Abstract

Recent approaches to data-to-text generation have adopted the very successful encoder-decoder architecture or variants thereof. These models generate text that is fluent (but often imprecise) and perform quite poorly at selecting appropriate content and ordering it coherently. To overcome some of these issues, we propose a neural model with a macro planning stage followed by a generation stage reminiscent of traditional methods which embrace separate modules for planning and surface realization. Macro plans represent high level organization of important content such as entities, events, and their interactions; they are learned from data and given as input to the generator. Extensive experiments on two data-to-text benchmarks (RotoWire and MLB) show that our approach outperforms competitive baselines in terms of automatic and human evaluation.

1 Introduction

Data-to-text generation refers to the task of generating textual output from non-linguistic input (Reiter and Dale, 1997, 2000; Gatt and Krahmer, 2018) such as databases of records, simulations of physical systems, accounting spreadsheets, or expert system knowledge bases. As an example, Figure 1 shows various statistics describing a major league baseball (MLB) game, including extracts from the box score (i.e., the performance of the two teams and individual team members who played as batters, pitchers or fielders; Table (A)), play-by-play (i.e., the detailed sequence of each play of the game as it occurred; Table (B)), and a human written game summary (Table (C)).

Figure 1: 

MLB statistics tables and game summary. Tables summarize the performance of teams and individual team members who played as batters and pitchers as well as the most important actions (and their actors) in each play (Tables (A) and (B)). Macro plan for the game summary is shown at the bottom (Table (E)). <P> indicates paragraph delimiters. There is a plan for every paragraph in the game summary (correspondence shown in same color); <V(entity)> verbalizes entities, while <V(inning-T/B)> verbalizes events related to the top/bottom side of an inning (see Section 3.1). Set of candidate paragraph plans are shown above macro plan (Table (D)) and grouped into two types: plans describing a single entity/event or their combinations. Best viewed in color.

Figure 1: 

MLB statistics tables and game summary. Tables summarize the performance of teams and individual team members who played as batters and pitchers as well as the most important actions (and their actors) in each play (Tables (A) and (B)). Macro plan for the game summary is shown at the bottom (Table (E)). <P> indicates paragraph delimiters. There is a plan for every paragraph in the game summary (correspondence shown in same color); <V(entity)> verbalizes entities, while <V(inning-T/B)> verbalizes events related to the top/bottom side of an inning (see Section 3.1). Set of candidate paragraph plans are shown above macro plan (Table (D)) and grouped into two types: plans describing a single entity/event or their combinations. Best viewed in color.

Traditional methods for data-to-text generation (Kukich, 1983; McKeown, 1992; Reiter and Dale, 1997) follow a pipeline architecture, adopting separate stages for text planning (determining which content to talk about and how it might be organized in discourse), sentence planning (aggregating content into sentences, deciding specific words to describe concepts and relations, and generating referring expressions), and linguistic realization (applying the rules of syntax, morphology, and orthographic processing to generate surface forms). Recent neural network–based approaches (Lebret et al., 2016; Mei et al., 2016; Wiseman et al., 2017) make use of the encoder-decoder architecture (Sutskever et al., 2014), are trained end-to-end, and have no special-purpose modules for how to best generate a text, aside from generic mechanisms such as attention and copy (Bahdanau et al., 2015; Gu et al., 2016). The popularity of end-to-end models has been further boosted by the release of new datasets with thousands of input-document training pairs. The example shown in Figure 1 is taken from the MLB dataset (Puduppully et al., 2019b), which contains baseball game statistics and human written summaries (∼25K instances). RotoWire (Wiseman et al., 2017) is another widely used benchmark, which contains NBA basketball game statistics and their descriptions (∼5K instances).

Wiseman et al. (2017) show that despite being able to generate fluent text, neural data-to-text generation models are often imprecise, prone to hallucination (i.e., generate text that is not supported by the input), and poor at content selection and document structuring. Attempts to remedy some of these issues focus on changing the way entities are represented (Puduppully et al., 2019b; Iso et al., 2019), allowing the decoder to skip low-confidence tokens to enhance faithful generation (Tian et al., 2019), and making the encoder-decoder architecture more modular by introducing micro planning (Puduppully et al., 2019a; Moryossef et al., 2019). Micro planning operates at the record level (see Table (A) in Figure 1; e.g., C.Mullins BH 2, J.Villar TEAM Orioles), it determines which facts should be mentioned within a textual unit (e.g., a sentence) and how these should be structured (e.g., the sequence of records). An explicit content planner essentially makes the job of the neural network less onerous allowing to concentrate on producing fluent natural language output, without expending too much effort on content organization.

In this work, we focus on macro planning, the high-level organization of information and how it should be presented which we argue is important for the generation of long, multi-paragraph documents (see text (C) in Figure 1). Problematically, modern datasets like MLB (Puduppully et al., 2019b; and also Figure 1) and RotoWire (Wiseman et al., 2017) do not naturally lend themselves to document planning as there is no explicit link between the summary and the content of the game (which is encoded in tabular form). In other words, the underlying plans are latent, and it is not clear how they might be best represented, namely, as sequences of records from a table, or simply words. Nevertheless, game summaries through their segmentation into paragraphs (and lexical overlap with the input) give clues as to how content might be organized. Paragraphs are a central element of discourse (Chafe, 1979; Longacre, 1979; Halliday and Hasan, 1976), the smallest domain where coherence and topic are defined and anaphora resolution is possible (Zadrozny and Jensen, 1991). We therefore operationalize the macro plan for a game summary as a sequence of paragraph plans.

Although resorting to paragraphs describes the summary plan at a coarse level, we still need to specify individual paragraph plans. In the sports domain, paragraphs typically mention entities (e.g., players important in the game), key events (e.g., scoring a run), and their interaction. And most of this information is encapsulated in the statistics accompanying game summaries (see Tables (A) and (B) in Figure 1). We thus define paragraph plans such that they contain verbalizations of entity and event records (see plan (E) in Figure 1). Given a set of paragraph plans and their corresponding game summary (see Table (D) and summary (C) in Figure 1), our task is twofold. At training time, we must learn how content was selected in order to give rise to specific game summaries (e.g., how input (D) led to plan (E) for summary (C) in Figure 1), while at test time, given input for a new game, we first predict a macro plan for the summary and then generate the corresponding document.

