_i))$, therefore a
difference of performance between the two models
would be a sign that using information from the
previous clauses could help to detect the hedged
formulation in the current clause. We tested a
LSTM model with the same representations for
clauses as for the MLP model.

**CNN with attention:** Goel et al. (2019) established their best performance on hedge detection using a CNN model with additive attention over word (and not clause) embeddings. Contrary to the MLP and LSTM models mentioned above, this model tries to infer $y_i$ using $y_i = \max_{y \in \text{Classes}} P(y|g(w_0), g(w_1), ..., g(w_L))$, with L representing the maximum clause length we allow, and g representing a function that turns the word $w_j$, $j \in [0, L]$ into a vector representation (for more details, please see Adel and Schütze (2017)).

**BERT:** To benefit from deep semantic and contextual representations of the utterances, we also fine-tuned BERT (Devlin et al., 2019) on our classification task. BERT is a pre-trained Transformers encoder (Vaswani et al., 2017) that has significantly improved the state of the art on a number of NLP tasks, including sentiment analysis. It produces a contextual representation of each word in a sentence, making it capable of disambiguating the meaning of words like "think" or "just" that are representative of certain classes of hedges. BERT, however, is notably hard to interpret.

## 4.4 Analysis tools

Looking at which features improve the perfor-
mance of our classification models tells us whether
these features are informative or not, but does not
explain how these features are used by the mod-
els to make a given prediction. We therefore pro-
duced a complementary analysis using an inter-
pretability tool. As demonstrated by (Lundberg
and Lee, 2017), LightGBM internal feature impor-
tance scores are inconsistent with both the model
behavior and human intuition, so we instead used
a model-agnostic tool. SHAP (Lundberg and Lee,
2017) assigns to each feature an importance value
(called Shapley values) for a particular prediction
depending on the extent of its contribution (a de-
tailed introduction to Shapley values and SHAP
can be found in Molnar (2020)). SHAP is a model-
agnostic framework, therefore the values associ-

ated with a set of features can be compared across
models. It should be noted that SHAP produces
explanations on a case-by-case basis, therefore it
can both provide local and global explanations. For
the Gradient Boosting model, we use an adapted
version of SHAP (Lundberg et al., 2018), called
TreeSHAP.

# 5 Experiments and results

**5.1 Experimental setting**

To detect the best set of features, we used Light-
GBM and proceeded incrementally, by adding the
group of features we thought to be most likely asso-
ciated with hedges. We did not consider the risk of
relying on a sub-optimal set of features through this
procedure because of the strong ability of Light-
GBM to ignore uninformative features. We use this
incremental approach as a way to test our intuition
about the performativity of groups of features (i.e.
does adding a feature improve the performance of
the model) with regard to the task of classifica-
tion. To compare our models, we trained them on
the 4-class task, and looked at the average of the
weighted F1-scores for the three hedge classes (i.e.
how well the models infer minority classes) that we
report here as "3-classes", and at the average of the
weighted F1-scores for the 4 classes, that we report
as "4-classes". Details of the hyperparameters and
experimental settings are provided in Appendix A.

## 5.2 Model comparison and feature analysis

**Overall results:** Table 4 presents the results obtained by the 6 models presented in Section 4.3 for the multi-class problem. Best performance (F1-score of 79.0) is obtained with LightGBM leveraging almost all the features. In the appendix (see Table 8 and Table 9) we indicate the confidence intervals to represent the significance of the differences between the models.

First, and perhaps surprisingly, we notice that
the use of "Knowledge-Driven" features based on
rules built from linguistic knowledge of hedges
in the LightGBM model outperforms the use of
pre-trained embeddings within a fine-tuned BERT
model (79.0 vs. 70.6), and in the neural baseline
from (Goel et al., 2019) (79.0 vs 64.5).

