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Pearson Correlation
In collaborative filtering, correlation is often used to predict a
feature from a highly similar mentor group of objects whose features
are known. The [0,1]normalized Pearson correlation is defined as

(4.3) 
where denotes the average feature value of
over
all dimensions. Note that this definition of Pearson correlation tends
to give a full matrix. Other important correlations have been
proposed, such as Spearman correlation [Spe06] which works well
on rank orders.
Alexander Strehl
20020503