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Machine Learning·Intermediate

Recommendation Fundamentals: Retrieval, Ranking, and Evaluation Basics

Build a strong foundation in recommendation systems: candidate retrieval, ranking, exploration-exploitation, and offline/online evaluation. Designed for ML and product-system interviews.

40 min read 2 sections 1 interview questions
Recommender SystemsRetrieval RankingCollaborative FilteringTwo-Tower ModelsNDCGExplore ExploitCold StartFeature Engineering

Core Mental Model: Retrieval then Ranking

Recommendation systems at scale use multi-stage pipelines:

  1. retrieval for high recall from massive catalogs,
  2. ranking for precision on a small candidate set,
  3. policy/reranking for business and diversity constraints.

This decomposition is both a latency and quality requirement.

IMPORTANT

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