The low scores obtained by the LGBM, LSTM
and MLP models with pre-trained sentence em-
beddings versus Knowledge-Driven features might
signal that the word patterns characterizing hedges
are not salient in these representations (i.e. the
<table><caption>Table 4: Averaged weighted F1-scores (and standard deviation) for the three minority classes and for the 4 classes, for all models. "KD" stands for "Knowledge-Driven", meaning that the features are derived from lexicon, n-gram models and annotations.</caption><thead><tr><th>Models</th><th>KD Feat. (KDF)</th><th>Pre-Trained Emb. (PTE)</th><th>KDF + PTE</th></tr></thead><tbody><tr><td>Rule-based (3-classes)</td><td>67.6</td><td>0</td><td>0</td></tr><tr><td>MLP (3-classes)</td><td>68.5 (1.6)</td><td>35.8 (3.1)</td><td>64.8 (1.1)</td></tr><tr><td>Attention-CNN (3-classes)</td><td>0</td><td>64.5 (3.0)</td><td>0</td></tr><tr><td>LSTM (3-classes)</td><td>65.1 (5.7)</td><td>39.8 (8.0)</td><td>65.2 (5.1)</td></tr><tr><td>BERT (3-classes)</td><td>0</td><td><strong>70.6 (2.3)</strong></td><td>0</td></tr><tr><td>LGBM (3-classes)</td><td><strong>79.0 (1.3)</strong></td><td>35.0 (2.2)</td><td><strong>70.1 (1.4)</strong></td></tr><tr><td>Rule-based (4-classes)</td><td>94.7</td><td>0</td><td>0</td></tr><tr><td>MLP (4-classes)</td><td>94.8 (0.3)</td><td>89.7 (0.4)</td><td>93.9 (0.4)</td></tr><tr><td>Attention-CNN (4-classes)</td><td>0</td><td>94.4 (0.2)</td><td>0</td></tr><tr><td>LSTM (4-classes)</td><td>93.9 (1.4)</td><td>89.1 (1.4)</td><td>94.1 (1.2)</td></tr><tr><td>BERT (4-classes)</td><td>0</td><td><strong>94.9 (0.4)</strong></td><td>0</td></tr><tr><td>LGBM (4-classes)</td><td><strong>96.7 (0.2)</strong></td><td>91.0 (0.2)</td><td><strong>95.4 (0.2)</strong></td></tr></tbody></table>

distance between "I think you should add 5." and "You should add 5." is short.). KD Features seem to provide a better separability of the classes. The combination of KD features and Pre-trained embeddings does not significantly improve the performance of the models compared to the KD Features only, which suggests that the information from the Pre-trained embeddings is redundant with the one from the KD Features. This result may be due to the high dimensionality of the input vector (868 with PCA on the KD Features; 2500 otherwise). A second finding is that the use of gradient boosting models on top of rule-based classifiers better models the hedge classes. The other machine learning models did not prove to be as effective, except for BERT.

**Feature analysis using LightGBM:** Using the best performing model, Table 5 shows the role of each feature set in the prediction task. The significance of the differences is shown in Table 10 and Table 11. Compared to the rule-based model, the introduction of n-grams significantly improved the performance of our classifier, suggesting that some lexical and syntactic information describing the hedge classes was not present in the rule-based model. Looking at Table 5, we do not observe significant differences between the LGBM model using only the label rule based + (1-grams and 2-grams) and the models incorporating more features. To our surprise, neither the tutoring moves nor the nonverbal features significantly improved the performance of the model. The 2 features were included to index the specific peer tutoring context of these hedges, so this indicates that in future work we might wish to apply the current model to another context of use to see if this model of hedges is more generally applicable than we originally thought. By combining this result

with the increased performance of the model using Knowledge-Driven (i.e. explicit) features compared to pre-trained embeddings, it would seem that hedges are above all a lexical phenomenon (i.e. produced by specific lexical elements).

