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5. Disambiguation techniques

We have underlined the main causes of ambiguity, but many others exists. Now I want to talk about the technique called of disambiguation. Disambiguation is used in order to solve these ambiguities in an automatic way. (For automatic I refer to techniques that can be translate in a programming language and therefore useful in order to construct an intelligent agent). We begin just from the creation of a " Model of the World ", to which we have already pointed out dealing of the ambiguities of semantic type.




Model of the World.

It is based on the possibility that a fact happens. A model of the world correctly developed would solve many semantic ambiguity like the one we talked about with the sentence "the car hit the pole while it is moving.". But how a model of the world is constructed? We can start by defining objects or classes from which we will derive other objects adding to each new class some more property that characterize it (are you familiar with the concept of heredity of modern programming languages?).

So we define the object Entity. From the object Entity we derive the objects Animal, Vegetable, Mineral. Continue the derivations like in figure, remembering that every derived object inherits all the property of the object from which it has been derived.






Fig.5.1: Model of the world.





Therefore we define a series of boolean predicate (assuming that all what it is not explicitly marked as True is False):

ItLives (Animal) = True;
ItLives (Vegetable) = True;
ItMoves (Animal) = True;
ItMoves (Vegetable) = False;

ItNurses (Mammalian) = True;
ItFlies (Bird) = True;
ItBarks (Dog) = True;


Now we can ask to our intelligent agent, equipped of the world model, a question of the type:

"Do the birds moves?"


The agent through some simple auto-generated rules of derivations working on its world model should answer correctly. Using a boolean logic True/False, the rule of derivation, expressed in a recursive form, generated to solve the problem "Do the birds moves?" would be the following:

Return (False Or ItMoves (Oviparous))


Which comes expanded as in fig. 5.2 and gives back the True value.






Fig.5.2: Derivation of the rule.





What we did is to verify if the birds derive from one class X, where X is a class so that ItMoves(X)=True exists. In such case, according to what we said previously on the heredity, we can conclude ItMoves(Birds)=True too.
...and know? Well, you probably remember we were dealing the problem of the ambiguity and the techniques of disambiguation. Look at the following sentence which should sound you familiar:

" The car hit the tree while it was moving."


We had said that a "well done" syntactic analyzer would have derived:

" the car, while it was moving, hit the tree. "
&
" the car hit the tree while the tree is moving. "


Two meant for the same phrase are too many. It is necessary to discard one of them. The world representation comes in aid of the agent. The agent analyzing his world model will deduce (between the other rules):

Return (False Or ItMoves(Vegetable));


Which returns False, indicating to the agent that the tree cannot perform the action of moving; consequently the second interpretation is wrong and it can be discarded.

There would be still many things to say about the world model implementation strategies, but I prefer not to be boring. Last observation: to create a representation of the world is not sure an easy work, and the problem is not the huge amount of data needed (which could be solved by automatic-learning techniques), but their organization, that is the type of model to adopt. The one I have proposed, is based on object and heredity, but it is not the only existing strategy (even if perhaps one of simplest to implement).





The Probabilistic Context Free Grammar (PCFG).

One of the crucial points to deal with in the implementation of an intelligent agent which can understand the natural language is choosing the grammar. I'm not going to talk about it in an accurate way. We just say that an intelligent agent can be equipped with several different types of grammars, some of simple implementation, others a little more complex. Obviously, those that work better are the most complex ones (always supposing they have been written in correct way). We need to have a fundamental distinguish between in grammars which are independent from the context (context free), and the grammars which are not independent from the context. The context free grammars are easier to define through rules just like the following:

...
...
...
the problem of this kind of grammar is just their independence from the context. I explain myself better with an example. We consider the phrase:

" Mark loaded all the force on the mace, hit the ball and the sent outside from the diamond. Great blow! "


Ours agent, equipped with a context free grammar, would find itself of forehead to the problem to attribute meant to the word "diamond". The diamond is a precious stone and the name of the baseball game field. Obvious this is the case the corrected meant is the second one, but that is deduced from the context in which " diamond " is inserted. A context free grammar analyzes the words one to the time, without considering the mean of the previous/successive words and sentences. To the contrary one grammatical employee from the context would analyze the word " diamond ", in function of the other words in the phrase (like " mace ", " ball ", " blow ") and it would probably get to the correct interpretation.

Someone has tried to solve this type of ambiguities extending the context free grammars through the introduction of a parameter which ties, to each production, the probability with which it is used. Such probability is not constant, but it varies according to the context of the sentence, to the type of the speaker, and so on. This new type of grammar takes the name of probabilistic context free grammar (PCFG). Using a PCFG, the probability that one determined interpretation is chosen is given from the product of the probabilities of all the rules that have been used to reach that particular interpretation. Bringing back here of under the grammar of the extended previous figure with the probabilistic parameters.




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