What is NDCG and how does it evaluate retrieval?

Normalized Discounted Cumulative Gain, or NDCG, evaluates ranked retrieval results by assigning higher weight to relevant documents at higher ranks, using a logarithmic discount for position. It handles graded relevance judgments where some documents are more relevant than others, not just binary relevant-or-not. NDCG at 10 is a standard metric in information retrieval benchmarks like BEIR and MTEB. It is the most rigorous retrieval metric when you have labeled relevance grades, though binary recall at k is often sufficient for RAG evaluation in practice.