## 5.3 In-depth analysis of the informative features

We trained the SHAP explanation models on Light-GBM with all features. The most informative features (in absolute value) for each class are shown in Table 6, and the plots by class are presented in the Appendix. The most important features seem to be the rule-based labels, which appear in at least the fourth position for three classes (see Table 6), and in the first position for **Propositional Hedges** and **Not hedged** classes. Surprisingly, the Rule-Based label does not appear in the top 20 features for **Apologizers**. However, given that the class rarely appears in the data, the rules seldom activate, so the feature may simply be informative for a very small number of clauses. Unigrams (*Oh*, *Sorry*, *just*, *Would*, and *I*) are also present in the 5 top-ranked features. This confirms the findings mentioned in related work for the characterization of the different hedge classes (*just* with **Propositional Hedges**, *sorry* with **Apologizer**, *I* with **Subjectivizers**). The presence of *Oh* also has high importance for the characterization of **Apologizer** (n=2), as illustrated in examples such as "Oh sorry, that's nine.". We note that the occurrences of "Oh sorry" as a stand-alone clause were excluded by our rule-based model because they do not correspond to an apologizer (they cannot mitigate the content of a proposition if there is no proposition associated). This example illustrates the interest of a machine learning model approach to disambiguate the function of conventional non-propositional phrases like "Oh sorry".

In addition, SHAP highlights the importance of novel features whose function was not identified in the hedges literature: (i) what LIWC classifies as **informal words** but that are mostly interjections like *ah* and *oh* are strongly associated with **Apologizer**, as are disfluencies (n=12); (ii) the use of **POS tags** seems to be very relevant for characterizing the different classes (2-gram of POS tag features<fnref n="3" /> occur in the top-ranked features of all the

<footnote n="3">³Note that there is strong redundancy between some features of LIWC and the spaCy POS tagger that both produce a "Pronoun" category, using a lexicon in the first case, and a neural inference in the second.</footnote>
<table><caption>Table 5: Averaged weighted F1-scores for the three classes of hedges and the four classes, with an additive integration of KDF features in the LightGBM model. The standard deviation is computed across five folds.</caption><thead><tr><th>Models</th><th>Label RB</th><th>+ 1-gram and 2-gram</th><th>+ POS</th><th>+ LIWC</th><th>+ TM</th><th>+ Nonverbal</th></tr></thead><tbody><tr><td>3-classes</td><td>68.8 (0.8)</td><td>78.2 (1.6)</td><td>78.1 (1.3)</td><td>79.0 (1.3)</td><td>78.5 (2.4)</td><td>78.7 (1.8)</td></tr><tr><td>4-classes</td><td>95.0 (0.2)</td><td>96.5 (0.3)</td><td>96.5 (0.2)</td><td>96.7 (0.2)</td><td>96.6 (0.4)</td><td>96.7 (0.3)</td></tr></tbody></table>

<table><caption>Table 6: Most important clause-level features for LightGBM according to the SHAP analysis.</caption><thead><tr><th>Rank</th><th>Apologizer</th><th>Subjectivizers</th><th>Prop. Hedges</th><th>Not hedged</th></tr></thead><tbody><tr><td>1</td><td>Function words (LIWC)</td><td>"I"</td><td>Class label</td><td>Class label</td></tr><tr><td>2</td><td>"Oh" (LIWC)</td><td>"Yeah"</td><td>"Would"</td><td>"Would"</td></tr><tr><td>3</td><td>"Sorry"</td><td>Noun (POS)</td><td>"Just"</td><td>"Yeah"</td></tr><tr><td>4</td><td>Affect (LIWC)</td><td>Class label</td><td>Function word (LIWC)</td><td>Noun (POS)</td></tr><tr><td>5</td><td>Clause length</td><td>Cognitive process (LIWC)</td><td>Netspeak (LIWC)</td><td>Cognitive process (LIWC)</td></tr></tbody></table>

classes (see Figures in the Appendix). It means that there are some recurring syntactic patterns in each class; (iii) Regarding the **utterance size**, a clause shorter than the mean is weakly associated with **directness** (n=17) while a longer clause suggests that it contains a **Subjectivizer** (n=6). **Apologizers** are characterized by a mean clause length (n=5), with few variations from it; (iv) Tutoring moves are not strong predictors of any classes: "Affirmation from tutor" is the only feature appearing as a predictor of Propositional hedges (n=20). This is consistent with the feature analysis in Table 5, suggesting that tutoring moves do not significantly improve the performance of the classifier; (v) **Non-verbal behaviors** do not appear as important features for the classification. This is coherent with results from (Goel et al., 2019). Note that prosody might play a role in detecting instructions that trail off, but, as described, paraverbal features were not available; (vi) *Would* plays an important role in the production of hedges, as it is strongly associated to **Propositional hedges** (n=2). It is interesting to note that, when designing the rule-based classifier, we saw it decrease in performance when we started to include *would* in our regular expression patterns, probably because the form is hard to disambiguate for a deterministic system.

