INLG 2022 — The 15th ACL International Conference on Natural Language Generation (INLG)

🏆 Best Long-Paper Award

Evaluating Referring Form Selection Models in Partially-Known Environments

Zhao Han, Polina Rygina, Tom Williams

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referring form selection data decision tree model

This paper won the Best Long-Paper Award at INLG 2022:


For autonomous agents such as robots to effectively communicate with humans, they must be able to refer to different entities in situated contexts. In service of this goal, researchers have recently attempted to model the selection of referring forms on the basis of cognitive status (informed by Giveness Hierarchy), and have shown promising results with over 80% accuracy. However, we argue that the task environments lack ecological validity, due to their use of a small number of objects that are constantly activated and easily uniquely identifiable. Accordingly, we present a novel building-construction task that we believe has increased ecological validity. We then show how training cognitive status informed referring form selection models on data collected within this novel task environments yields substantially different results from those found in previous work, providing key insights and directions for future work.

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Video Presentation (10m)


One of the most studied dimensions of natural language pragmatics is reference: the process by which speakers pick out, or refer, to things of interest in the environment, and how hearers interpret, or resolve those references. The generation (or production) side of this problem has attracted sustained attention across a variety of communities, including philosophy of language, psycholinguistics, and artificial intelligence – so much so that referring has been called the “fruit fly” of language (?).

The vast majority of research on referring, however, has been focused on problems like Referring Expression Generation (?), in which the goal is to select the properties that will be used in a generated expression (e.g., choosing to highlight the redness, or the boxiness, of a red box, among other possible properties). In contrast, very little research has been done on the problem of computationally modeling Referring Form Selection, in which a speaker must select a more general referring form, such as “it”, “that”, or “the ⟨N′⟩”1 , despite its accepted status as an important initial step during language production (?).

While the computational modeling of referring expression generation has been heavily under-studied, it has been a key question of interest in the linguistics community, with a number of competing theories making different predictions, including Accessibility Theory (?) and Givenness Hierarchy Theory (?). Such theories thus provide natural starting points for computational modeling work. Yet while these theories provide critical linguistic insights about the nature of referring form selection, they provide little direct input into the cognitive processes, mechanisms, or algorithms that govern this process.

Recently, this has begun to change, with researchers like seeking to directly computationally model the mechanics of these underlying theories of reference (in their case, Givenness or Cognitive Status), and then build higher-level computational models of referring form selection that leverage those more fundamental models (?). These recent works have provided promising results, with over 80% accuracy in predicting the referring forms used by interactants in human-human and human-robot interaction scenarios.

Yet despite the promise of these results, concerns may be raised about the task environments in which those results were produced. Specifically, we argue that the task environment used in that previous work was not ideally suited for training or evaluating Cognitive Status informed Referring Form Selection models.

In this paper, we thus measure the performance of these previously published models using a better collection of tasks, making three key contributions in the process: (1) we present a novel task context that we argue is well designed for the studying of referring form selection; (2) we assess the performance of ?’s Referring Form Selection model in this setting to obtain a better estimate of its true performance in realistic task contexts; and (3) we use these results to motivate arguments as to how underlying models of cognitive status must be adapted to enable better performance on Referring Form Selection tasks.

Related Work

We will now describe prior what work has been done on Referring Form Selection, including the Cognitive Status informed work of ?. We will then provide our specific critiques of the task context in which that work was trained and evaluated.

Referring Form Selection models fall into two main categories (?). Rational models seek to explain how speakers egocentrically decide whether or not to use pronouns, e.g. for reasons of ease of production (??). In contrast, pragmatic models seek to explain how speakers allocentrically decide to use pronouns on the basis of their status as activated or focused within a conversation (????). These pragmatic models share an assumption that referring form selection is grounded in a relationship between discourse context and mnemonic or attentional states. For example, suggest that referring forms are selected based on which of a hierarchically nested set of Cognitive Statuses ({in focus ⊆ activated ⊆ familiar ⊆ uniquely identifiable ⊆ referential ⊆ type identifiable}) can be assumed to hold for the target referent.

While these models have shown promise in predicting whether or not someone chooses to use a definite noun phrase or a more reduced form, neither class of model is terribly effective at predicting precisely which form a speaker will choose to use. Rational models, for example, predict much more frequent use of reduced forms than are actually seen in practice, and fail to predict differential usage of “equally short” referring forms (?). To make matters work, models in both categories tend to focus on specific referential phenomena, rather than trying to comprehensively model the entire process of reference production (??); and indeed often do not really try to model cognitive mechanisms or psycholinguistic processes at all (?). And, of critical importance to those studying situated interaction, the vast majority of this previous work, in both camps, has predominantly been assessed on corpora not collected in or encoding any features of situated domains.

Work in the Artificial Intelligence community on Referring Form Selection suffers from similar problems. Most such work (???????) falls under multi-factorial process modeling, in which the process of referring is modeled as a classification problem performed on the basis of a variety of features (???). Like the linguistic models discussed above, these models often do not attempt to select between referring forms at a fine-grained level, instead choosing to predict pronoun use as a whole. And, like the linguistic models discussed above, these models are often trained and evaluated in purely textual domains, such as the Wall Street Journal corpus (?), thus avoiding many of the nuanced challenges that arise in situated domains, which are highly ambiguous and open worlds, and in which agents must make decisions on the basis of features that can be readily and immediately assessed, which may well go beyond purely linguistic features, including features of the environment in which dialogue is situated.

Some recent research efforts have attempted to fix these problems. ?, for example, have presented models for dynamic modeling of Cognitive Status (a construct underlying Givenness Hierarchy theoretic accounts of referring (?)), and have then used these models as informative features for Referring Form Selection, with apparently good results (?). Pal et al.’s work is also notable in that it is trained on data collected in situated interaction contexts. However, even this work suffers from certain flaws that may raise similar questions about generalizability to situated domains. Specifically, we argue that Pal et al.’s work was conducted in a task environment that may not have been well suited for training or evaluation of cognitive status informed models. Pal et al.’s model was trained and evaluated using videos collected by ?, in which humans give instructions to humans or robot interactants as to how to re-arrange a large-scale environment to match a pre-determined pattern.

This task domain may be ill-suited to studying Cognitive Status informed Referring Form Selection for several reasons. First, this domain contains a relatively small number of candidate referents, i.e., three towers of cans and four labeled boxes. This could result in an irregular situation in which the majority of task-relevant objects are constantly at least activated (which, in a Givenness Hierarchy theoretic account, would enable the use of referring forms such as this), and are likely to remain so regardless of dialogue context merely due to the small number of observed task relevant objects. Second, all task-relevant objects in this domain are easily uniquely discriminable. Each of the “towers” has a unique context, and each of the boxes is labeled with a unique letter. This means that speakers may be able to over-rely on proper nouns and simple single-property descriptions, and would not need to seriously consider their choice of referring expression. Third, all task-relevant objects in this domain are visible at all times. This is likely to exacerbate the challenges listed above. Moreover, it is likely to completely preclude the need for indefinite descriptions, which are often used when the speaker assumes that the listener does not already have knowledge of their target referent.

In this work, we seek to address these challenges. We begin by collecting a new corpus of sequential referring expressions in a task context that does not have these shortcomings. Then, we re-assess the performance of ?’s Referring Form Selection models on the data collected in this more ideally suited task domain.

3 Environment and Task Design

[Annotated] Two of four quadrants of the task environment to study referring behaviors for linguistic HRI. Two rule cards are placed on each table, with learners' on the left hand side and instructors' (dashed box) on the right hand side (solid box). Objects are intentionally placed at the intersections of a 3×3 grid to encourage use of different referring forms whose use varies according to distance. Instructors teach learners to construct buildings whose constituents blocks are distributed across the visible and non-visible quadrants.
Figure 1: Two of four quadrants of the task environment. To promote a wide variety of referring forms, we placed objects in different quadrants with careful manipulation of target referent visibility (thus leading to course-grained variance in cognitive status) and by requiring repeated reference to task referents (thus leading to fine-grained variance in cognitive status).

To collect a wider variety of referring forms in a situated context, we designed a dyadic interaction task (Shown in Figure 1) in which pairs of participants perform four tower constructions tasks in four visually separated quadrants of a larger task environment. The task environment is separated into four quadrants to create a partially-observable environment in which participants can readily observe their current quadrant, but not the other quadrants. Each quadrant is filled with block shapes, including triangles, cubes, cuboids, cylinders, arches, and half-circles. All blocks are distributed to the corners and intersections of a 3 × 3 grid.

