Page "Collaborative filtering" Paragraph 18

Another form of collaborative filtering can be based on implicit observations of normal user behavior ( as opposed to the artificial behavior imposed by a rating task ).

In these systems you observe what a user has done together with what all users have done ( what music they have listened to, what items they have bought ) and use that data to predict the user's behavior in the future or to predict how a user might like to behave if only they were given a chance.

These predictions then have to be filtered through business logic to determine how these predictions might affect what a business system ought to do.

It is, for instance, not useful to offer to sell somebody some music if they already have demonstrated that they own that music or, considering another example, it is not useful to suggest more travel guides for Paris to someone who already bought a travel guide for this city.