What did Amazon apply collaborative filtering to starting in the late 1990s?

Answer

Suggesting books based on co-purchase patterns

Amazon is highlighted as a key driver in demonstrating the commercial viability of recommender systems, beginning their major application of collaborative filtering in the late 1990s. Their focus at that time was on leveraging the purchasing behavior of their massive customer base to suggest physical goods, primarily books. This involved using item-to-item CF, which compares items based on how frequently they were bought together by the same customers, proving that these algorithms could effectively influence real-world purchasing decisions for tangible products.

What did Amazon apply collaborative filtering to starting in the late 1990s?
inventionrecommendationsystemrecipe