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<article xmlns:xlink="http://www.w3.org/1999/xlink">
  <front>
    <journal-meta />
    <article-meta>
      <title-group>
        <article-title>Knowledge Graph Embeddings for Causal Relation Prediction</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Aamod Khatiwada</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Sola Shirai</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
          <xref ref-type="aff" rid="aff2">2</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Kavitha Srinivas</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Oktie Hassanzadeh</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>IBM Research</institution>
          ,
          <addr-line>Yorktown Heights, NY</addr-line>
          ,
          <country country="US">USA</country>
        </aff>
        <aff id="aff1">
          <label>1</label>
          <institution>Khoury College of Computer Sciences, Northeastern University</institution>
          ,
          <addr-line>Boston, MA</addr-line>
          ,
          <country country="US">USA</country>
        </aff>
        <aff id="aff2">
          <label>2</label>
          <institution>Rensselaer Polytechnic Institute</institution>
          ,
          <addr-line>Troy, NY</addr-line>
          ,
          <country country="US">United States</country>
        </aff>
      </contrib-group>
      <abstract>
        <p>Recently, there has been an increasing interest in knowledge graphs (KGs) of causal relations between events. Such KGs can be used for event analysis and forecasting in a variety of applications. In this paper, we study the problem of enriching an existing causal KG of news events using KG embeddings-based link prediction techniques. We perform a thorough evaluation of the performance of five diferent methods using classic accuracy measures as well as a novel scheme for manual evaluation. Our study provides insights on the strengths and weaknesses of diferent link prediction methods.</p>
      </abstract>
      <kwd-group>
        <kwd>eol&gt;Link Prediction</kwd>
        <kwd>Causal Knowledge</kwd>
        <kwd>Knowledge Graphs</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>1. Introduction</title>
      <p>Volcanic eruption of</p>
      <p>Krakatoa in 1883
instanceOf
injury
hasCause
structural failure
Type level entity
Instance level entity
2004 Indian Ocean</p>
      <p>Earthquake
volcanic eruption</p>
      <p>instanceOf
hasEffect
tsunami instanceOf acOf</p>
      <p>e
istn
n
hasCause hasEffect
hasEffect
instanceOf</p>
      <p>hasEffect
earthquake
instance</p>
      <p>Of
2015 Nepal Earthquake
natural disaster
instanceOf
slide
subclassOf
landslide
captured in our sample KG. Such sparsity of causal relations limits the capability of predicting
potential consequences of various kinds of events.</p>
      <p>
        There is a large body of work on link prediction, but most focus on the problem of predicting
missing relations in general between the entities in the KGs [
        <xref ref-type="bibr" rid="ref6 ref7 ref8">6, 7, 8</xref>
        ]. The task is formulated
as a target entity prediction problem – i.e., given a source entity and a relation, the objective
is to find the target entity in the KG as &lt;source entity, relation, ? &gt;. However, to the best
of our knowledge, predicting causal relations in KGs has not been explored yet. Apart from
sparsity, certain properties diferentiate causal relations from the majority of relations in existing
benchmarks. For example, causal relations are generally many-to-many i.e., the same event can
have multiple efects and vice-versa. Like in Fig. 1, earthquake has multiple efects ( tsunamis,
landslides, etc) and if we formulate this as a link prediction problem (&lt;earthquake, has efect, ?&gt; ),
we have multiple possible answers (e.g. tsunami, landslide, etc). But for some popular relations
like place of birth, (e.g., &lt;barack obama, place of birth, ?&gt;), there is a single possible answer
i.e., Honolulu. Existing link prediction benchmarks like FB15k-237, WN18RR, CODEX-L do
not contain event entities and causal relations. Hence, a proper evaluation of link prediction
techniques on causal relation prediction is yet to be explored. Prior literature suggests that
embedding and graph-based models are efective for general link prediction tasks [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ]. In this
paper, we study the performance of current and prior state-of-the-art embedding and Graph
Convolutional Network (GCN)-based link prediction techniques on causal relation prediction
and analyze their performance.
