Mostly, in recommendation system/collaborative filtering, cosine similarity approach is used. But, this metric is the angle between the two vectors and does not consider the magnitude of the values. Users (rating) on other hand, dont conider the angle between the vector. For example: [1 1 10 9] and [10 10 100 90] are more/exactly similar while [1 1 10 9] and [1 2 9 10] are less similar than the previous comparison. Which may lead to incorrectly result.

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