Four buildings to be constructed, with 18 blocks for each. To help participants identify individual blocks, two angles were provided. For any building, half (9) of its blocks are in the current quadrant while the other half are distributed to the remaining three quadrants, allowing us to collect references to non-present objects.
Figure 2: The buildings in the construction task. Two angles were provided to help participants to recognize constituent block shapes.

The described task environment is used as the setting for a series of four dyadic construction tasks, one in each of the four quadrants. Each task requires one participant (the Teacher) to instruct the other participant (the Learner) to construct a building based on a given image (Figure 2). The Learner, in turn, must work to construct the tower piece by piece as it is described to them, without speaking themselves, using only the resources available in their current quadrant unless the Teacher instructs them to seek a block in a different quadrant. Note that participants do not statically provide or listened to monolithic multi-minute monologues. In fact, the task is highly interactive, with teachers giving instructions, learners following instructions, and then teachers providing corrections or proceeding. While learners were mostly silent while completing their tasks, this is perfectly reasonable given the particular domain we investigate in this work, i.e., deciding how to deliver multi-step task instructions.

Each building has 18 blocks, nine (50%) of which are placed in the quadrant where the building is being constructed, the other half of which are distributed in the other three quadrants. The large number of blocks in this task context ensure that, unlike in Pal et al.’s work, there are a large number of candidate referents that are not trivially distinguishable. The separation between quadrants ensures that, unlike in Pal et al.’s work, not all objects are visible at any given time. And, the distribution of blocks throughout the four quadrants ensures that the Teacher will need to refer to blocks that have not yet have been observed or which were observed in a previous construction task but which are no longer visible in the current quadrant, further diversifying the expected set of referring expressions used by Teachers.

Corpus Collection Procedure

The described environmental and task context were used to collect a new corpus of referring expressions, through the following IRB-approved procedure. Eleven pairs of participants were recruited from the campus of The Colorado School of Mines. Upon arrival, each pair of participants provided informed consent and were provided instructions about the structure of the tower construction task. Participants were then led to the task environment and seated in the first quadrant, where a photo of the target buildingwas available to the participant assigned to be the Teacher, as seen in Figure 1. Participants were then videorecorded completing each of the four tower construction tasks in sequence. Each participant was paid $10 USD.

Corpus Annotation

The collected eleven-dyad corpus was comprised of eleven collections of four monologues each. These eleven collections averaged 27:32 minutes in length, with a minimum of 16:26 and maximum of 34:03. The average monologue length was 6:53. We first transcribed these recordings automatically online using the Dovetail qualitative analysis software2 . The first two authors then manually verified and corrected these transcripts. The collected transcripts were then divided into a total of 1992 utterance clauses.

After removing clauses that contained no referring forms (e.g., utterances made when switching quadrants and at study conclusion) or only plural referring forms (e.g., them, they), which we did not aim to model in this work, 1867 referring expressions remained, including from the corrective instructions. Each participant contributed an average of 169.7 referring forms, which is significantly more than the average of 18 (60333) referring forms per participant in the situated interaction corpus (?) used by ?. The data does not have information that names or uniquely identifies individual people or offensive content. Below, we provide two sample utterance sequences from the collected corpus.

Sample 1

  • Alright. Do you see the red block over there?
  • We need that but the blue.
  • Awesome. Put that leg on that side to the left of the green cube, just like that.
  • And you see the red thing I was talking about?
  • Put that right on top of the blue thing. Perfect.

Sample 2

  • And then the blue cylinder is going to go there.
  • Okay. So for the next, we need this one.
  • And you can go ahead and set that up right next to the triangle.
  • And put it vertically on the inside.
refer form bar
Figure 3: Distribution of a wide variety of referring forms. In addition to the six definitive nouns (right six columns), we also found participant frequently used indefinite nouns of “a ⟨N′⟩” and “another ⟨N′⟩” (left two columns).

After translating each of the corpus’ 44 monologues into a sequence of (non-plural) referring forms, we categorized each into one of eight categories (See Figure 3), and annotated, at each reference point, key features of each candidate object in the environment. Critically, we ensured that the features used could all be assessed on the fly, to ensure they could actually be used in future robotics applications. In the following subsections, we detail both of these types of annotation.