      </p>
      <p>Summarizing our contributions, to the best of our knowledge, we are the first to evaluate
embeddings and GCN based link prediction techniques on causal relation prediction task. Since
the existing benchmarks do not contain events and causal relations, we create two new causal
KG datasets using events in Wikidata. We perform a thorough evaluation of link predictions
methods over the datasets using classic accuracy metrics as well as a novel manual evaluation
scheme to analyze the limitations of the classic metrics.</p>
    </sec>
    <sec id="sec-2">
      <title>2. Related Work</title>
      <p>
        Diferent embedding-based approaches are proposed and shown to be efective for Link
Prediction task [
        <xref ref-type="bibr" rid="ref6 ref7">6, 7</xref>
        ]. Bordes et al. introduces a linear additive model called TransE that learns the
embeddings for entities and relationships in KGs by translating them to low-dimensional
vectors [
        <xref ref-type="bibr" rid="ref9">9</xref>
        ]. Precisely, for a triple &lt;source entity, relation, target entity&gt;, TransE translates the source
entity to target entity using their relation as a translation vector. Balazevic et al. proposes a
simple linear model (TuckER) based on tucker decomposition of the binary tensors of triples [
        <xref ref-type="bibr" rid="ref10">10</xref>
        ].
TuckER nearly achieves state-of-the-art performance in some existing benchmarks. Moreover,
Trouillon et al. proposes ComplEx, a semantic matching model, that represents each entity
using a pair of complex conjugate vectors: one for each entity as a source and as a target [
        <xref ref-type="bibr" rid="ref11">11</xref>
        ].
This helps ComplEx to deal with the assymetric relations and it shows promising results in
the existing link prediction benchmarks [
        <xref ref-type="bibr" rid="ref7">7</xref>
        ]. There are non-linear models like ConvE that use
Convolutional Neural Network to predict missing relations in the KGs [
        <xref ref-type="bibr" rid="ref12">12</xref>
        ]. ConvE uses 2D
convolutional operation on the source entity and relation to infer the target entity after
processing the convolution result. In existing datasets, ConvE achieves state-of-the-art performance
in terms of mean reciprocal rank (MRR) [
        <xref ref-type="bibr" rid="ref12">12</xref>
        ]. Recently, Graph Convolution Network (GCN)
based models are seen to be efective in the link prediction task [
        <xref ref-type="bibr" rid="ref13">13</xref>
        ]. For instance, Nguyen et
al. presents NoGE that uses co-occurrence information between entities and relations in the
encoder module. NoGE achieves state-of-the-art performance against linear models, CNN-based
models and semantic matching models in CODEX-M and CODEX-L benchmarks [
        <xref ref-type="bibr" rid="ref13 ref14">13, 14</xref>
        ]. To the
best of our knowledge, the above link prediction techniques have not been applied to causal KGs.
While there are a number of causal KGs such as ATOMIC [
        <xref ref-type="bibr" rid="ref2">2</xref>
        ], CauseNet [
        <xref ref-type="bibr" rid="ref3">3</xref>
        ], and CausalKG [
        <xref ref-type="bibr" rid="ref15">15</xref>
        ],
our focus in this paper is on a Wikidata-based KG that captures news events and has application
in news event analysis and forecasting [
        <xref ref-type="bibr" rid="ref16 ref5">5, 16</xref>
        ]. Our work is orthogonal to prior work on using
textual sources to enrich causal KGs [
        <xref ref-type="bibr" rid="ref1 ref4">1, 4</xref>
        ].
      </p>
    </sec>
    <sec id="sec-3">
      <title>3. Background</title>
      <sec id="sec-3-1">
        <title>3.1. Preliminaries</title>
        <p>We use ℰ and ℛ to denote set of all entities and all relations in Knowledge Graph  respectively.
