In which year was the first commercial product recommendation engine implemented by a major online retailer?

Product recommendation engines revolutionized online retail by personalizing the shopping experience. These sophisticated algorithms analyze customer behavior, purchase history, and browsing patterns to suggest products customers might like. This technological innovation has not only transformed how we shop but also significantly boosted conversion rates for retailers worldwide. Test your knowledge about when this game-changing retail technology first emerged commercially!

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1995 (Amazon's item-to-item collaborative filtering)
2001 (Netflix's Cinematch system)
1999 (eBay's related items feature)
2005 (Spotify's music recommendation algorithm)