Explicit feedback has the advantage of simplicity, albeit the adoption of nu-
meric/symbolic scales increases the cognitive load on the user, and may not be
adequate for catching user’s feeling about items. Implicit feedback methods are
based on assigning a relevance score to specific user actions on an item, such as
saving, discarding, printing, bookmarking, etc. The main advantage is that they do
not require a direct user involvement, even though biasing is likely to occur, e.g.
interruption of phone calls while reading.