Definition
- Systems that suggest items (products, content, services) tailored to an individual’s tastes, behavior, or context.
Main approaches
- Collaborative filtering: uses patterns of user–item interactions (user-based or item-based). Strength: captures community tastes. Weakness: cold start, sparsity. (Goldberg et al., 1992; Sarwar et al., 2001)
- Content-based filtering: matches item features to user profiles built from previously liked items. Strength: handles new items; interpretable. Weakness: limited novelty, feature engineering required.
- Hybrid methods: combine collaborative and content cues to mitigate weaknesses (Burke, 2002).
- Model-based methods: matrix factorization, deep learning (neural collaborative filtering, sequence models) for scalable, latent representations. (Koren et al., 2009; He et al., 2017)
Evaluation metrics
- Accuracy: precision, recall, RMSE, NDCG.
- Beyond-accuracy: diversity, novelty, serendipity, coverage, fairness, calibration.
- Online metrics: click-through rate (CTR), conversion, dwell time, retention, A/B testing.
Key practical issues
- Cold start: new users/items need profiling or side information.
- Sparsity: few interactions per user; use smoothing, side data, matrix factorization.
- Scalability: approximate nearest neighbors, hashing, embedding retrieval.
- Privacy: data minimization, differential privacy, federated learning.
- Bias and fairness: popularity bias, feedback loops; need debiasing, counterfactual evaluation.
Ethical and societal concerns
- Filter bubbles and echo chambers.
- Manipulation and persuasion.
- Transparency and explainability demands.
- Regulatory compliance (GDPR, consumer protection).
Design recommendations (concise)
- Combine signals (collab + content + contextual).
- Optimize for long-term engagement and user satisfaction, not only clicks.
- Monitor and mitigate biases; log counterfactuals for offline evaluation.
- Offer user control & explanations; allow exploration controls.
References (select)
- Goldberg et al., “Using collaborative filtering to weave an information tapestry” (1992).
- Sarwar et al., “Item-based collaborative filtering recommendation algorithms” (2001).
- Koren, Bell, and Volinsky, “Matrix factorization techniques for recommender systems” (2009).
- Burke, “Hybrid recommender systems: Survey and experiments” (2002).
- He et al., “Neural Collaborative Filtering” (2017).
If you want, I can summarize architectures, show sample evaluation code, or list datasets and libraries (e.g., MovieLens, Amazon, RecSys, Surprise, LightFM, Spotlight).