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Machine Learning - DISCo

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C<br />

[2+,3-I I(<br />

i<br />

\<br />

Cup C Fragile ...<br />

Cup C BottamlsFlal,<br />

I2++l<br />

Light,<br />

HmConcnvity,<br />

ConcavifyPointsUp<br />

[4+.2-I<br />

Cup<br />

C<br />

HasConcavity,<br />

ConcavityPointsUp<br />

HandleOnTop<br />

[0+,2-I<br />

Cup<br />

C BonomlsFlat,<br />

Light,<br />

HasConcaviry,<br />

ConcavifyPoinfsUp,<br />

1 ~andleon~o~<br />

Cup<br />

C BottomlsFlof,<br />

Light,<br />

HasConcavity,<br />

W+&I<br />

FIGURE 12.8<br />

Hypothesis space search in FOCL. To learn a single rule, FOCL searches from general to increasingly<br />

specific hypotheses. Two kinds of operators generate specializations of the current hypothesis. One<br />

kind adds a single new literal (solid lines.in the figure). A second kind of operator specializes the<br />

rule by adding a set of literals that constitute logically sufficient conditions for the target concept,<br />

according to the domain theory (dashed lines in the figure). FOCL selects among all these candidate<br />

specializations, based on their performance over the data. Therefore, imperfect domain theories will<br />

impact the hypothesis only if the evidence supports the theory. This example is based on the same<br />

training data and domain theory as the earlier KBANN example.<br />

primitive attributes of the instances, are considered nonoperational. An example<br />

of a nonoperational attribute in this case is the attribute Stable.<br />

At each point in its general-to-specific search, FOCL expands its current<br />

hypothesis h using the following two operators:<br />

,<br />

1. For each operational literal that is not part of h, create a specialization of h<br />

by adding this single literal to the preconditio s. This is also the method used<br />

by FOIL to generate candidate successors. P he solid arrows in Figure 12.8<br />

denote this type of specialization.

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