Consider the following list of simple NPs selected as candidates: 1. conference workshop announcement 2. international conference 3. workshop description 4. conference deadline In the case of the first noun phrase, for example, its importance is found in the two noun modifiers: conference and workshop as much as in the head announcement, due to their presence as heads or modifiers in the candidate NPs 2-4.
the axis-parallel decision trees check at each node the value of a single attribute.