We present a two-stage approach where macro plans are induced from training data (by taking the table and corresponding summaries into account) and then fed to the text generation stage. Aside from making data-to-text generation more interpretable, the task of generating a document from a macro plan (rather than a table) affords greater control over the output text and plays to the advantage of encoder-decoder architectures which excel at modeling sequences. We evaluate model performance on the RotoWire (Wiseman et al., 2017) and MLB (Puduppully et al., 2019b) benchmarks. Experimental results show that our plan-and-generate approach produces output that is more factual, coherent, and fluent compared with existing state-of-the-art models. Our code, trained models, and dataset with macro plans can be found at https://github.com/ratishsp/data2text-macro-plan-py.

2 Related Work

Content planning has been traditionally considered a fundamental component in natural language generation. Not only does it determine which information-bearing units to talk about, but also arranges them into a structure that creates coherent output. Many content planners have been based on theories of discourse coherence (Hovy, 1993), schemas (McKeown et al., 1997), or have relied on generic planners (Dale, 1989). Plans are mostly based on hand-crafted rules after analyzing the target text, although a few approaches have recognized the need for learning-based methods. For example, Duboue and McKeown (2001) learn ordering constraints in a content plan, Konstas and Lapata (2013) represent plans as grammar rules whose probabilities are estimated empirically, while others make use of semantically annotated corpora to bootstrap content planners (Duboue and McKeown, 2002; Kan and McKeown, 2002).

More recently, various attempts have been made to improve neural generation models (Wiseman et al., 2017) based on the encoder-decoder architecture (Bahdanau et al., 2015) by adding various planning modules. Puduppully et al. (2019a) propose a model for data-to-text that first learns a plan from the records in the input table and then generates a summary conditioned on this plan. Shao et al. (2019) introduce a Planning-based Hierarchical Variational Model where a plan is a sequence of groups, each of which contains a subset of input items to be covered in a sentence. The content of each sentence is verbalized, conditioned on the plan and previously generated context. In their case, input items are a relatively small list of attributes (∼28) and the output document is also short (∼110 words).

There have also been attempts to incorporate neural modules in a pipeline architecture for data-to-text generation. Moryossef et al. (2019) develop a model with a symbolic text planning stage followed by a neural realization stage. They experiment with the WebNLG dataset (Gardent et al., 2017) which consists of RDF 〈 Subject, Object, Predicate 〉 triples paired with corresponding text. Their document plan is a sequence of sentence plans that in turn determine the division of facts into sentences and their order. Along similar lines, Castro Ferreira et al. (2019) propose an architecture composed of multiple steps including discourse ordering, text structuring, lexicalization, referring expression generation, and surface realization. Both approaches show the effectiveness of pipeline architectures, however, their task does not require content selection and the output texts are relatively short (24 tokens on average).

Although it is generally assumed that task-specific parallel data is available for model training, Laha et al. (2020) do away with this assumption and present a three-stage pipeline model which learns from monolingual corpora. They first convert the input to a form of tuples, which in turn are expressed in simple sentences, followed by the third stage of merging simple sentences to form more complex ones by aggregation and referring expression generation. They also evaluate on data-to-text tasks which have relatively short outputs. There have also been efforts to improve the coherence of the output, especially when dealing with longer documents. Puduppully et al. (2019b) make use of hierarchical attention over entity representations which are updated dynamically, while Iso et al. (2019) explicitly keep track of salient entities and memorize which ones have been mentioned.

Our work also attempts to alleviate deficiencies in neural data-to-text generation models. In contrast to previous approaches, (Puduppully et al., 2019a; Moryossef et al., 2019; Laha et al., 2020), we place emphasis on macro planning and create plans representing high-level organization of a document including both its content and structure. We share with previous work (e.g., Moryossef et al. 2019) the use of a two-stage architecture. We show that macro planning can be successfully applied to long document data-to-text generation resulting in improved factuality, coherence, and fluency without any postprocessing (e.g., to smooth referring expressions) or recourse to additional tools (e.g., parsing or information extraction).

3 Problem Formulation

We hypothesize that generation based on plans should fare better compared to generating from a set of records, since macro plans offer a bird’s-eye view, a high-level organization of the document content and structure. We also believe that macro planning will work well for long-form text generation, that is, for datasets that have multi-paragraph target texts, a large vocabulary space, and require content selection.

We assume the input to our model is a set of paragraph plans E={ei}i=1|E| where ei is a paragraph plan. We model the process of generating output summary y given E as a two step process, namely, the construction of a macro plan x based on the set of paragraph plans, followed by the generation of a summary given a macro plan as input. We now explain how E is obtained and each step is realized. We discuss our model considering mainly an example from the MLB dataset (Puduppully et al., 2019b) but also touch on how the approach can be straightforwardly adapted to RotoWire (Wiseman et al., 2017).

3.1 Macro Plan Definition

A macro plan consists of a sequence of paragraph plans separated by a paragraph discourse marker <P>, that is, x = ei<P>ej<P>ek where ei,ej,ekE. A paragraph plan in turn is a sequence of entities and events describing the game. By entities we mean individual players or teams and the information provided about them in box score statistics (see rows and column headings in Figure 1 Table (A)), while events refer to information described in play-by-play (see Table (B)). In baseball, plays are grouped in half-innings. During each half of an inning, a team takes its turn to bat (the visiting team bats in the top half and the home team in the bottom half). An example macro plan is shown at the bottom of Figure 1. Within a paragraph plan, entities and events are verbalized into a text sequence along the lines of Saleh et al. (2019). We make use of special tokens for the <TYPE> of record followed by the value of record from the table. We retain the same position for each record type and value. For example, batter C.Mullins from Figure 1 would be verbalized as <PLAYER> C.Mullins <H/V> H <AB> 4 <BR> 2 <BH> 2 <RBI> 1 <TEAM> Orioles …. For the sake of brevity we use shorthand <V(C.Mullins)> for the full entity.

Paragraph Plan for Entities

For a paragraph containing entities, the corresponding plan will be a verbalization of the entities in sequence. For paragraphs with multiple mentions of the same entity, the plan will verbalize an entity only once and at its first position of mention. Paragraph “Keller gave up a home run … the teams with the worst records in the majors” from the summary in Figure 1 describes four entities including B. Keller, C. Mullins, Royals and Orioles. The respective plan is the verbalization of the four entities in sequence: <V(B.Keller)> <V(C.Mullins)> <V(Royals)> <V(Orioles)>, where V stands for verbalization and <V(B. Keller)> is ashorthand for <PLAYER> B.Keller <H/V> V <W> 7 <L> 5 <IP> 8 …, <V(Royals)> is a shorthand for the team <TEAM> Royals <TR>9 <TH> 14 <E> 1, and so on.

