What is item-based collaborative filtering and how does it differ from user-based?

Item-based collaborative filtering is a recommendation system technique that predicts a user’s preferences by analyzing similarities between items. Instead of focusing on user relationships, it identifies items that are frequently liked or interacted with together. For example, if users who buy a sci-fi novel often buy another specific sci-fi book, those two items are deemed similar. The system then recommends items similar to those a user has already shown interest in. This approach relies on precomputing item-item similarity scores, often using metrics like cosine similarity or Pearson correlation, which measure how closely two items align in terms of user interactions. Once similarities are established, recommendations are generated by aggregating scores from items the target user has already engaged with.

User-based collaborative filtering, in contrast, predicts preferences by finding users with similar behavior to the target user. For instance, if User A and User B have historically liked the same movies, the system might recommend movies User B liked that User A hasn’t seen yet. This method calculates user-user similarity scores, often using the same metrics as item-based approaches, and generates recommendations based on the preferences of the most similar users. A key challenge is scalability: user similarity calculations become computationally expensive as the number of users grows, since comparing every user pair becomes impractical for large datasets.

The main differences lie in focus, scalability, and performance. Item-based filtering centers on item relationships, making it more stable over time because item similarities (e.g., books in a genre) change less frequently than user preferences. This stability allows item-based systems to precompute similarities offline, improving real-time recommendation efficiency. User-based systems, however, require frequent updates to user similarity matrices as preferences evolve, which can be resource-intensive. Additionally, item-based approaches often handle sparse datasets better—if users interact with few items, finding reliable user-user similarities is harder, whereas item-item patterns can still emerge from limited data. For example, Netflix might use item-based filtering to suggest shows similar to ones you’ve watched, while a social media app might use user-based filtering to recommend posts popular among users with overlapping interests. The choice between the two depends on factors like dataset size, update frequency, and the need for real-time performance.