Which recommendation technique did Spotify NOT use in its early algorithm development that later became crucial to its personalization strategy?

Music streaming platforms have revolutionized how we discover and consume music. Behind these platforms are sophisticated recommendation systems that analyze your listening habits and suggest new artists and songs. These algorithms have evolved dramatically over the years, becoming increasingly personalized and influential in shaping music consumption patterns worldwide. Test your knowledge about how these music recommendation systems actually work!

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Collaborative filtering based on similar users' preferences
Audio signal analysis to identify musical attributes
Deep neural networks for contextual recommendations
Matrix factorization for implicit feedback processing