Paragraph Plan for Events

A paragraph may also describe one or more events. For example, the paragraph “With the score tied 1–1 in the fourth… 423-foot home run to left field to make it 3-1” discusses what happened in the bottom halves of the fourth and fifth innings. We verbalize an event by first describing the participating entities followed by the plays in the event. Entities are described in the order in which they appear in a play, and within the same play we list the batter followed by the pitcher, fielder, scorer, and basemen. The paragraph plan corresponding to the bottom halves of the fourth and fifth inning is <V(4-B, 5-B)>. Here, <V(4-B, 5-B)> is a shorthand for <V(W.Merrifield)> <V(A.Cashner)> <V(B.Goodwin)> < V(H.Dozier) >… <V(4-B,1)> <V(4-B,2)><V(5-B,1)> <V(5-B,2)>, and so on. The entities <V(W.Merrifield)>, <V(A.Cashner)>, <V(B.Goodwin)>, and <V(H.Dozier)> correspond in turn to W. Merrifield, A. Cashner, B. Goodwin, and H. Dozier while <V(5-B,1)> refers to the first play in the bottom half of the fifth inning (see the play-by-play table in Figure 1) and abbreviates the following detailed plan: <INN> 5 <HALF> B <BATTING> Royals <PITCHING> Orioles <PL-ID> 1 <BATTER> H.Dozier <PITCHER> A. Cashner > <ACTION> Home-run <SCORES> Royals-3-Orioles-1, and so forth.

The procedure described above is not specific to MLB and can be ported to other datasets with similar characteristics such as RotoWire. However, RotoWire does not provide play-by-play information, and as a result there is no event verbalization for this dataset.

3.2 Macro Plan Construction

We provided our definition for macro plans in the previous sections, however, it is important to note that such macro plans are not readily available in data-to-text benchmarks like MLB (Puduppully et al., 2019b) and RotoWire (Wiseman et al., 2017) which consist of tables of records r paired with a gold summary y (see Tables (A)–(C) in Figure 1). We now describe our method for obtaining macro plans x from r and y.

Similar to Moryossef et al. (2019), we define macro plans to be conformant with gold summaries such that (1) they have the same splits into paragraphs—entities and events within a paragraph in y are grouped into a paragraph plan in x; and (2) the order of events and entities in a paragraph and its corresponding plan are identical. We construct macro plans by matching entities and events in the summary to records in the tables. Furthermore, paragraph delimiters within summaries form natural units which taken together give rise to a high-level document plan.

We match entities in summaries with entities in tables using exact string match, allowing for some degree of variation in the expression of team names (e.g., A’s for Athletics and D-backs for Diamondbacks). Information pertaining to innings appears in the summaries in the form of ordinal numbers (e.g., first, ninth) modifying the noun inning and can be relatively easily identified via pattern matching (e.g., in sentences like “Dozier led off the fifth inning”). However, there are instances where the mention of innings is more ambiguous (e.g., “With the scored tied 1–1 in the fourth, Andrew Cashner (4–13) gave up a sacrifice fly”). We could disambiguate such mentions manually and then train a classifier to learn to predict whether an inning is mentioned. Instead, we explore a novel annotation-free method that makes use of the pretrained language model GPT2 (Radford et al., 2019). Specifically, we feed the context preceding the ordinal number to GPT2 (i.e., the current paragraph up to the ordinal number and the paragraph preceding it) and if inning appears in the top 10 next word predictions, we consider it a positive match. On a held-out dataset, this method achieves 98% precision and 98% recall at disambiguating inning mentions.

To resolve whether the summary discusses the top or bottom side of an inning, we compare the entities in the paragraph with the entities in each half-inning (play-by-play Table (B) in Figure 1) and choose the side with the greater number of entity matches. For instance, Andrew Cashner, Merrifield and fourth inning uniquely resolves to the bottom half of the fourth inning.

3.3 Paragraph Plan Construction

Figure 1 shows the macro plan we obtain for game summary (C). Importantly, macro plan (E) is the outcome of a content selection process after considering several candidate paragraph plans as input. So, what are the candidate paragraph plans that give rise to macro plan (E)? To answer this question, we examined the empirical distribution of paragraph plans in MLB and RotoWire (training portion). Interestingly, we found that ∼79% of the paragraph plans in MLB refer to a single event or a single player (and team(s)). In RotoWire, ∼92% of paragraphs are about a singleton player (and team(s)) or a pair of players.

Based on this analysis, we assume that paragraph plans can be either one (verbalized) entity/event or a combination of at most two. Under this assumption, we explicitly enumerate the set of candidate paragraph plans in a game. For the game in Figure 1, candidate paragraph plans are shown in Table (D). The first table groups plans based on individual verbalizations describing the team(s), players, and events taking place in specific innings. The second table groups pairwise combinations thereof. In MLB, such combinations are between team(s) and players. In RotoWire, we also create combinations between players. Such paragraph plans form set E based on which macro plan x is constructed to give rise to game summary y.

4 Model Description

The input to our model is a set of paragraph plans, each of which is a sequence of tokens. We first compute paragraph plan representations ∈ℝn, and then apply a contextualization and content planning mechanism similar to planning modules introduced in earlier work (Puduppully et al., 2019a; Chen and Bansal, 2018). Predicted macro plans serve as input to our text generation model, which adopts an encoder-decoder architecture (Bahdanau et al., 2015; Luong et al., 2015).

4.1 Macro Planning

Paragraph Plan Representation

We encode tokens in a verbalized paragraph plan ei as {ei,j}j=1|ei| with a BiLSTM (Figure 2, bottom part). To reflect the fact that some records will be more important than others, we compute an attention weighted sum of {ei,j}j=1|ei| following Yang et al. (2016). Let d ∈ℝn denote a randomly initialized query vector learnt jointly with the rest of parameters. We compute attention values αi,j over d and paragraph plan token representation ei,j:
αi,jexp(dei,j)
(1)
Paragraph plan vector ei is the attention weighted sum of ei,j (with jαi,j=1):
ei=jαi,jei,j
(2)
Figure 2: 

Paragraph plan representation and contextualization for macro planning. Computation of e3 is detailed in Equations (1) and (2), e3att in Equation (3), and e3c in Equation (4).

