candidate training point xa with each possible rating y ∈ Y is added to the training
set T , then an approximation of the user’s preferences f is obtained and its accuracy
is evaluated (i.e. cross-validated) on the training items X (Train). It is assumed that
when the candidate training item is paired with its correct rating, the cross-validated
accuracy will improve the most. The usefulness of the candidate training point is
measured by the improvement in the cross-validated accuracy as following: