where L is a loss function such as MAE or MSE (Section 23.4), and f (x) is the
actual rating of the item x, and fT ∪(xa,y)(x) is the approximated rating (where a
function f is learned from the training set T ∪ (xa, y)) .
A potential drawback is that training points selected by this AL method could be
overfitted to the training set.