Figure 2: 

Paragraph plan representation and contextualization for macro planning. Computation of e3 is detailed in Equations (1) and (2), e3att in Equation (3), and e3c in Equation (4).

Next, we contextualize each paragraph plan representation vis-a-vis other paragraph plans (Figure 2, top left part). First, we compute attention scores βi,k over paragraph plan representations to obtain an attentional vector eiatt for each:
βi,kexp(eiWaek)ci=kiβi,kekeiatt=Wg[ei;ci]
(3)
where Wa ∈ℝn×n,Wg ∈ℝn×2n are parameter matrices, and kiβi,k=1. Then, we compute a content selection gate, and apply this gate to ei to obtain new paragraph plan representation eic:
gi=sigmoideiatteic=giei
(4)
where ⊙ denotes element-wise multiplication. Thus, each element in ei is weighted by corresponding element of gi ∈[0,1]n to obtain a contextualized paragraph plan representation eic.

Content Planning

Our model learns to predict macro plans, after having been trained on pairs of sets of paragraph plans and corresponding macro plans (Sections 3.2 and 3.3 explain how we obtain these for data-to-text datasets like RotoWire and MLB). More formally, we model macro plan z = z1z|z| as a sequence of pointers, with each zk pointing to an input paragraph plan, i.e., zk{ei}i=1|E|. We decompose p(z|E), the probability of macro plan z given paragraph plans E, as:
p(z|E)=k=1|z|p(zk|z<k,E)
(5)
where z<k = z1zk−1.
We use Pointer Networks (Vinyals et al., 2015) to model p(zk|z<k,E) as:
p(zk=ei|z<k,E)exp(hkWbeic)
(6)
where p(zk|z<k,E) is normalized to 1 and Wb ∈ℝn×n. Rather than computing a weighted representation, Pointer Networks make use of attention to point to specific elements in the input (see Figure 3). We use a decoder LSTM to compute hidden representation hk at time step k. We initialize h0 with the mean paragraph plan representation, avg({eic}i=1|E|). Once the output points to ei, its representation eic is used as input to the next step of the LSTM decoder. The process stops when the model points to EOM, a token indicating end of the macro plan.
Figure 3: 

Macro planning model; paragraph plan representation and contextualization mechanism are detailed in Figure 2. The output points to e3, e|E|, and e1 (see Equations (5) and (6)). EOM is end of macro plan token.

Figure 3: 

Macro planning model; paragraph plan representation and contextualization mechanism are detailed in Figure 2. The output points to e3, e|E|, and e1 (see Equations (5) and (6)). EOM is end of macro plan token.

4.2 Text Generation

Recall that z is a sequence of pointers with each entry zk pointing to a paragraph plan, namely, zk{ei}i=1|E|. We can deterministically obtain macro plan x from z by retrieving the paragraph plans being pointed to, adding <P> separators in between. The conditional output probability p(y|x) is modeled as:
p(y|x)=t=1|y|p(yt|y<t,x)
where y<t = y1yt−1.

To compute p(y|x), we use an encoder-decoder architecture enhanced with an attention mechanism (Bahdanau et al., 2015; Luong et al., 2015). We encode macro plan x with a bidirectional LSTM (Hochreiter and Schmidhuber, 1997). At time step t, we lookup the embedding of the previously predicted word yt−1 and feed it as input to the decoder, which is another LSTM unit. The decoder attends over hidden states of the macro plan to predict yt. We further incorporate a copy mechanism (Gulcehre et al., 2016) in the decoder to enable copying values directly from the macro plan.

We expect the text generation model to learn to generate summary tokens while focusing on the corresponding macro plan and that the output summary will indeed follow the plan in terms of the entities and events being described and their order. At the same time, we believe that text generation is relatively easier as the encoder-decoder model is relieved from the tasks of document structuring and information selection.

4.3 Training and Inference

We train two independent models for macro planning and text generation. Our training objective for macro planning aims to maximize the log likelihood of the macro plan given the paragraph plans:
maxθ(E,z)Dlogpz|E;θ
where D is the training set consisting of pairs of (sets of) paragraph plans and macro plans, and θ are model parameters.
Our training objective for text generation aims to maximize the log likelihood of the output text given the macro plan:
maxϕ(x,y)Flogpy|x;ϕ
where F is the training set consisting of pairs of macro plans and game summaries, and ϕ are model parameters.
During inference, we employ beam search to find the most likely macro plan z^ among candidate macro plans z′ given paragraph plans as input.
z^=argmaxzp(z|E;θ)
We deterministically obtain x^ from z^, and output summary ŷ among candidate outputs y′ given macro plan x^ as input:
y^=argmaxyp(y|x^;ϕ)

5 Experimental Setup

Data

We performed experiments on the RotoWire (Wiseman et al., 2017) and MLB (Puduppully et al., 2019b) benchmarks. The details of these two datasets are given in Table 1. We can see that MLB is around 5 times bigger, has a richer vocabulary, and has longer game summaries. We use the official splits of 3,398/727/728 for RotoWire and 22,821/1,739/1,744 for MLB. We make use of a tokenization script1 to detokenize and retokenize the summaries in both RotoWire and MLB.

Table 1: 

Dataset statistics for RotoWire and MLB. Vocabulary size, number of tokens, number of instances (i.e., table-summary pairs), number of record types, average number of records, average number of paragraph plans, and average summary length.

RotoWireMLB
Vocab Size 11.3K 38.9K 
# Tokens 1.5M 14.3M 
# Instances 4.9K 26.3K 
# Record Types 39 53 
Avg Records 628 565 
Avg Paragraph Plans 10.7 15.1 
Avg Length 337.1 542.05 
RotoWireMLB
Vocab Size 11.3K 38.9K 
# Tokens 1.5M 14.3M 
# Instances 4.9K 26.3K 
# Record Types 39 53 
Avg Records 628 565 
Avg Paragraph Plans 10.7 15.1 
Avg Length 337.1 542.05 

We reconstructed the MLB dataset, as the version released by Puduppully et al. (2019b) had removed all paragraph delimiters from game summaries. Specifically, we followed their methodology and downloaded the same summaries from the ESPN Web site2 and added the <P> delimiter to paragraphs in the summaries.3RotoWire does not have paragraph delimiters in game summaries either. We reverse engineered these as follows: (1) we split summaries into sentences using the NLTK (Bird et al., 2009) sentence tokenizer; (2) initialized each paragraph with a separate sentence; (3) merged two paragraphs into one if the entities in the former were a superset of entities in the latter; (4) repeated Step 3 until no merges were possible.

