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If we were going to use collaborative filtering to group customers and recommend films, we needed to know what customers enjoyed rather than just what they rented. We needed a reviews system: a movie rating system. Grouping customers by ratings—by “clustering” users according to overlapping positive or negative reviews—meant that we could efficiently recommend films to users based not on what they’d rented but what they liked. Ultimately, the algorithm would become much more complex than that. But for it to work at all, we needed users to review movies—lots of them.
