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Recommendation Systems — Collaborative and Content-Based Filtering

Core MLRecommendations🟢 Free Lesson

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Specialized Topics

Recommendation Systems — The Algorithm Behind 'You Might Also Like'

Recommendation systems predict what users will like based on past behavior, powering billions of dollars in e-commerce and content revenue.

  • Collaborative Filtering — finds patterns in user behavior to recommend items liked by similar users
  • Content-Based Filtering — recommends items similar to what a user has already enjoyed using item features
  • Matrix Factorization — decomposes sparse user-item matrices into dense latent factor representations

"Our head is a recommendation engine." — Jeff Bezos

Recommendation Systems — Complete Guide

Recommendation systems predict what users will like based on past behavior.


Mathematical Foundations

Cosine Similarity

Matrix Factorization Objective

Precision@K

NDCG@K


Types

Collaborative vs Content-Based Filtering

Collaborative vs Content-Based FilteringCollaborative Filtering"Users like you also liked..."User-ItemABCDUser 1✓✓User 2✓✓✓User 3✓✓?User 1 and 3 are similar → Recommend DUses: User-Item interaction matrixProblem: Cold-start for new usersContent-Based Filtering"Items similar to what you liked..."Movie AAction, Sci-FiMovie BAction, ThrillerMovie CRomance, Drama[0.9, 0.2, 0.1][0.8, 0.7, 0.1][0.1, 0.1, 0.9]Similar features → high similarityUses: Item metadata (genre, tags)Problem: Filter bubble, no discovery

Collaborative Filtering

Matrix Factorization Diagram

Matrix Factorization — Decomposing User-Item MatrixUser-Item Matrix RItems →←Users5 3 ?4 ? 2? 1 52 ? 4? 4 ?? = missing ratingsUser Factors P5 × k×Item Factors Qk × 3=Predicted RÌ‚5 × 3R ≈ P × Q^T | min Σ (r_ij - p_i · q_j)^2 + λ(||p_i||² + ||q_j||²)k = latent factors (typically 50-200) | SVD or ALS for optimization

Cold-Start Problem

The Cold-Start Problem in RecommendationsNew User Cold-Start• No interaction history• Cannot find similar users• Collaborative failsSolution: Use content-basedor ask for preferencesonboarding surveyNew Item Cold-Start• No ratings yet• Cannot find similar items• Content-based worksSolution: Use item featuresmetadata, descriptiontext for similaritySystem Cold-Start• Brand new system• No data at all• Need bootstrappingSolution: Popularity-basedthen transition tocollaborative as data grows

Evaluation


Key Takeaways


What to Learn Next

-> Clustering Group similar users or items using K-Means, DBSCAN, and hierarchical methods.

-> Dimensionality Reduction Reduce sparse user-item matrices to dense representations with PCA and autoencoders.

-> Neural Networks Build deep learning models for neural collaborative filtering and representation learning.

-> Model Evaluation Master precision, recall, and ranking metrics for evaluating recommendation quality.

-> A/B Testing Design online experiments to measure the real-world impact of recommendation changes.

-> NLP Fundamentals Process item descriptions and user reviews with text mining for content-based recommendations.

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