In this case, similarity between two items *i* and *j* is measured by computing
the *Pearson-r* correlation
*corr*_{i,j}. To make the correlation
computation accurate we must first isolate the co-rated cases (i.e.,
cases where the users rated both *i* and *j*) as shown in
Figure 2. Let the set of users who both rated *i* and *j* are
denoted by *U* then the correlation similarity is given by

Here *R*_{u,i} denotes the rating of user *u* on item *i*,
is the average rating of the *i*-th item.