Unni Cathrine Eiken February 2005

which is the desired output category. The comma-separated values format was used for the
EPAS classification. In order to classify the constituents of each EPAS based on the contextual
patterns in the structure, each part of the EPAS was classified with reference to the other
constituents in it. Somewhat analogous to the way that a screw can be described as being small,
long and containing no holes, the argument åsted (crime scene) can be described through its cooccurrence
with the predicate ankomme (arrive) and the argument etterforsker (investigator)
(example (4-1)). This makes it possible to train a classifier on the EPAS list, using the argument
which’s environment is to be learned as category label, and each constituent in the EPAS as
features. To avoid that the category was explicitly present in the training material and ensure
that the classifier was trained only on the environment of the desired category, the relevant
feature was ignored using TiMBL’s ignore option. In order to classify the structures once for
each argument type, two different data sets were prepared. Example (4-1) shows the format of
the three-feature dataset that was used. The parentheses indicate that the feature in question was
ignored when training and classifying.
(4- 1)
features category
a. predicate, (argument 1), argument 2 argument 1
b. predicate, argument 1, (argument 2) argument 2
Example (4-2) shows excerpts of the two input files: (4-2a) shows the structures with argument
1 as category, while (4-2b) shows the same structures with argument 2 as the category. The
classifier is given two constituents of an EPAS to learn from and the target constituent is given
as the EPAS’ category.
(4- 2)
a. ankomme,etterforsker,?,etterforsker
ankomme,etterforsker,?,etterforsker
ankomme,etterforsker,åsted,etterforsker
antyde,politi,?,politi
avhøre,?,person,?
avhøre,?,vedkommende,?
avhøre,politi,vitne,politi
62