Temporal Information Extraction

Temporal information extraction is centered around mining factual knowl-edge along with a time annotation from free text. Hence, all challenges of information extraction (see Section 2.5) arise, but additionally time expres-sions in text have to be recognized. For instance, the sentence “In 1997, De Niro married his second wife, actress Grace Hightower, at their Marbletown home.”² contains the temporal fact that De Niro and Hightower got mar-ried in 1997. As case study for our data model, we implement this kind of temporal information extraction problem in it.

Competitors We compare our model against state-of-the-art methods for temporal tuple extraction, that is constraint solving via an integer linear program [109] and probabilistic inference via Markov logic networks [120].

2http://en.wikipedia.org/wiki/Deniro(accessed January 14th, 2014)

To solve integer linear programs we deploy the Gurobi software³ imple-menting the constraints from [140, 153]. For Markov logic networks, we employ “Markov TheBeast”⁴ (TheBeast), which performs cutting plane in-ference [121]. Since Markov logic networks do not natively support time, we encode time-intervals by adding two arguments on the relational predi-cates denoting their begin and end time points. Both competitors perform maximum-a-posteriori [64] inference, i.e. they return the single most likely possible world, rather than computing probabilities as in our model. Fur-thermore, we tried Markov logic networks implementations which compute probabilities [102, 120] but even after extensive tuning they either exceeded the available memory, disk space, or ran for several days without terminat-ing. All systems operate on the same set of deduction rules and constraints.

Dataset Our dataset consists of 1,817 tuples which we extracted from free-text biographies of 272 celebrities crawled from wikipedia.org, imdb.com, and biography.com. The tuples correspond to nine temporal relations, namely AttendedSchool^T, Born^T, Died^T, Divorce^T, IsDating^T, Founded^T, GraduatedFrom^T, MovedTo^T, and Wedding^T. Our extractor employs textual patterns from [101] to recognize potential facts, regular expressions to find dates, and the Stanford named entity recognizer [50] to identify names in text, which we then disambiguate heuristically to entities of YAGO2 (see Appendix A.2.1). We assign each pattern and extraction step a probability, such that the probability of each extracted tuple is obtained by multiplying the probabilities of all steps involved in its extraction process.

Ground Truth We manually label 100 randomly chosen tuples from each relation by determining their correct time-intervals from the text and labeled them as false if the extracted tuple was erroneous. Additionally, we equally divide the labeled tuples into a training set for development and a test set for evaluation.

Deduction Rules and Constraints We employed a set of 11 hand-crafted temporal deduction rules, including the ones shown in Example 3.7.

Besides deducing the Marriage^T relation from Wedding^T and Divorce^T, we aggregate the relations over their observations (as distinguished by their id) by writing for example:

MovedTo^T(P, L, T_b, T_e) ← MovedToExtraction^T(P, L, Id , T_b, T_e) These different observations arise as the same fact can be extracted repeat-edly, i.e. from different sources. Besides the deduction rules, there are 21

3http://www.gurobi.com

4https://code.google.com/p/thebeast/

constraints to enforce relations to be irreflexive, temporally preceding, or temporally disjoint. For instance, by writing

¬(Divorce(P1, P₂) ∧ P₁ = P₂)

we rule out that a person is getting divorced from him or herself, which assures that Divorce irreflexive. As for temporal constraints we ensure, for instance, that people are born before they attend a school:

¬(Born^T(P, T_b, Te) ∧ AttendedSchool^T(P, S, T_b^′, T_e^′) ∧ T_b^′ <^T Te) Besides the formulas above, all remaining deduction rules and constraints can be found in Appendix A.3.1.

Metrics To evaluate the result quality, we rely on the established metrics precision, recall, and F₁ [95], but tailored towards temporal data. Hence, our precision and recall metrics reflect the overlap between the obtained tuples’

time-intervals and the time-intervals of the ground-truth. For this, we define the function Valid . For a set of tuples in a temporal relation instance R^T with identical non-temporal arguments ¯a and a probability threshold θ_p, Valid returns the set of time points at which these tuples are valid with a probability of at least θ_p.

