Shortcomings

One of the biggest prerequisites of a collaborative filtering system is the availability of data of past activity. Amazon is able to leverage collaborative filters so well because it has access to data concerning millions of purchases from millions of users.

Therefore, collaborative filters suffer from what we call the cold start problem. Imagine you have started an e-commerce website – to build a good collaborative filtering system, you need data on a large number of purchases from a large number of users. However, you don't have either, and it's therefore difficult to build such a system from the start.