In this discussion, we must not overlook the fact that relevance is a complex
concept. It is quite difficult for a person to explain why one document is more
relevant than another, and when people are asked to judge the relevance of documents
for a given query, they can often disagree. Information scientists have written
volumes about the nature of relevance, but we will not dive into that material
here. Instead, we discuss two key aspects of relevance that are important for both
retrieval models and evaluation measures.