1. A system with a central processing unit and a memory for calculating subtree features for subtrees having root documents, a subtree being a hierarchical organization of documents in which documents have ancestor/descendant relationships, comprising: a calculate feature component that calculates a feature for each document within a subtree; and a calculate subtree feature component that calculates a subtree feature for the subtree wherein a contribution of a descendant document of the root document decreases as an ancestral distance between the descendant document and the root document increases and as a number of sibling documents of the descendant document increases; wherein the subtree feature of a subtree represents an aggregate feature for the root document of the subtree, wherein the contribution is decreased as represented by the following: 1  R ⁡ ( p s )  ⁢ ∑ u = 1 h ⁡ ( p s ) - 1 ⁢ [ ∑ p i 1 ∈ R ⁡ ( p s ) ⁢ ∑ p i 2 ∈ R ⁡ ( p i 1 ) ⁢ ⋯ ⁢ ∑ p i u ∈ R ⁡ ( p i u - 1 ) ⁢ f ⁡ ( p i u ) ∏ k = 1 u ⁢  R ⁡ ( p i k )  ] where h(p s ) represents height of a subtree with the root document of p s , R(p s ) represents the child document of p s , ∥a∥ represents the number of elements of a, and f(p iu ) represents the feature of document p iu , and wherein the components are implemented as computer-executable instructions stored in memory for execution by the central processing unit.