Training Configuration

We tuned the model hyperparameters on the development set. For training the macro planning and the text generation stages, we used the Adagrad (Duchi et al., 2011) optimizer. Furthermore, the text generation stage made use of truncated BPTT (Williams and Peng, 1990) with truncation length 100. We learn subword vocabulary (Sennrich et al., 2016) for paragraph plans in the macro planning stage. We used 2.5K merge operations for RotoWire and 8K merge operations for MLB. In text generation, we learn a joint subword vocabulary for the macro plan and game summaries. We used 6K merge operations for RotoWire and 16K merge operations for MLB. All models were implemented on OpenNMT-py (Klein et al., 2017). We add to set E the paragraph plans corresponding to the output summary paragraphs, to ensure full coverage during training of the macro planner. During inference for predicting macro plans, we employ length normalization (Bahdanau et al., 2015) to avoid penalizing longer outputs; specifically, we divide the scores of beam search by the length of the output. In addition, we adopt bigram blocking (Paulus et al., 2018). For MLB, we further block beams containing more than two repetitions of a unigram. This helps improve the diversity of the predicted macro plans.

System Comparisons

We compared our model against the following systems: (1) the Template-based generators from Wiseman et al. (2017) for RotoWire and Puduppully et al. (2019b) for MLB. Both systems apply the same principle, they emit a sentence about the teams playing in the game, followed by player-specific sentences, and a closing sentence. MLB additionally contains a description of play-by-play; (2) ED+CC, the best performing system in Wiseman et al. (2017), is a vanilla encoder-decoder model equipped with an attention and copy mechanism; (3) NCP+CC, the micro planning model of Puduppully et al. (2019a), generates content plans from the table by making use of Pointer networks (Vinyals et al., 2015) to point to records; content plans are encoded with a BiLSTM and the game summary is decoded using another LSTM with attention and copy; (4) ENT, the entity-based model of Puduppully et al. (2019b), creates dynamically updated entity-specific representations; the text is generated conditioned on the data input and entity memory representations using hierarchical attention at each time step.

6 Results

Automatic Evaluation

For automatic evaluation, following earlier work (Wiseman et al., 2017; Puduppully et al., 2019a, b, inter alia) we report BLEU (Papineni et al., 2002) with the gold summary as reference but also make use of the Information Extraction (IE) metrics from Wiseman et al. (2017), which are defined over the output of an IE system; the latter extracts entity (players, teams) and value (numbers) pairs in a summary, and then predicts the type of relation. For instance, given the pair Kansas City Royals, 9, it would predict their relation as TR (i.e., Team Runs). Training data for the IE system is obtained by checking for matches between entity, value pairs in the gold summary and entity, value, record type triplets in the table.

Let ŷ be the gold summary and y the model output. Relation Generation (RG) measures the precision and count of relations extracted from y that also appear in records r. Content Selection (CS) measures the precision and recall of relations extracted from y that are also extracted from ŷ. Content Ordering (CO) measures the normalized Damerau-Levenshtein distance between the sequences of relations extracted from y and ŷ.

We reused the IE model from Puduppully et al. (2019a) for RotoWire but retrained it for MLB to improve its precision and recall. Furthermore, the implementation of Wiseman et al. (2017) computes RG, CS, and CO excluding duplicate relations. This artificially inflates the performance of models whose outputs contain repetition. We include duplicates in the computation of the IE metrics (and recreate them for all comparison systems).

Table 2 (top) presents our results on the RotoWire test set. In addition to Templ, NCP+CC, ENT, and ED+CC we include the best performing model of Wiseman et al. (2017) (WS-2017; note that ED+CC is an improved re-implementation of their model), and the model of Rebuffel et al. (2020) (RBF-2020), which represents the state of the art on RotoWire. This model has a Transformer encoder (Vaswani et al., 2017) with a hierarchical attention mechanism over entities and records within entities. The models of Saleh et al. (2019), Iso et al. (2019), and Gong et al. (2019) make use of additional information not present in the input (e.g., previous/next games, summary writer) and are not directly comparable to the systems in Table 2. Results for the MLB test set are in the bottom portion of Table 2.

Table 2: 

Evaluation on RotoWire and MLB test sets; relation generation (RG) count (#) and precision (P%), content selection (CS) precision (P%), recall (R%) and F-measure (F%), content ordering (CO) in normalized Damerau-Levenshtein distance (DLD%), and BLEU.

RotoWireRGCSCOBLEU
#P%P%R%F%DLD%
Templ 54.3 99.9 27.1 57.7 36.9 13.1 8.46 
WS-2017 34.1 75.1 20.3 36.3 26.1 12.4 14.19 
ED+CC 35.9 82.6 19.8 33.8 24.9 12.0 14.99 
NCP+CC 40.8 87.6 28.0 51.1 36.2 15.8 16.50 
ENT 32.7 91.7 34.7 48.5 40.5 16.6 16.12 
RBF-2020 44.9 89.5 23.9 47.0 31.7 14.3 17.16 
 
Macro−Plan(4) 42.1 97.6 34.1 57.8 42.9 17.7 15.46 
36.2 81.3 22.1 38.6 28.1 12.1 14.00 
 
MLB RG CS CO BLEU 
P% P% R% F% DLD% 
Templ 62.3 99.9 21.6 55.2 31.0 11.0 4.12 
ED+CC 32.5 91.3 27.8 40.6 33.0 17.1 9.68 
NCP+CC 19.6 81.3 44.5 44.1 44.3 21.9 9.68 
ENT 23.8 81.1 40.9 49.5 44.8 20.7 11.50 
 
Macro−Plan(SP,4) 30.8 94.4 40.8 54.9 46.8 21.8 12.62 
25.1 92.7 40.0 44.6 42.2 21.9 11.09 
RotoWireRGCSCOBLEU
#P%P%R%F%DLD%
Templ 54.3 99.9 27.1 57.7 36.9 13.1 8.46 
WS-2017 34.1 75.1 20.3 36.3 26.1 12.4 14.19 
ED+CC 35.9 82.6 19.8 33.8 24.9 12.0 14.99 
NCP+CC 40.8 87.6 28.0 51.1 36.2 15.8 16.50 
ENT 32.7 91.7 34.7 48.5 40.5 16.6 16.12 
RBF-2020 44.9 89.5 23.9 47.0 31.7 14.3 17.16 
 
