4. The method of claim 1 , wherein the step of maximizing a likelihood includes the use of a specific function that processes data describing parent-child relationships between the documents.
4. The method of claim 1 , wherein the step of maximizing a likelihood includes the use of a specific function that processes data describing parent-child relationships between the documents. 5. The method of claim 4 wherein the specific function utilizes: g(y i ,y j ,x)=R i,j (y i −y j ), 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.