5.1 Referring Forms

We categorized referring expressions into seven types of referring forms: itthisthatthis N’that N’the N’〉 and ⟨indefinite NP⟩. While indefinite noun phrases took multiple forms (e.g., “a ⟨N′⟩”, which accounted for 16.3% of all referring forms, and “another ⟨N′⟩”, which accounted for 4.2% of referring forms (per Figure 3). Similarly, like (?), we take a descriptivist view (???) and merge bare noun phrases together with definite noun phrases.

5.2 Object Features

Next, we discuss the features annotated for each object in the scene at each reference point. We used the same four simple features used with great success by ?, both because they are easily assessable by autonomous agents like robots, and to facilitate direct comparison with ?. Each of these four features is described in a subsection below.

5.2.1 Cognitive Status

The first feature used was Cognitive Status, which was, unsurprisingly, the most informative feature used in ?’s Cognitive Status informed approach. To annotate the cognitive status of each object in the scene at each point of reference, we used the Cognitive Status model used by ?, as defined in ?. This approach uses a Cognitive Status Engine comprised of a set of Cognitive Status Filters, one for each object. Each Cognitive Status Filter is a Bayesian filter of the form:

t       t− 1    t    t   t−1   t
p(So) = p(S o  )p(L o)p(So | So ,L o)

Here, is a cognitive status in {I,A,F} (where I is “In Focus”, is “Activated”, and is “Familiar”, predicted from an object’s cognitive status at the previous time point and the object’s “linguistic status” at the previous timepoint ∈{N,M,T} (where is “Not Mentioned”, is “Mentioned”, and is “Mentioned in a Topic Role”. To compare directly with and ?, we have made the same assumption that all objects are initially at lease familiar. This is a simplifying assumption that we will return to later.

Initially, using this model identically to how it was used by failed to predict any objects in the scene to be “In Focus” at any timepoint. To diagnose this problem, we created a blended model by linearly combining a non-probabilistic model HL directly derived from linguistic rules (cp. (?)) with the probabilistic model trained by ?C′ = w1HL w2C, where wl = 0.1, wc = 0.9. Here, HL is encoded as a 9 × 3 matrix (×L) where each row (column, as transposed below) represented a combination of a cognitive status and linguistic status at time − 1, and each column (row, as transposed below) represented a cognitive status at time t:

      ⌊1  0  0  1  0  0  1  0  0⌋
LT =  ⌈0  1  0  0  1  0  0  1  0⌉

       0  0  1  0  0  1  0  0  1

This blended model was then used as the basis for each Cognitive Status Filter.

5.2.2 Number of Distractors

Next, we considered the number of distractors, which are the number of objects that have a cognitive status at the same GH-theoretic tier or higher than the target. We believe the number of distractors affects how people determine referential choice, as evidenced by ?.

5.2.3 Physical distance

FL: 6FM: 5FR: 6
ML: 4MM: 3MR: 4
NL: 2NM: 1NR: 2

Table 1: Codes for physical distance.

During our task design phase, we have intentionally placed blocks at a 3 × 3 grid, allowing us to classify each referring form in at least nine categories, a combination of {near (N), middle (M), far (F)} and {left (L), middle (M), right (R)}. The nine grid points are coded as shows in Table 1 (note that a participant sits below NM).

Additionally, each object can be in one of four distance-relevant task categories at any time: on-table (T), in-building (B), in-hand (H), and in-other-quadrant (O). Although B and H are specific to our task scenario, they can be generalized: B can be seen as objects at the task goal location, and H can be generalized to invisible locations. Because T is a general term, we coded it the same as MM, i.e., 3. B and H do not have distance comparisons, we coded them as 0. As O is furthest, we coded it as 10. This was a simplifying assumption to best compare with prior work.

5.2.4 Temporal Distance

Similar to ?, we annotated recency of mention, i.e., temporal distance, for each object by indexing the previous occurrence of the object. TD is coded as 0 when an object is not yet mentioned in a monolog, 1 when the object is the last mentioned object, and 1∕n where is the number of objects referred since the object was mentioned.

Computational Modeling

As we intended to interrogate the performance of previous published models, we use the same decision tree algorithm by for explainability and theory-building purposes. Specifically, we used the same decision tree implementation in Weka 3.8.6 (?): REPTree (Reduced Error Pruning Tree) (?), an extension of the C4.5 algorithm. REPTree builds a decision tree using information gain and prunes the tree using reduced-error pruning (REP) with backfitting (?).

ModelRemoved Feature
M1N/A (full model)
M2Cognitive status
M3Number of distractors
M4Physical distance
M5Temporal distance

Table 2: Five model types.

