18. The non-transitory computer-readable storage media of claim 17 , wherein the specific function utilizes: 
 g ( y i ,y j ,x )= R i,j ( y i −y i ), where R i,j denotes the parent-child relation: R i,j =1 if document i is the parent of j, and R i,j =0 for other cases.
17. The non-transitory computer-readable storage media of claim 14 , wherein the process of maximizing a likelihood includes the use of a specific function that processes data describing parent-child relationships between the documents.