While exploring the Shapley values associated to each clause, we observed that features like tutoring moves are extremely informative for a very small number of clauses (therefore not significantly influencing the overall performance of the prediction), and more or less not informative for the rest. Inferring the global importance of a feature as a mean across the shapley values in the dataset may not be the only way to explore the behavior of gradi-

ent boosting methods. It might be more useful to cluster clauses based on the importance that SHAP gives to that feature in its classification, as this could help discover sub-classes of hedges that are differentiated from the rest by their interaction with a specific feature (in the way that some **Apologizers** are characterized by an "oh"). We also note that the explanation model is sensitive to spurious correlations in the dataset, caused by the small representation of some class: for example, "nine" (n=7) and "four" (n=20) are positive predictors of **Apologizers**.

## 6 Conclusion and future work

Through our classification performance experiments, we showed that it is possible to use machine learning methods to diminish the ambiguity of hedges, and that the hybrid approach of using rule-based label features derived from social science (including linguistics) literature within a machine learning model helped significantly to increase the model's performance. Nonverbal behaviors and tutoring moves did not provide information at the sentence level; both the performance of the model and the feature contribution analysis suggested that their impact on the model output was not strong. This is consistent with results from Goel et al. (2019). However, in future work we would like to investigate the potential of multimodal patterns when we are able to better model sequentiality (e.g., negative feedback followed by a smile). Regarding the SHAP analysis, most of the features that are considered as important are coherent with the definition of the classes (*I* for subjectivizers, *sorry* for apologizers, *just* for propositional hedges). However, we discovered that features like utterance
size can also serve as indicators of certain classes of hedges. A limitation of SHAP is that it makes a feature independence assumption, which prompts the explanatory model to underestimate the importance of redundant features (like pronouns in our work). In the future we will explore explanatory models capable of taking into account the correlation between features in the dataset like SAGE (Covert et al., 2020), but suited for very imbalanced datasets. In the domain of peer-tutoring, we would like to be able to further test the link between hedges and rapport, and the link between hedges and learning gains in the subject being tutored. As noted above, this kind of study requires a fine-grained control of the language produced by one of the interlocutors, which is difficult to achieve in a human-human experience.

We note that the hedge classifier can be used not just to classify, but also to work towards improving the generation of hedges for tutor agents. In future work we will explore using the classifier to re-rank generation outputs, taking advantage of the recurring syntactic patterns (see (ii) in Section 5.3) to improve the generation process of hedges, and regenerating clauses that don't contain one of these syntactic patterns.

## Acknowledgments

Many thanks to members of the ArticuLabo at INRIA Paris for their precious assistance. This work was supported in part by the French government under management of Agence Nationale de la Recherche as part of the "Investissements d'avenir" program, reference ANR-19-P3IA-0001 (PRAIRIE 3IA Institute).

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## A Additional information on the experimental settings

We used PyTorch (Paszke et al., 2019) to implement the neural models. For each set of features, hyperparameters were selected using Optuna (Akiba, 2019), a parameter search framework. We reimplemented the Attention-CNN with Glove (Pennington et al., 2014) 300-D words embeddings as the vector representation. For each models, the results are cross-validated using 5 folds (we chose 5 instead of 10 to avoid having folds with too few samples per class). We corrected the loss function for class imbalance to force the model to adapt more to the less frequent classes. The strength of this correction depended on the model, and was selected because it provided a satisfying compromise between favoring recall and precision in the classification results of that model. For LightGBM, a "square root of the square root of the inverse class proportion" correction was selected. Neural models were trained using AdamW as an optimizer (Loshchilov and Hutter, 2018), and used a reduced feature vector, obtained with the application of PCA ($d_{init} = 1800$; $d = 100$; 99.8% of the information is conserved). No significant performance differences were observed between the original vector and the reduced vector for training the models. To compute the SHAP values mentioned in the paper, we kept one split to perform the 5-split of the dataset, and leave 1 split to validate and early stop the model, in order to avoid overfitting. A complete configuration of hyperparameters used for each model is reported in the GitHub repository with the code of the paper: https://github.com/YannRaphalen/Hedges-Detection.

