The earliest recommenders used nearest neighbor collaborative filtering algorithms (Resnick et al. 1994, Shardanand et al. 1995).Nearest neighbor algorithms are based on computing the distance between consumers based on their preference history. Predictions of how much a consumer will like a product are computed by taking the weighted average of the opinions of a set of nearest neighbors for that product. Neighbors who have expressed no opinion on the product in question are ignored. Opinions should be scaled to adjust for differences in ratings tendencies between users (Herlocker et al., 1999). Nearest neighbor algorithms have the advantage of being able to rapidly incorporate the most up to date information, but the search for neighbors is slow in large databases. Practical algorithms use heuristics to search for good neighbors and may use opportunistic sampling when faced with very large populations.