The same authors  introduce PPCF problem over vertically partitioned data where there are ratings for disjoint set of items from common set of users.
There have been PPCF studies examining how data is distributed among more than two parties.
In another PPCF study, arbitrarily partitioned data is evaluated with other partitioning cases .
In the context of PPCF , the private denotes each rating values and also denotes which items are rated by which user.
It is experimentally shown that while the PPCF algorithms are very robust against a couple of attacks, they are still as vulnerable as traditional CF schemes against other types of attacks.
In Section 2, we review the related work in CF, PPCF, and shilling attacks, and we explain the need for and significance of this study.
CF and PPCF schemes are deployed primarily by e-commerce companies to increase sales by attracting and influencing customers.