The BERT model was fine-tuned on a Nvidia Quadro RTX 8000 GPU.

## B Tables
<table><caption>Table 7: Regexp rules used for the classifier.</caption><thead><tr><th>Class</th><th>Rule (regexp)</th></tr></thead><tbody><tr><td>Subj.</td><td>(?!what).* (ilwe) ?(don't|didn't|did)? ?(not)? (guess|guessed|thought|think|believe|believed|suppose|supposed) ?(whether|if|is|that|lit|this)?.*</td></tr><tr><td>Subj.</td><td>.* (ili'm|we) ?(was|aml|wasn't)? ?(not)? (sure|certain).*</td></tr><tr><td>Subj.</td><td>.* (i feel like you).*</td></tr><tr><td>Subj.</td><td>.* (you (might|m|ay) (believel|think)).*</td></tr><tr><td>Subj.</td><td>.* (according to|presumably).*</td></tr><tr><td>Subj.</td><td>.* (ilyou|we) have to (check|look|verify).*</td></tr><tr><td>Subj.</td><td>.* (if i'm not wrong|if i'm right|if that's true).*</td></tr><tr><td>Subj.</td><td>.* (unless i).*</td></tr><tr><td>Apol.</td><td>.* (i'm|lilwe're) (amlare)? ?(apologize|sorry).*</td></tr><tr><td>Apol.</td><td>(?!.* (bel|been|was) like excuse me)((excuse me|sorry)[w,]+|[w,]+(excuse me|sorry))</td></tr><tr><td>Prop.</td><td>.* (just|a little|may|believe|actually|sort of|kind of|pretty much|somewhat|exactly|almost|little bit|quitet|regular|regularly|actually|almost|as it were|basically| probably|can be view as|crypto|es|pecially|essentially| exceptionally|for the most part|in a manner of speaking| in a real sense|in a sense|in a way|largely|literally| loosely speaking|kind|also more or less|mostly|often| on the tall side|par excellence|particularly| pretty much|principally|pseudo-l|quintessentially| relatively|roughly|so to say|strictly speaking| technically|typically|virtually|approximately| something between|essentially|only).*</td></tr><tr><td>Prop.</td><td>.* (ili'm|lyou|it's) (amlare) (apparently|surely)[,]?.*</td></tr><tr><td>Prop.</td><td>.* (it) (looks|seems|appears)[,]?.*", ".* (or|land) (that|something|stuff|so forth)</td></tr></tbody></table>

<table><caption>Table 8: Significance table for the 3-classes part of Table 4. "Yes" means that the difference is statistically significant.</caption><thead><tr><th>Models</th><th>RB</th><th>MLP (KDF)</th><th>MLP (PTE)</th><th>MLP (K+P)</th><th>CNN (PTE)</th><th>LSTM (KDF)</th><th>LSTM (PTE)</th><th>LSTM (K+P)</th><th>BERT (PTE)</th><th>LGB (KDF)</th><th>LGB (PTE)</th><th>LGB (K+P)</th></tr></thead><tbody><tr><td>Rule-based MLP (KDF)</td><td>No</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>Yes</td><td>No</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>No</td></tr><tr><td>MLP (PTE)</td><td>Yes</td><td>Yes</td><td>No</td><td>No</td><td>Yes</td><td>Yes</td><td>No</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>No</td></tr><tr><td>MLP (KDF + PTE)</td><td>No</td><td>No</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>Attention-CNN (PTE)</td><td>No</td><td>No</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>LSTM (KDF)</td><td>No</td><td>No</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>LSTM (PTE)</td><td>Yes</td><td>Yes</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>No</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>LSTM (KDF + PTE)</td><td>No</td><td>No</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>BERT (PTE)</td><td>No</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>No</td></tr><tr><td>LGBM (KDF)</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>LGBM (PTE)</td><td>Yes</td><td>Yes</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>LGBM (KDF + PTE)</td><td>No</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr></tbody></table>

