What is the difference between nDCG and rank correlation methods?
A central problem in ranking is to design a measure for evaluation of ranking functions. In this paper we study, from a theoretical perspective, the Normalized Discounted Cumulative
Gain (NDCG) which is a family of ranking measures widely used in practice. Although
there are extensive empirical studies of the NDCG family, little is known about its theoretical properties

NDCG has two advantages compared to many other measures. First, NDCG allows each retrieved document has graded relevance while most traditional ranking measures only allow binary relevance. ... Second, NDCG involves a discount function over the rank while many other measures uniformly weight all positions.

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