Furthermore, we use  to denote an entity in ℰ and  to denote a relation in ℛ. Also, we
denote a triple in  as &lt; , ,  &gt; where ,  ∈ ℰ and  ∈ ℛ. Here,  denotes the source
entity,  denotes the target entity and  denotes the relation between  and . Whenever
mathematical calculations are involved, we use , , and  also to denote their respective
vector representations. Accordingly, we will now define our problem. 1
Definition 1 (Causal Relation Prediction Problem). Given a Knowledge Graph  with a
set of entities ℰ and a set of relations ℛ, source event entity  ∈ ℰ , causal relation  ∈ , and
an integer , the causal relation prediction problem is find the set of top-k target event entities
ℰ = {1, 2, . . . }, such that &lt; , ,  &gt;∈  for all  ∈ ℰ.
1Note that since we are interested in causal relations, the input source entity must be an event.</p>
      </sec>
      <sec id="sec-3-2">
        <title>3.2. Link Prediction Techniques</title>
        <p>
          In this work, we study the performance of link prediction techniques on the scope of causal
relation prediction. There are many link prediction techniques and their extensions in the
literature [
          <xref ref-type="bibr" rid="ref6 ref7">6, 7</xref>
          ]. Here, we describe techniques that we include in our analysis. Note that there
is not a single technique that achieves state-of-the-art performance in all the link prediction
benchmarks [
          <xref ref-type="bibr" rid="ref6">6</xref>
          ]. Therefore, for comprehensive analysis, we study the best performing models
from diferent categories like linear models, semantic matching models, CNN-based models and
GCN-based models. From linear, we select TransE [
          <xref ref-type="bibr" rid="ref9">9</xref>
          ], one of the first embedding-based model,
and TuckER [
          <xref ref-type="bibr" rid="ref10">10</xref>
          ] that improves over TransE and performs similar to state-of-the-art techniques.
Similarly, we use ComplEx [
          <xref ref-type="bibr" rid="ref11">11</xref>
          ] among semantic matching models and ConvE [
          <xref ref-type="bibr" rid="ref12">12</xref>
          ] among
CNN-based models. Moreover, we use NoGE [
          <xref ref-type="bibr" rid="ref13">13</xref>
          ] among GCN models that is shown to perform
better than other Graph based models in the existing benchmarks.
        </p>
      </sec>
    </sec>
    <sec id="sec-4">
      <title>4. Evaluation</title>
      <p>
        Now, we evaluate the link prediction techniques on the scope of causal relation prediction
empirically. We run all the experiments in Python 3.8 using a computing cluster (Intel Supermicro
SYS-4029GP-TVRT, 64 GB memory and 8 × 768 MB V100-SXM2 GPU). We implement all the
techniques (see Section 3.2) using publicly available codes. We reproduce TransE and ComplEx
using their distributed implementation provided in PyTorch-BigGraph [
        <xref ref-type="bibr" rid="ref17">17</xref>
        ]. Furthermore, we
implement TuckER, ConvE and NoGE using the codes in their respective github repository (see
appendix Fig. 5).
      </p>
      <sec id="sec-4-1">
        <title>4.1. Link Prediction Efectiveness</title>
        <p>
          As we analyze link prediction techniques for causal relation prediction, we start our experiments
by evaluating their performance on link prediction task. Specifically, we report Mean
Reciprocal Rank (MRR) and Hits@k [
          <xref ref-type="bibr" rid="ref7">7</xref>
          ] for k = 1, 5, 10 and 50 in three existing link prediction
benchmarks: FB15k-237, WN18RR and CODEX-L. We train each technique using train set
for at least 500 epochs using the hyperparameters suggested in the respective papers and select
a model from the checkpoint with minimum validation loss.
        </p>
        <p>
          The performance of each technique on each benchmark is reported in Fig. 2. Similar to what
previous works have reported [
          <xref ref-type="bibr" rid="ref13 ref7">7, 13</xref>
          ], ComplEx and NoGE are the best performing models with
ConvE also showing reasonable performance. ComplEx is benefitted by its ability to handle the
assymetric relations. NoGE captures the graphical properties like co-occurrence of the nodes
and performs better than other techniques. TransE, being a simple translation model, performs
well on the FB15k-237 benchmark in terms of MRR but its performance drops significantly on
WN18RR and CODEX-L in comparison to other techniques.