Similarly, we followed the same training procedure as ?, training five distinct models (Table 2): a full model (M1), and four ablated models, removing either cognitive status (M2), distractors (M3), physical distance (M4), or temporal distance (M5). We initially set the maximum depth of the tree to six, the same as Pal et al.’s model, but the decision tree became complex and difficult to interpret/explain, we thus set the maximum allowed depth of the tree to five. Similar performance was observed at depth 5 vs. 6.

The performance of these five models (for unpruned and pruned trees) were evaluated using five-fold cross validation to further avoid over-fitting (note that the tree pruning is also for this purpose). To quantify the models’ performance, five common scoring metrics are used, as in (?): accuracy, root mean squared error (RMSE), precision, recall, and F1 score. The latter three are weighted by class size. Additionally, we used coverage (modeled as number of classes included in model predictions) and number of leaves to quantify model simplicity and explainability.

All data (which will be licensed under CC-BY 4.0) and code are attached to this submission.

Six GH informed referring formsWith two indefinite forms
F1 score0.5970.5710.5960.5460.5600.5440.5390.5450.4540.522

Table 3: Evaluation metrics and results for unpruned trees.

Six GH informed referring formsWith two indefinite forms
F1 score0.5890.5760.5890.5420.5560.5360.5280.5360.4450.514

Table 4: Evaluation metrics and results.

6.1 Results

Table 3 shows the results for the full, unpruned trees; Table 4 shows the results for the pruned trees, which had lower coverage but are more readily interpretable. In both tables, the left and right sides show results with and without indefinite forms included. We consider these separately as did not consider indefinite noun phrases.

In this section we will more deeply interrogate the results of the pruned trees, as they are more readily interpretable. As seen in Table 4 left, we achieved 61%–66% accuracy for M1-M5. M1 and M3 (removing the number of distractors) are top-performing on all metrics. For M5 where the temporal distance feature was not used, the performance is slightly dropped to 61.72%. All models scored similarly in other metrics.

tree M1 6
Figure 4: The decision tree visualization for the best-performing M1 (six main referring form categories informed by Givenness Hierarchy). {I,A,F}={In Focus, Activated, Familiar}.

Fig. 4 shows a tree visualization for the M1 model. From the top node, it branches at the temporal distance (TD) at 2.5 (root) and 0.5 (depth 1). When TD ∈ [1,2] (left branch), the model looks at the cognitive status, where “it” is used if an object is in focus (I), “the ⟨N′⟩” is used otherwise. When TD = 0 (i.e., TD < 0.5), “the ⟨N′⟩” is used. When TD is far (TD ≥ 3), i.e., the right side of the tree, physical distance (PD) is used to differentiate between “this ⟨N′⟩” and “the ⟨N′⟩” of PD ≥ 3. When the objects are closer (PD ≤ 1, i.e., the objects are in near middle (NM), in hand or in building), cognitive status and temporal distance plays a more important role. Specifically, “that” is used when the cognitive status is in focus and mentioned a few utterances ago (TD ≥ 4). The number of distractors was not selected as a decision node.

For the eight-class referring form classification, the accuracy score dropped up to approximately 10% to 51.30%–59.83%. M3’, without the number of distractors feature, performed as well as full model M1’.

tree M1 8
Figure 5: The decision tree visualization for the best-performing M1’ model (eight referring form categories). Because indefinite nouns are included, physical distance became the root node.

Figure 5 shows the visualization of the M1’ model. Because indefinite referring forms were added and they were used to refer to non-present objects, the physical distance feature determines when to use “a ⟨N′⟩”, as seen in the rightmost traversal. Within the task environment, on the far side, physical distance separates the usage of “this ⟨N′⟩” and “the N” (the third-right most and the second-right most leaves); this is exactly the same as M1 model, as seen in the rightmost subtree in Figure 4. For the cognitive status, “the ⟨N′⟩” is used when it is lower than In Focus (I) and the object is in front of the instructor (PD = 1, i.e., PD < 1.5). For In Focus, if the object is less temporally distant (TD >= 3), “it” is used as expected; Otherwise, “the ⟨N′⟩” is used. When the cognitive status is Activated or Familiar, “the ⟨N′⟩” is used.

The unpruned trees are available in Appendix 9. As there are many branches and leaves, we do not step through them here. To show the simplicity of the tree with maximum allowed depth 5, Appendix 9 shows the trees with maximum depth 6 and Appendix 9 shows those trees without maximum depth set.