Valid (R^T, ¯a, θ_p) :=

½ t

¯¯

¯¯

I = R^T(¯a, t_b, t_e) ∈ R^T, t ∈ [t_b, t_e), P (λ(I)) ≥ θ_p

¾

(3.7) Based on this set we define our quality metrics.

Definition 3.9. For a temporal relation instance R^T, a corresponding ground truth instance R^T_truth, non-temporal arguments ¯a and a probability threshold θ_p, we define precision and recall as follows:

Precision(R^T, ¯a, θ_p) := ^{|Valid (RT,¯}_|Valid(R^{a,θp)∩Valid(R}T,¯a,θp)|^{Ttruth,¯a,θp))|}

Recall (R^T, ¯a, θ_p) := ^{|Valid (RT,¯a,θp)∩Valid(RTtruth,¯a,θp))|}

|Valid(R^T_truth,¯a,θp)|

The harmonic mean of precision and recall establishes the F₁ measure:

F1(R^T, ¯a, θp) := 2 · Precision(R^T, ¯a, θ_p) · Recall (R^T, ¯a, θ_p) Precision(R^T, ¯a, θ_p) + Recall (R^T, ¯a, θ_p)

Intuitively, precision measures what fraction of time-points is correct.

Recall calculates the fraction of correct time-points covered by the extracted data. Finally, F1 combines both into one value.

Example 3.17. Consider the ground truth tuple I_g : Divorce^T(DeNiro, Abbott, 1988-09-01, 1988-09-02) and I₅ from Figure 3.1. For θ_p = 0.7, we obtain a precision of about 0.01 and recall of 1.0 which yields a F₁ value of approximately 0.02.

To establish precision and recall for sets of tuples with different non-temporal arguments, we report the macro-average of the individual tuples’

values.

Results In the resulting plots we distinguish between the setups of system with constraints (+c) and without constraints (-c). In Figure 3.4(a) we

0.0 0

(a) Precision & Recall (varying θp)

TPDB-c TPDB+c Gurobi+c TheBeast-c TheBeast+c

Marriage 0.76 0.81 0.52 0.44 0.46

AttendedSchool 0.72 0.72 0.54 0.66 0.68

Born 0.83 0.84 0.87 0.80 0.80

Died 0.70 0.62 0.48 0.46 0.53

Founded 0.80 0.80 0.38 0.73 0.80

GraduatedFrom 0.74 0.74 0.70 0.68 0.67

IsDating 0.62 0.66 0.54 0.51 0.50

MovedTo 0.75 0.77 0.79 0.69 0.74

Average 0.74 0.75 0.61 0.62 0.65

(b) F1 measure (best θp)

Figure 3.4: Temporal Information Extraction: Quality

depict precision over recall for the Marriage^T and IsDating^T relation, where each point of TPDB corresponds to a different threshold θ_p. The plots of the remaining six relations can be found in Appendix A.3.1. As an overview, we additionally provide Figure 3.4(b), which reports the best F₁ for each method within each relation. Moreover, we measured the runtimes of the competing systems which we display in Figure 3.5.

Discussion To demonstrate the challenges of the dataset, about 700 of the 1,817 tuples violated at least one constraint and about 500 of the 700 tuples contradicted more than one constraint. Considering the F₁ metric, TPDB performs best, where the addition of constraints shifts the focus from recall towards precision and sometimes yields major quality gains (see

Fig-0

Figure 3.5: Temporal Information Extraction: Runtimes

ure 3.4(a)). Gurobi and TheBeast perform well, but the single possible world returned by their maximum-a-posteriori inference results in less stable F1

values (see Figure 3.4(b)). Considering runtimes, in the case without con-straints TPDB is the fastest system. When adding the concon-straints, TheBeast and TPDB take about the same time, whereas Gurobi is slower.