History

• Ants, Cavemen, and Early Recommender Systems – The emergence of critics
• Information Retrieval and Filtering
• Manual Collaborative Filtering
• Automated Collaborative Filtering
• The Commercial Era

Information Retrieval

• Concept: An information retrieval process begins when a user enters a query into the system. Queries are formal statements of information needs, for example search strings in web search engines. In information retrieval a query does not uniquely identify a single object in the collection. Instead, several objects may match the query, perhaps with different degrees of relevancy.
• Static content base – Invest time in indexing content
• Dynamic information need – Queries presented in “real time”
• Common approach: TFIDF (tf-idf, term frequency–inverse document frequency)
  • Rank documents by term overlap
  • Rank terms by frequency

Information Filtering

• Concept: a approach that select relevant information for a user based on the user's profile
• Characteristic:
  • Reverse assumptions from Information Retrieval:
    • Static information need
    • Dynamic content base
  • Invest effort in modeling user need
    • Hand‐created “profile”
    • Machine learned profile
    • Feedback/updates
  • Pass new content through filters

Collaborative Filtering

In the newer, narrower sense, collaborative filtering is a method of making automatic predictions (filtering) about the interests of a user by collecting preferences or taste information from many users (collaborating).

The most common application of collaborative filtering is user-based k-nearest neighbors algorithm.

Preference

Explicit preference:

• Rating
• Review
• Vote

Implicit preference:

• Click
• Purchase
• Follow

Ratings: star, thumbs / like, etc.

Predictions and Recommendations

• Predictions: Estimates of how much you’ll like an item
  • Often scaled to match some rating scale
  • Often tied to search or browsing for specific products
• Recommendations: suggestions for items you might like (or might fit what you’re doing)
  • Often presented in the form of “top-n lists”
  • Also sometimes just placed in front of you