# k-NN Collaborative Filtering: User-User vs Item-Item — When to Use Which?

> Source: https://sukruyusufkaya.com/en/learn/oneri-sistemleri/knn-cf-user-user-vs-item-item
> Updated: 2026-05-13T13:29:35.026Z
> Category: Öneri Sistemleri
> Module: Module 5: Memory-Based Collaborative Filtering (k-NN)
**TLDR:** Recommender's birth paper, GroupLens 1994. After 30 years, still the baseline of every recommender system. This lesson covers philosophical differences between user-user and item-item CF, mathematical formulation of each, and when each wins.