Compared with ?’s model performance (72%-86%), the performance of those models trained with the new dataset, especially with the frequently-used indefinite nouns, yielded approximately 20% drop in performance.


By designing a new task, we were able to collect a situated corpus with a wide variety of referring forms. The corpus also include two frequently used indefinite nouns that were not observed in previous corpora, thanks to careful manipulation of object visibility and the partially observable environment with four quadrants. Using this more ecologically valid task environment, we were able to show that the high performance of ?’s work may have been artificially inflated by the nature of their task environment.

Before continuing, we would like to state that this work is not a simple replication of ?’s paper. Our work contributes a novel situated building-construction task that is much improved over the task used by ?. Moreover, we expand significantly beyond their work, dealing with more difficult issues such as significantlymore objects, their visibility and cognitive status, and ambiguity. In the rest of this section we detail and further interrogate why we believe we observed this performance difference.

First, ?’s model was trained on a small dataset of referring forms, in which all have similar cognitive status (activated or in focus) due to the small set of 11 objects (compared to 72 objects in this work). ?’s task environment also involved very short dialogues, whereas our tower construction task took an average of half an hour to finish. The small dataset used by ? may have resulted in over-fitting.

Second, in ?’s task, all objects were either labeled or uniquely distinguishable. In contrast, our tower construction task had only a few shapes of blocks used across 72 blocks, significantly increasing ambiguity.

Third, indefinite nouns were not considered by ?, who only used visible objects. As we see from Figure 3 (the left two bars), indefinite nouns were common in our task. In the previous modeling effort, the cognitive status filters (CSFs) assume all object are at least activated and do not attempt to reason about what is “not known of” to the interlocutor, as the assumption was that both interlocutor and autonomous agents such as robots know of the same objects in the scene. Future work should weaken this assumption to model Theory of Mind reasoning.

Limitations and Future Work

The observed performance gaps motivate possible improvements. During task design, we explicitly intended to collect a multimodel situated dataset, not only language but also gestures, which are particularly informative and suited for situated contexts. We plan to analyze our collected videos and extract gestures, which will likely serve as informative features, as deictic gestures will likely be used on objects’ first reference to facilitate use of “this” and “that”. In contrast, abstract gestures may be used when objects are in previous quadrants (?).

As mentioned in Section 5.2.1, all objects were annotated as at least Familiar to best compare with Pal et al.’s work. Yet, this assumption is clearly violated, especially for objects not yet seen in the task. How to ascribe cognitive status to not-yet-seen objects is a challenging philosophical question, though. We plan to address this in future work.

Finally, to minimize differences between the model trained in this work and that trained by ?, we excluded a feature that would likely have been informative: referent visibility. As discussed, non-visibility was coded as a physical distance of 10; in future work this should be treated as a separate feature.


We presented a new interaction-based task design to collect a new situated corpus to advance the computational modelling for referring form selection. Specifically, we adapted the modelling technique used by and reassess its performance on the new corpus. In future work, we plan to annotate the gestures used in our experiment and improve the computational modelling trained on the new multimodal dataset, moving beyond pure replication.

Supplementary Materials

The data and decision tree code can be found at


This work has been supported in part by Office of Naval Research grant N00014-21-1-2418.

Please see PDF for appendices.

Appendix A: Unpruned Decision Trees

tree M1 6 unprune
Figure 6: The unpruned decision tree (with six major referring forms).
tree M1 8 noprune
Figure 7: The unpruned decision tree (with six major referring forms and two indefinite referring forms).

Appendix B: Pruned Decision Trees With Maximum Allowed Depth 6

tree M1 6 d6
Figure 8: The decision tree visualization for M1 with maximum allowed depth (six main referring form categories informed by Givenness Hierarchy).
tree M1 8 d6
Figure 9: The decision tree visualization for M1 with maximum allowed depth (eight main referring form categories). Note that this is exactly the same as Figure 5.

Appendix C: Pruned Decision Trees With Maximum Allowed Depth Unset

tree M1 6 d unset
Figure 10: The decision tree visualization for M1 with maximum allowed depth unset (six main referring form categories informed by Givenness Hierarchy).
tree M1 8 d unset
Figure 11: The decision tree visualization for M1 with maximum allowed depth unset (eight main referring form categories).