Knowledge about the World

This section explores information about the world that can be useful to temporal relation classification. In particular, some temporal relations between events are more expected than others just due to lexical semantics. The isolated meaning of the words involved may provide temporal clues that are worth exploring.

Temporal Direction Consider the two following examples, from the point of view of classifying the temporal relations between the events and times that are highlighted in boldface in each sentence (i.e. Task A Event-Timex):

(28) a. Os analistas previam [que em 1990 a BellSouth visse lucros na casa

dos 3,90 dólares por ação.]

Analysts were predicting [ that in 1990 BellSouth would see earnings in the range of $3.90 a share.]

b. A N.V. DSM informou [que o lucro líquido no terceiro trimestre

subiu 63%.]

N.V. DSM said [net income in the third quarter jumped 63%].

In the case of the example in (28a), the fact that the temporal expression occurs

in the complement of this verb (enclosed in square brackets) is a good indication that the event precedes the date, because of what the verb means: predictions are made before what is predicted happens, and since what is predicted is the 1990 BellSouth earnings, the predict event should have occurred earlier than 1990 (or at least earlier than the time at which these earning are announced).

In (28b), with the verb informar “inform, say”, we find the inverse temporal

relation. Here, reporting events are expected to temporally follow the reported

events. In this sentence, there is an annotated temporal relation between the event denoted by the term informou “said” and the time expression o terceiro trimestre “the third quarter.” This time expression locates the time of the event described in the embedded clause (inside brackets) in the timeline. The annotated temporal relation is thus dependent on the temporal relation between the two events. Since that is a temporal relation between a reporting event and a reported event, the expectation is that the reporting event temporally follows the reported event.

The idea is to record this sort of information in a feature for the classifiers. Although the classifiers do not know what the complement of the verb is, they do now that in these two examples the verb precedes the temporal expression (because

of the classifier feature order-event-first presented above in Section4.2), which can

be regarded as a hint that the temporal expression occurs inside the complement of the verb (which is the case in these two examples), as complements follow their heads in a language like Portuguese.

We call this sort of temporal information between a word and its complement “temporal direction”, for lack of a better expression.

In order to obtain this information, all event lemmas present in the training data were extracted and a mapping was manually created between them and the expected temporal relation with its complement. For many of these words the associated value is NONE, since they impose no temporal constraint with respect to the material mentioned in their complement. The other possible values are AFTER and BEFORE. A few examples:

• acusar “accuse, charge” AFTER • atrasar “stall, delay” BEFORE • organizar “organize” BEFORE • prever “predict” BEFORE • relatar “report,post” AFTER

• tentar “try, seek, attempt” BEFORE

This feature thus records knowledge about the world. According to the ex-

amples provided, events of accusing follow the events that someone is accused of doing, events of delaying precede delayed events, events of organizing precede orga- nized events, reporting events follow reported events, and trying events precede tried

events. AppendixIIIshows the full list of manually annotated lemmas of the event

words found in the training data.

This annotated information is not evaluated independently, and some error is likely. Rather, a classifier feature is employed, with these values, related to the

4.4 Classifier Features

lemma of the event that is the first argument of the temporal relation to be classified, in the hope that it will be useful despite its imperfections.

It must be mentioned that this manual annotation was performed without look- ing at contexts where the words occur, but rather by just taking into account what one would expect to see in the data, based on the word. The justification for that choice is so that the resulting mapping does not overfit the training data.

It must be once again stressed that the classifiers do not know that e.g. in (28b)

lucros “earnings” is the syntactic complement of the verb. They only know that the event denoted by the verb probably precedes the event denoted by whatever event is mentioned in the verb’s complement. Other features can, however, provide clues for this syntactic relation, like the feature order-event-first already mentioned, although this is just a hint. This is another limitation of this feature.

Even though this feature records expected temporal relations between events, it should be useful for Task A Event-Timex, as a means to classify temporal relations in those cases where the time in that relation is given by an expression that is not a syntactic dependent of the word denoting the event in the temporal relation, but rather modifies another event denoting word or phrase in the appropriate syntactic

relation with the other event term, as the example in (28a) above. Based on clas-

sifier performance, this feature does seem to be somewhat useful for the problem of temporal relation classification (Section 4.5.2).