        </p>
      </sec>
      <sec id="sec-4-2">
        <title>4.2. Causal Relation Prediction Benchmarks</title>
        <p>Existing link prediction benchmarks: FB15k-237, WN18RR and CODEX-L mostly contain
non-event related triples and they are suitable only for general link prediction evaluation.
Therefore, we create two new causal relation benchmarks by extracting the event-related triples
from Wikidata for our evaluation. Our objective is to represent Causal Knowledge Graphs of
news events and their causal relations in these benchmarks.</p>
        <p>
          Event selection. We use the triples containing event entities in Wikidata to create the
causal relation prediction benchmarks. To select such triples, we first determine event entities in
Wikidata. Unfortunately, there are no properties or classes (types) in Wikidata that distinguishes
event entities from other entities. Generally, the day-to-day events happening around the world
are covered by news articles and their headlines [
          <xref ref-type="bibr" rid="ref4">4</xref>
          ]. Therefore, we consider the Wikidata classes
having a mapping to the news articles in Wikinews as event classes. Note that all the events in
news articles do not have a mapping to Wikidata classes. Using mapping, we identify 50 event
classes (manually verified after extraction) such as earthquake, tsunami, and disease outbreak.
We consider all such classes, along with their subclasses (connected by subclass of relation) and
instances (connected by instance of relation), as event entities.
        </p>
        <p>
          Triple Extraction. After determining the event entities, we query Wikidata and extract
all the triples that contain the event entities as either head or tail entities. To understand the
impact of literals, we include the triples representing numerical properties of the events. Notice
however, we exclude images and those literals that represent metadata like wikibase:statements,
wikibase:identifiers, wikibase:sitelinks, schema:version, schema:description and schema:about.
Furthermore, some event properties are not captured by triples containing the event instead
may be indirect. The embedding models can infer such properties from the graph pattern [
          <xref ref-type="bibr" rid="ref7">7</xref>
          ].
Therefore, we also extract the entities that are two hops away from each event. In total, we collect
around 1M triples from Wikidata. Then we remove the cyclic triples (having the same head and
tail entities), which we consider as noise in the KG and they do not add interesting information for
causal analysis. This leave us with around 980k triples, among which around 6k are cause-efect
triples (based on relations listed in wikidata.org/wiki/Wikidata:List_of_properties/causality).
We create two benchmarks using these triples.
        </p>
        <p>Benchmark Creation. We split the collected event related triples into test, train and
validation set. Recall that we want to study the performance for causal relation prediction
task. Therefore, we ensure that the test set contains only the causal relations. In validation
set, we create two variations: one contains only the causal triples (similar to test set) and other
contains the mixture of causal triples and other event related triples (similar to train set). This
gives an insight on dataset preparation and helps us to understand the role of event related but
non-causal relations in learning the link prediction models for causal relation task. The train
set, which resembles causal KGs, contains causal triples along with other event-related triples.
(i) Wiki data Causal validation (WikiCV) Benchmark contains only causal triples in</p>
        <sec id="sec-4-2-1">
          <title>Best score</title>
          <p>
            Second Best score
the validation set. Specifically, out of around 6k causal relation triples, we randomly select
1000 triples each for test and validation set, and remaining triples are used for train set. For
convenience, we convert all the causal relations (hasCause, hasContributingFactor, etc) into
hasEfect relation in test and validation set. Furthermore, there is an issue of triple leakage–
the test triples are visible in training sets in the inverse form– identified on the previous link
prediction benchmarks [
            <xref ref-type="bibr" rid="ref8">8</xref>
            ]. So to avoid leakage, we remove inverse relations from the train set
which gives train set having around 941k triples. Other details like number of triples, number of
causal relation triples, number of entities and number of relations are shown in appendix (Fig. 6).
