328 Chapter 12 Natural laNguage INterfaCe: QuestIoN aNswerINg system
Class 3 Systems generate answers to list, script, or template-like questions
from parts found in several documents.
Class 4 Interactive systems answer questions in the context of previous in-
teractions, which involve complex reference resolution.
Class 5 Systems answer speculative questions, which involve knowledge
extraction from relevant documents and case-based, temporal, spa-
tial, and evidential reasoning.
12.1.2 Closed-Domain Question Answering
Closed-domain question answering has recently regained the interest of re-
searchers. The definition of the closed domain ranges from working in a
specific domain to using closed document collections restricted in ...