Macro−Plan(4) 42.1 97.6 34.1 57.8 42.9 17.7 15.46 
36.2 81.3 22.1 38.6 28.1 12.1 14.00 
 
MLB RG CS CO BLEU 
P% P% R% F% DLD% 
Templ 62.3 99.9 21.6 55.2 31.0 11.0 4.12 
ED+CC 32.5 91.3 27.8 40.6 33.0 17.1 9.68 
NCP+CC 19.6 81.3 44.5 44.1 44.3 21.9 9.68 
ENT 23.8 81.1 40.9 49.5 44.8 20.7 11.50 
 
Macro−Plan(SP,4) 30.8 94.4 40.8 54.9 46.8 21.8 12.62 
25.1 92.7 40.0 44.6 42.2 21.9 11.09 

Templ has the highest RG precision and count on both datasets. This is not surprising, by design Templ is always faithful to the input. However, notice that it achieves the lowest BLEU among comparison systems, indicating that it mostly regurgitates facts with low fluency. Macro achieves the highest RG precision among all neural models for RotoWire and MLB. We obtain an absolute improvement of 5.9% over ENT for RotoWire and 13.3% for MLB. In addition, Macro achieves the highest CS F-measure for both datasets. On RotoWire, Macro achieves the highest CO score, and the highest BLEU on MLB. On RotoWire, in terms of BLEU, Macro is worse than comparison models (e.g., NCP+CC or ENT). Inspection of the output showed that the opening paragraph, which mostly describes how the two teams fared, is generally shorter in Macro, leading to shorter summaries and thus lower BLEU. There is high variance in the length of the opening paragraph in the training data and Macro verbalizes the corresponding plan conservatively. Ideas such as length normalization (Wu et al., 2016) or length control (Kikuchi et al., 2016; Takeno et al., 2017; Fan et al., 2018) could help alleviate this; however, we do not pursue them further for fair comparison with the other models.

The Contribution of Macro Planning

To study the effect of macro planning in more detail, we further compared Macro against text generation models (see Section 4.2) which are trained on verbalizations of the tabular data (and gold summaries) but do not make use of document plans or a document planning mechanism. On RotoWire, the model was trained on verbalizations of players and teams, with the input arranged such that the verbalization of the home team was followed by the visiting team, the home team players and the visiting team players. Mention of players was limited to the four best ones, following Saleh et al. (2019) (see −Plan(4) in Table 2). For MLB, we additionally include verbalizations of innings focusing on scoring plays which are likely to be discussed in game summaries (see −Plan(SP,4) in Table 2). Note that by preprocessing the input in such a way some simple form of content selection takes place simply by removing extraneous information which the model does not need to consider.

Across both datasets, −Plan variants appear competitive. On RotoWire −Plan(4) is better than ED+CC in terms of content selection but worse compared to ENT. On MLB, −Plan(SP,4) is again superior to ED+CC in terms of content selection but not ENT whose performance lags behind when considering RG precision. Taken together, these results confirm that verbalizing entities and events into a text sequence is effective. At the same time, we see that −Plan variants are worse than Macro across most metrics which underlines the importance of an explicit planning component.

Table 3 presents intrinsic evaluation of the macro planning stage. Here, we compare the inferred macro plan with the gold macro plans, CS and CO metrics with regard to entities and events instead of relations. We see that our macro planning model (Macro) achieves high scores for CS and CO for both RotoWire and MLB. We further used the CS and CO metrics to check how well the generated summary follows the (predicted) plan. We followed the steps in Section 3.2 and reverse engineered macro plans from the model summaries and compared these extracted plans with the original macro plans with regard to entities and events. We found that Macro creates summaries that follow the plan closely: For RotoWire, the CS F-score and CO are greater than 98%; for MLB, the CS F-score is greater than 94% and CO is greater than 89%. We show an output summary for Macro in Table 4, together with the predicted document plan.

Table 3: 

Evaluation of macro planning stage; content selection precision (CS-P), recall (CS-R), F-measure (CS-F), and content ordering (CO) between the inferred plans and gold plans in terms of entities and events for RotoWire (RW) and MLB test sets.

MacroCS-PCS-RCS-FCO
RotoWire 81.3 73.2 77.0 45.8 
MLB 80.6 63.3 70.9 31.4 
MacroCS-PCS-RCS-FCO
RotoWire 81.3 73.2 77.0 45.8 
MLB 80.6 63.3 70.9 31.4 
Table 4: 

Predicted macro plan (top) with corresponding model output (bottom). Entities and events in summary corresponding to those in the macro plan are boldfaced.