          </p>
          <p>(ii) Wiki data Mixed Validation (WikiMV) Benchmark is created on the same way as
WikiCV. The diference is the validation set which contains the mixture of causal as well as
other event related triples. Also, to accommodate event related triples along with causal relation
triples, we use larger validation set and equal test set (1, 500 triples each). Due to this change,
the number of removed inverse relations changes. So, the number of triples in train set (∼ 951k)
on WikiMV is diferent than that on WikiCV.</p>
        </sec>
      </sec>
      <sec id="sec-4-3">
        <title>4.3. Causal Relation Prediction Efectiveness</title>
        <p>Now we present the efectiveness of five diferent link prediction techniques on the causal
relation prediction task. Fig. 3 shows MRR and Hits@k for k = 1, 5, 10 and 50 on WikiCV
and WikiMV benchmarks. While there isn’t a clear winner for the link prediction task, NoGE
outperforms other techniques in all but Hits@1 in both causal relation benchmarks. Specifically,
it outperforms second best technique–ComplEx– in terms of MRR by around 33 % in WikiCV
and slightly outperforms the second best technique–ConvE– in WikiMV benchmark. In terms
of Hits@10, NoGE is better by around 11 % and almost two times than ComplEx in WikiCV and
WikiMV respectively. NoGE seems to capture the graph structure better even for the sparse
graph. We observed that TuckER and TransE are able to capture the semantic similarity but
could not infer the causal information well. Hence, most of their predictions are other events of
the same class rather than consequence or efect events.</p>
        <p>
          Furthermore, we observe that the techniques perform better when we use causal relations,
together with other event related triples, for validation. This is because the validation output is
used to select the model for evaluation and the non-causal relations appearing with the events
(instance of, subclass of, etc) act as negative samples to the model. This helps the models to
better distinguish causal relations from other relations.
4.4. Manual Result Analysis
The classical metrics show that NoGE performs relatively better in link prediction tasks, followed
by ConvE and ComplEx. However, looking at numbers, we see weaker performance on causal
relation prediction task. For example, the MRR of NoGE is over 30 % in all link prediction
benchmarks (see Fig. 2). But it drops significantly in both Causal Relation Benchmarks (see
Fig. 3). To understand this issue, we verify the results of each technique manually in both
benchmarks. We observe that the techniques are penalized by the evaluation metrics due to
sparsity and missing relations in KG that we use to create the benchmark. For instance, there
is a causal relation &lt;civil disorder, hasEfect, curfew&gt; in WikiMV test set. When we use this as
a test case (querying &lt;civil disorder, hasEfect, ?target event&gt;), NoGE returns demonstration
as the top-1 result which seems to be true. However, since this information is not available
in the groundtruth, the classical evaluation measure, which uses closed-world assumption [
          <xref ref-type="bibr" rid="ref6">6</xref>
          ],
considers demonstration as incorrect prediction.
        </p>
        <p>It is impractically time consuming to evaluate each result manually or to label the complete
groundtruth. Therefore, to understand the performance of each technique even better, we
develop a novel manual evaluation strategy that compares the success of each technique on
causal relation prediction task with reduced efort. Instead of evaluating instance-level results,
our idea is to evaluate the results on type level by mapping each instance level prediction into
their most granular types.2 We illustrate this with an example.</p>
        <p>Example 1. Consider causal relation prediction task i.e., &lt;source event, hasEfect, ?target event&gt;
where the objective is to predict the efect (target event). Consider two instance level predictions:
&lt;Murder of George Floyd (Q95579249), hasEfect, George Floyd protests&gt; and &lt;Death Of Javier
Ordóñez (Q99194919), hasEfect, Javier Ordóñez protests&gt;. Here, both input source events belong to
type murder and both predicted target events belong to type protest. So we map all instances to
their respective types and evaluate correctness of &lt;murder, hasEfect, protest&gt;.</p>
        <p>Mapping to type level prediction. Our objective is to evaluate each technique on type level
and observe their relative performance. For that we generate top-k type level target events
for each source event and evaluate them manually. At first, we find a ranked list of 50 target
predictions for each source event in the test set by each technique. The ranking is based on
confidence score that each technique assign to the target entity while making a prediction.
Recall that the train set contains: (i) literals such as numbers, dates, etc. which are not events
but may help in predicting causal relations, (ii) entities having no labels in Wikidata and (iii)
entities having more than one most granular type that creates ambiguity during evaluation.
Some techniques may predict such literals, unlabeled entities and entities having ambiguous
types as target events. We consider such predictions as incorrect and filter them out. Our
evaluation metrics (recall), to be discussed later, will penalize such predictions. After filtration,
if a predicted entity is already a type, we keep it as it is; else, we map each source event and its
predictions to their respective types as discussed in Example 1. Note that there can be multiple
instance level predictions having the same type level predictions (see Example 1). In such case,
we record the maximum confidence score among them. This is because the highest score among
each instance level predictions signifies the best confidence of the technique for that prediction.
2Here onwards, we simply use type to denote an entity’s most granular type unless mentioned otherwise.
TransE
TuckER
ComplEx
ConvE
NoGE</p>
        <p>WikiCV WikiMV
Precision Recall F1-Score F1-Score (at  ) Precision Recall F-score F-score (at  )
0.277 0.141 0.187 0.188 0.330 0.177 0.230 0.231
0.200 0.108 0.140 0.141 0.219 0.130 0.163 0.164
0.469 0.146 0.223 0.224 0.548 0.188 0.280 0.280
0.299 0.110 0.161 0.164 0.348 0.171 0.230 0.238
0.386 0.155 0.221 0.221 0.386 0.188 0.253 0.255</p>
        <p>Groundtruth creation. We record top-10 type level target events based on confidence score
for each type level source event. Of course, all source event may not find 10 target events
because either they do not have at least 10 target events in train set or they do not predict events
instead predict literals and ambiguous entities. The details on the number of source events,
target events and (source event, target event) pairs generated by each technique is reported in
appendix (Fig. 7). It is seen that TuckER and TransE produce the largest number of (source event,
target event) pairs whereas ComplEx and ConvE are the most selective techniques producing
least pairs. Next, we manually label–either true or false–(source event, target event) pairs
produced by each technique and use result to create a groundtruth. In groundtruth, the true
causal relations are all (source event, target event) pairs labeled as true. All other pairs are false
causal relations. Note that there can be true causal relation not produced by any techniques, and
hence do not make it to the groundtruth as true. However, since we are interested in relative
comparison of the techniques, we assume that each true causal relation is predicted by at least
one technique. Note that an event may have more than 10 target events in the groundtruth if
diferent techniques predict diferent set of target types for the same source event.</p>
        <p>Manual Evaluation metrics. After creating the groundtruth based on manual annotation,
we report precision (P), recall (R) and F-score for each technique. Let  be the set of predictions
made by a technique whose size depends on the number of target events that are queried for
each source event. Let  be the set of true causal relations in the groundtruth. Then, P, R and
F-score are given by:
 = | ∩ | ,  = | ∩ | ,  -score = 2 ·  · 
|  | ||  + 
(1)</p>
        <p>Manual Evaluation Efectiveness Results. We evaluate precision, recall and F-score at
diferent value of k i.e. diferent number of target events per source event. Considering events
in both WikiCV and WikiMV, the median and average target event per source event types is 7
and 7.5 respectively. Therefore, we select a nearby value k = 5 for our discussion. The results
on other values of k are shown in appendix (Fig. 8).</p>
        <p>Fig. 4 shows efectiveness of diferent techniques in both WikiCV and WikiMV benchmarks
for top-5 prediction per source event. Here, we focus on Precision, Recall and F-score. We
will explain F-score (at  ) in Section 4.5. We observe that ComplEx and NoGE are the best
methods on both benchmarks with ComplEx having the best precision and NoGE having the
best recall. In terms of F-score, ComplEx slightly outperforms NoGE in WikiCV benchmark
and by around 10 % in WikiMV benchmark. This shows that although ComplEx produces less
results, they are highly precise. On the other hand, TransE and TuckER produces larger number
of predictions (see Fig. 7) which favors their recall but penalizes precision. Note that even
though ComplEx and NoGE produce less number of prediction results, they have much higher
precision than other methods and comparable recall. When we look at the recall at diferent k
in both benchmarks (Fig. 8), we observe that ComplEx (blue square line) performs better until k
= 5 but its recall starts getting saturated after that. This shows that ComplEx performs better for
the source events having fewer target events. TransE and TuckER, on the other hand, has higher
recall with increase in k because their correct predictions are spread over all ranking positions
rather than compressed at top rankings. We also observed in our manual evaluation that each
method make accurate predictions that cannot be found in the output of other methods.</p>
      </sec>
      <sec id="sec-4-4">
        <title>4.5. Efect of threshold</title>
        <p>Finally, we see if applying threshold on confidence scores increases efectiveness (F-score) over
top-k approach. A higher threshold increases precision but decreases recall and vice-versa.
