Cross-sell [73] uses pairwise conditional probabilities with the
na¨ıve Bayes assumption to do recommendation in unary e-commerce
domains. Based on user purchase histories, the algorithm estimates
P(a|b) (the probability that a user purchases a given that they have
purchased b) for each pair of items a, b. The user’s currently-viewed item
or shopping basket is combined with these pairwise probabilities to rec-
ommend items optimizing the expected value of site-defined objective
functions (such as profit or sales lift).