Recommender Systems — How User Activity Creates Phantom Item Similarity

Phantom correlation (Favier et al., University of Antwerp): user activity alone makes items look similar. On MovieLens-1M, an activity-only model removes 81% of the independence model's co-occurrence mismatch.
Key takeaways:

• The toy example. 100 equally liked films; half the users watch each film with probability 0.01, half with 0.20, all independently. Lift still says every pair co-occurs about 1.82 times more often than chance.
• The model. Three assumptions (independent consumption, more activity raises every item, activity is the only difference) yield exactly one model, the Rasch…