Thus, we consider maximization of F-score, the combination of both precision and recall. For fair
comparison, we first generate the top-k results and then apply diferent threshold to see if there
is an improvement over F-score. F-scores in both WikiCV and WikiMV before and after applying
threshold to the top-5 results is shown under (F-Score) and F-Score (at  ) column respectively
(Fig. 4). All but NoGE in WikiCV and ComplEx in WikiMV show small improvement in F-score
when applying the best threshold. We also analyze the precision, recall and F-score trend for
diferent thresholds (appendix Fig. 9). We only show the trend for TransE, ComplEx and NoGE
in WikiMV benchmark as each technique follows similar trend on both benchmarks. Also, the
trend for TuckER and ConvE resembles to that of TransE. For TransE, TuckER and ConvE,
the precision trend is random. But for better performing techniques (ComplEx and NoGE),
precision increases with increase in threshold. For all techniques, recall goes up with decrease in
threshold and seems to saturate for further decrease. This shows that there could be a threshold
value for each technique where we get the best F-score which we will explore in future work.</p>
      </sec>
    </sec>
    <sec id="sec-5">
      <title>5. Conclusion</title>
      <p>
        We evaluated existing link prediction techniques for causal relation prediction in
Wikidatabased causal Knowledge Graphs that contain highly sparse and generally many-to-many causal
relations. Based on classical link prediction metrics, we observe that the techniques perform
better for the well-studied link prediction task but show weaker performance in the causal
relation prediction task. Furthermore, we observed that our model trained on a dataset with
a mixture of causal and other non-causal but event-related triples performed better than one
trained on a datasets of causal relation triples only. We also studied the drawbacks of existing
metrics and proposed a novel manual evaluation strategy. Our results show that the techniques
generally perform better than what the classic metrics indicate, although there is still plenty of
room for improvements. We also observed that each of the methods, regardless of their accuracy
scores, make accurate predictions that cannot be found in the output of the other methods. In
the future, we will explore using a combination of diferent techniques, including rule-based link
prediction methods [
        <xref ref-type="bibr" rid="ref18">18</xref>
        ], to get better overall prediction results. Also, we will further explore
the use of threshold instead of top-k ranking as a robust mechanism of KG enrichment.
      </p>
    </sec>
    <sec id="sec-6">
      <title>A. Appendix</title>
      <sec id="sec-6-1">
        <title>Technique</title>
        <p>TransE
TuckER
ComplEx
ConvE
NoGE</p>
      </sec>
      <sec id="sec-6-2">
        <title>Source code link</title>
        <p>https://github.com/facebookresearch/PyTorch-BigGraph
https://github.com/ibalazevic/TuckER
https://github.com/facebookresearch/PyTorch-BigGraph
https://github.com/TimDettmers/ConvE
https://github.com/daiquocnguyen/GNN-NoGE
0.20
llca0.15
e
R
0.10
1 2 3 4 5 6 7 8 9 10</p>
        <p>k
(b) Recall on WikiCV
1.0
0.8
0.6
0.4
0.2
1.0
0.6
0.4
0.0 1 2 3 4 5 6 7 8 9 10</p>
        <p>k
(a) Precision on WikiCV
0.0 1 2 3 4 5 6 7 8 9 10
k
0.7
0.6
0.5
ion0.4
s
i
rce0.3
P
0.2
0.1
1.0
0.8
0.6
0.4
0.2
0.0
6</p>
      </sec>
    </sec>
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