Spearman rank correlation. The Spearman rank correlation coeffi-
cient is another candidate for a similarity function [58]. For the
Spearman correlation, the items a user has rated are ranked
such that their highest-rated item is at rank 1 and lowerrated
items have higher ranks. Items with the same rating are
assigned the average rank for their position. The computation
is then the same as that of the Pearson correlation, except that
ranks are used in place of ratings