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.