<table><caption>Table 9: Significance table for the 4-classes part of Table 4. "Yes" means that the difference is statistically significant.</caption><thead><tr><th>Models</th><th>RB</th><th>MLP (KDF)</th><th>MLP (PTE)</th><th>MLP (K+P)</th><th>CNN (PTE)</th><th>LSTM (KDF)</th><th>LSTM (PTE)</th><th>LSTM (K+P)</th><th>BERT (PTE)</th><th>LGB (KDF)</th><th>LGB (PTE)</th><th>LGB (K+P)</th></tr></thead><tbody><tr><td>Rule-based MLP (KDF)</td><td>No</td><td>Yes</td><td>Yes</td><td>No</td><td>Yes</td><td>Yes</td><td>No</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>MLP (PTE)</td><td>Yes</td><td>Yes</td><td>Yes</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>No</td></tr><tr><td>MLP (KDF + PTE)</td><td>Yes</td><td>Yes</td><td>Yes</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>Attention-CNN (PTE)</td><td>No</td><td>No</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>LSTM (KDF)</td><td>Yes</td><td>Yes</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>LSTM (PTE)</td><td>Yes</td><td>Yes</td><td>Yes</td><td>No</td><td>No</td><td>Yes</td><td>Yes</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>LSTM (KDF + PTE)</td><td>Yes</td><td>Yes</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>BERT (PTE)</td><td>No</td><td>No</td><td>Yes</td><td>Yes</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>No</td></tr><tr><td>LGBM (KDF)</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>LGBM (PTE)</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>LGBM (KDF + PTE)</td><td>Yes</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr></tbody></table>

<table><caption>Table 10: Significance table for the 3-classes part of Table 5. "Yes" means that the difference is statistically significant.</caption><thead><tr><th>Models</th><th>Label RB</th><th>+ 1-gram and 2-gram</th><th>+ POS</th><th>+ LIWC</th><th>+ TM</th><th>+ Nonverbal</th></tr></thead><tbody><tr><td>Label RB</td><td></td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>+ 1-gram and 2-gram</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td></tr><tr><td>+ POS</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td></tr><tr><td>+ LIWC</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td></tr><tr><td>+ TM</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td></tr><tr><td>+ Nonverbal</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td></tr></tbody></table>
<figure><img src="image_3.png" /><figcaption>Figure 2: Absolute averaged feature contribution, as indicated by SHAP. The longer the bar is for one color, the more the feature is associated with the class represented by that color.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 3: Averaged contribution of features to the detection of the "Not indirect" class, as indicated by SHAP. Each dot corresponds to a classified clause. A red dot indicates that the feature is present in the clause, while a blue dot indicates that the feature is absent. The farther on the right the dot is, the more the feature contributed to its classification as a hedge.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 4: Averaged contribution of features to the detection of "Apologizers", as indicated by SHAP.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 5: Averaged contribution of features to the detection of "Propositional hedges", as indicated by SHAP.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 6: Averaged contribution of features to the detection of "Subjectivizers", as indicated by SHAP.</figcaption></figure>

<table><caption>Table 11: Significance table for the 4-classes part of Table 5. "Yes" means that the difference is statistically significant.</caption><thead><tr><th>Models</th><th>Label RB</th><th>+ 1-gram and 2-gram</th><th>+ POS</th><th>+ LIWC</th><th>+ TM</th><th>+ Nonverbal</th></tr></thead><tbody><tr><td>Label RB</td><td></td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td></tr><tr><td>+ 1-gram and 2-gram</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td></tr><tr><td>+ POS</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td></tr><tr><td>+ LIWC</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td></tr><tr><td>+ TM</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td></tr><tr><td>+ Nonverbal</td><td>Yes</td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td></tr></tbody></table>