レコメンドシステムで、Explicit と Implicit とは（レコメンド例のSlide）
レビューとかでユーザが明示的に評価したら、Explicit。
PVとかCVとかのlogで評価を判断するのがImplicit。らしい。
There are two ways to gather the data. The first method is to ask for explicit ratings from a user, typically on a concrete rating scale (such as rating a movie from one to five stars). The second is to gather data implicitly as the user is in the domain of the system - that is, to log the actions of a user on the site.
Recommendation Systems :: General Collaborative Filtering Algorithm Ideas
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