Implementations of this interface define a notion of similarity between two items. Implementations should return values in the range -1.0 to 1.0, with 1.0 representing perfect similarity.

Implementations of this interface compute an inferred preference for a user and an item that the user has not expressed any preference for. This might be an average of other preferences scores from that user, for example. This technique is sometimes called "default voting".

Implementations of this interface define a notion of similarity between two users. Implementations should return values in the range -1.0 to 1.0, with 1.0 representing perfect similarity.