abstract = "We analyse generalisation in the eXtended Classifier
System (XCS) with symbolic conditions, based on genetic
programming, briefly XCSGP. We start from the results
presented in the literature, which showed that XCSGP
could not reach optimality in Boolean problems when
classifier conditions involved logical disjunctions. We
apply a new implementation of XCSGP to the learning of
Boolean functions and show that our version can
actually reach optimality even when disjunctions are
allowed in classifier conditions. We analyse the
evolved generalisations and explain why logical
disjunctions can make the learning more difficult in
XCS models and why our version performs better than the
earlier one. Then, we show that in problems that allow
many generalizations, so that or clauses are less
'convenient', XCSGP tends to develop solutions that do
not exploit logical disjunctions as much as one might
expect. However, when the problems allow few
generalizations, so that or clauses become an
interesting way to introduce simple generalizations,
XCSGP exploit them so as to evolve more compact
solutions.",
notes = "CEC 2007 - A joint meeting of the IEEE, the EPS, and
the IET.