<V(Rays)> <P> <V(8-B)> <P> <V(Rays)> <P> <V(Rays)> <V(Red Sox)> <P> <V(8-B)> <P> <V(8-T)> <P> <V(9-T)> <P> <V(Clay Buchholz)> <P> <V(5-T)> <P> <V(Edwin Jackson)> <P> <V(5-T)> <P> <V(8-T)>
ST. PETERSBURG, Fla. (AP) – The Tampa Bay Rays are making the most of it. <P> Akinori Iwamura hit a two-run homer in the eighth inning and the Rays beat the Boston Red Sox 2-1 on Sunday to complete a three-game sweep. <P> The Rays, who have the best record in the majors, have won six of their last seven games. <P> The Rays have won four of their last five series, including three in a row against the Red Sox, who have won six of their last seven overall. <P> Dioner Navarro singled with one out in the eighth off Clay Buchholz (1-2) and moved to third on Jason Bartlett’s flyout to center. Iwamura then drove a 1-1 pitch into the left-field stands for his second homer of the season. <P> Scott Dohmann (2-0) got the win in relief, striking out Manny Ramirez with runners on first and third to end the eighth. <P> Troy Percival worked the ninth for his fifth save in five opportunities. <P>Clay Buchholz (1-2) gave up two runs and three hits in eight innings. He struck out nine and walked two. <P> The Red Sox loaded the bases with one out in the fifth on a single by Coco Crisp, a wild pitch and a walk to Jed Lowrie. Jacoby Ellsbury drove in Crisp with a two-out single to center. <P>Jackson struck out four and walked three. <P> The Red Sox loaded the bases with one out in the fifth on a single by Coco Crisp, a walk to Jed Lowrie and a one-out walk to Jed Lowrie. Jackson struck out Julio Lugo, but Jacoby Ellsbury singled to center to put the Red Sox up 1-0. <P> The Red Sox threatened in the eighth when J. D. Drew drew a two-out walk against Trever Miller, but Ramirez struck out to end the inning. 
<V(Rays)> <P> <V(8-B)> <P> <V(Rays)> <P> <V(Rays)> <V(Red Sox)> <P> <V(8-B)> <P> <V(8-T)> <P> <V(9-T)> <P> <V(Clay Buchholz)> <P> <V(5-T)> <P> <V(Edwin Jackson)> <P> <V(5-T)> <P> <V(8-T)>
ST. PETERSBURG, Fla. (AP) – The Tampa Bay Rays are making the most of it. <P> Akinori Iwamura hit a two-run homer in the eighth inning and the Rays beat the Boston Red Sox 2-1 on Sunday to complete a three-game sweep. <P> The Rays, who have the best record in the majors, have won six of their last seven games. <P> The Rays have won four of their last five series, including three in a row against the Red Sox, who have won six of their last seven overall. <P> Dioner Navarro singled with one out in the eighth off Clay Buchholz (1-2) and moved to third on Jason Bartlett’s flyout to center. Iwamura then drove a 1-1 pitch into the left-field stands for his second homer of the season. <P> Scott Dohmann (2-0) got the win in relief, striking out Manny Ramirez with runners on first and third to end the eighth. <P> Troy Percival worked the ninth for his fifth save in five opportunities. <P>Clay Buchholz (1-2) gave up two runs and three hits in eight innings. He struck out nine and walked two. <P> The Red Sox loaded the bases with one out in the fifth on a single by Coco Crisp, a wild pitch and a walk to Jed Lowrie. Jacoby Ellsbury drove in Crisp with a two-out single to center. <P>Jackson struck out four and walked three. <P> The Red Sox loaded the bases with one out in the fifth on a single by Coco Crisp, a walk to Jed Lowrie and a one-out walk to Jed Lowrie. Jackson struck out Julio Lugo, but Jacoby Ellsbury singled to center to put the Red Sox up 1-0. <P> The Red Sox threatened in the eighth when J. D. Drew drew a two-out walk against Trever Miller, but Ramirez struck out to end the inning. 

Human-Based Evaluation

We also asked participants to assess model output in terms of relation generation, grammaticality, coherence, and conciseness (Wiseman et al., 2017; Puduppully et al., 2019a; Puduppully et al., 2019b), For RotoWire, we compared Macro against RBF-2020,4 ED+CC, Gold, and Templ. For MLB, we compared Macro against ENT, ED+CC, Gold, and Templ.

We conducted our study on the Amazon Mechanical Turk (AMT) crowdsourcing platform, following best practices for human evaluation in NLG (van der Lee et al., 2019). Specifically, to ensure consistent ratings, we required crowdworkers to have an approval rating greater than 98% and a minimum of 1,000 previously completed tasks. Raters were restricted to English-speaking countries (i.e., US, UK, Canada, Ireland, Australia, or NZ). Participants were allowed to provide feedback on the task or field questions (our interface accepts free text).

In our first study, we presented crowdworkers with sentences randomly selected from summaries along with their corresponding box score (and play-by-play in case of MLB) and asked them to count supported and contradicting facts (ignoring hallucinations, i.e., unsupported facts). We did not require crowdworkers to be familiar with NBA or MLB. Instead, we provided a cheat sheet explaining the semantics of box score tables. In addition, we provided examples of sentences with supported/contradicting facts. We evaluated 40 summaries from the test set (20 per dataset), 4 sentences from each summary and elicited 3 responses per summary. This resulted in 40 summaries × 5 systems × 3 raters, for a total of 600 tasks. Altogether, 131 crowdworkers participated in this study (agreement using Krippendorff’s α was 0.44 for supported and 0.42 for contradicting facts).

As shown in Table 5, Macro yields the smallest number of contradicting facts among neural models on both datasets. On RotoWire the number of contradicting facts for Macro is comparable to Gold and Templ (the difference is not statistically significant) and significantly smaller compared to RBF-2020 and ED+CC. The count of supported facts for Macro is comparable to Gold, and ED+CC, and significantly lower than Templ and RBF-2020. On MLB, Macro has significantly fewer contradicting facts than ENT and ED+CC and is comparable to Templ and Gold (the difference is not statistically significant). The count of supported facts for Macro is comparable to Gold, ENT, ED+CC, and Templ. For both datasets, Templ has the lowest number of contradicting facts. This is expected as Templ essentially parrots facts (aka records) from the table.

Table 5: 

Average number of supported (#Supp) and contradicting (#Contra) facts in game summaries and best-worst scaling evaluation (higher is better). Systems significantly different from Macro are marked with an asterisk * (using a one-way ANOVA with post hoc Tukey HSD tests; p ≤ 0.05).

RotoWire#Supp#ContraGramCoherConcis
Gold 3.63 0.07 38.33 46.25* 30.83 
Templ 7.57* 0.08 −61.67* −52.92* −36.67* 
ED+CC 3.92 0.91* 5.0 −8.33 −4.58 
RBF-2020 5.08* 0.67* 13.33 4.58 3.75 
Macro 4.00 0.27 5.0 10.42 6.67 
 
MLB #Supp #Contra Gram Coher Concis 
Gold 3.59 0.14 21.67 30.0 26.67 
Templ 4.21 0.04 −51.25* −43.75* 7.5 
ED+CC 3.42 0.72* −22.5* −12.08* −39.17* 
ENT 3.71 0.73* 5.83* −0.83* −22.08* 
Macro 3.76 0.25 46.25 26.67 27.08 
RotoWire#Supp#ContraGramCoherConcis
Gold 3.63 0.07 38.33 46.25* 30.83 
Templ 7.57* 0.08 −61.67* −52.92* −36.67* 
ED+CC 3.92 0.91* 5.0 −8.33 −4.58 
RBF-2020 5.08* 0.67* 13.33 4.58 3.75 
Macro 4.00 0.27 5.0 10.42 6.67 
 
MLB #Supp #Contra Gram Coher Concis 
Gold 3.59 0.14 21.67 30.0 26.67 
Templ 4.21 0.04 −51.25* −43.75* 7.5 
ED+CC 3.42 0.72* −22.5* −12.08* −39.17* 
ENT 3.71 0.73* 5.83* −0.83* −22.08* 
Macro 3.76 0.25 46.25 26.67 27.08 

We also conducted a second study to evaluate the quality of the generated summaries. We presented crowdworkers with a pair of summaries and asked them to choose the better one in terms of Grammaticality (is the summary written in well- formed English?), Coherence (is the summary well structured and well organized and does it have a natural ordering of the facts?), and Conciseness (does the summary avoid unnecessary repetition including whole sentences, facts or phrases?). We provided example summaries showcasing good and bad output. For this task, we required that the crowdworkers be able to comfortably comprehend NBA/MLB game summaries. We elicited preferences with Best-Worst Scaling (Louviere and Woodworth, 1991; Louviere et al., 2015), a method shown to be more reliable than rating scales. The score of a system is computed as the number of times it is rated best minus the number of times it is rated worst (Orme, 2009). The scores range from −100 (absolutely worst) to +100 (absolutely best). We divided the five competing systems into ten pairs of summaries and elicited ratings for 40 summaries (20 per dataset). Each summary pair was rated by 3 raters. This resulted in 40 summaries × 10 system pairs × 3 evaluation criteria × 3 raters, for a total of 3,600 tasks. A total of 206 crowdworkers participated in this task (agreement using Krippendorff’s α was 0.47).

As shown in Table 5, on RotoWire, Macro is comparable to Gold, RBF-2020, and ED+CC in terms of Grammaticality but significantly better than Templ. In terms of Coherence, Macro is comparable to RBF-2020 and ED+CC but significantly better than Templ and significantly worse than Gold. With regard to Conciseness, Macro is comparable to Gold, RBF-2020, and ED+CC, and significantly better than Templ. On MLB, Macro is comparable to Gold in terms of Grammaticality and significantly better than ED+CC, ENT, and Templ. Macro is comparable to Gold in terms of Coherence and significantly better than ED+CC, ENT and Templ. In terms of Conciseness, raters found Macro comparable to Gold and Templ and significantly better than ED+CC and ENT. Taken together, our results show that macro planning leads to improvement in data-to-text generation in comparison to other systems for both RotoWire and MLB datasets.

7 Discussion

In this work we presented a plan-and-generate approach for data-to-text generation that consists of a macro planning stage representing high-level document organization in terms of structure and content, followed by a text generation stage. Extensive automatic and human evaluation shows that our approach achieves better results than existing state-of-the-art models and generates summaries which are factual, coherent, and concise.

Our results show that macro planning is more advantageous for generation tasks expected to produce longer texts with multiple discourse units, and could be easily extended to other sports domains such as cricket (Kelly et al., 2009) or American football (Barzilay and Lapata, 2005). Other approaches focusing on micro planning (Puduppully et al., 2019a; Moryossef et al., 2019) might be better tailored for generating shorter texts. There has been a surge of datasets recently focusing on single-paragraph outputs and the task of content selection such as E2E (Novikova et al., 2017), WebNLG (Gardent et al., 2017), and WikiBio (Lebret et al., 2016; Perez-Beltrachini and Lapata, 2018). We note that in our model content selection takes place during macro planning and text generation. The results in Table 2 show that Macro achieves the highest CS F-measure on both datasets, indicating that the document as a whole and individual sentences discuss appropriate content.

Throughout our experiments we observed that template-based systems score poorly in terms of CS (but also CO and BLEU). This is primarily due to the inflexibility of the template approach which is limited to the discussion of a fixed number of (high-scoring) players. Yet, human writers (and neural models to a certain extent), synthesize summaries taking into account the particulars of a specific game (where some players might be more important than others even if they scored less) and are able to override global defaults. Template sentences are fluent on their own, but since it is not possible to perform aggregation (Reiter, 1995), the whole summary appears stilted, it lacks coherence and variability, contributing to low BLEU scores. The template baseline is worse for MLB than RotoWire which reflects the greater difficulty to manually create a good template for MLB. Overall, we observe that neural models are more fluent and coherent, being able to learn a better ordering of facts which is in turn reflected in better CO scores.

Despite promising results, there is ample room to improve macro planning, especially in terms of the precision of RG (see Table 2, P% column of RG). We should not underestimate that Macro must handle relatively long inputs (the average input length in the MLB development set is ∼3100 tokens) which are challenging for the attention mechanism. Consider the following output of our model on the MLB dataset: Ramirez’s two-run double off Joe Blanton tied it in the sixth, and Brandon Moss added a two-out RBI single off Alan Embree to give Boston a 3-2 lead. Here, the name of the pitcher should have been Joe Blanton instead of Alan Embree. In fact, Alan Embree is the pitcher for the following play in the half inning. In this case, attention diffuses over the relatively long MLB macro plan, leading to inaccurate content selection. We could alleviate this problem by adopting a noisy channel decomposition (Yee et al., 2019; Yu et al., 2020), that is, by learning two different distributions: a conditional model that provides the probability of translating a paragraph plan to text and a language model that provides an unconditional estimate of the output (i.e., the whole game summary). However, we leave this to future work.

For RotoWire, the main source of errors is the model’s inability to understand numbers. For example, Macro generates the following output: The Lakers were the superior shooters in this game, going 48 percent from the field and30 percent from the three-point line, while the Jazz went 47 percent from the floor and 30 percent from beyond the arc. Here, 30 percent should have been 24 percent for the Lakers but the language model expects a higher score for the three-point line, and since 24 is low (especially compared to 30 scored by the Jazz), it simply copies 30 scored by the Jazz instead. A mechanism for learning better representations for numbers (Wallace et al., 2019) or executing operations such as argmax or minus (Nie et al., 2018) should help alleviate this problem.

Finally, although our focus so far has been on learning document plans from data, the decoupling of planning from generation allows to flexibly generate output according to specification. For example, we could feed the model with manually constructed macro plans, consequently controlling the information content and structure of the output summary (e.g., for generating short or long texts, or focusing on specific aspects of the game).

Acknowledgments

We thank the Action Editor, Claire Gardent, and the three anonymous reviewers for their constructive feedback. We also thank Laura Perez-Beltrachini for her comments on an earlier draft of this paper, and Parag Jain, Hao Zheng, Stefanos Angelidis and Yang Liu for helpful discussions. We acknowledge the financial support of the European Research Council (Lapata; award number 681760, “Translating Multiple Modalities into Text”).

Notes

3

Although our model is trained on game summaries with paragraph delimiters, and also predicts these at generation time, for evaluation we strip <P> from model output.

4

We are grateful to Clément Rebuffel for providing us with the output of their system.

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