The primary output of IBL algorithms is a concept description (or concept). This is a function
that maps instances to categories: given an instance drawn from the instance space, it yields
a classification, which is the predicted value for this instance's category attribute. 2
An instance-based concept description includes a set of stored instances and, possibly,
some information concerning their past performances during classification (e.g., their number
of correct and incorrect classification predictions). This set of instances can change
after each training instance is processed. However, IBL algorithms do not construct extensional
concept descriptions. Instead, concept descriptions are determined by how the IBL
algorithm's selected similarity and classification functions use the current set of saved instances.
These functions are two of the three components in the following framework that
describes all IBL algorithms: