20. The system of claim 19 , wherein the best facet drill-down model comprises a model according to the function: U q M B ⁡ ( F q ) = E ⁡ [ X | q ] - E M B ⁡ [ X | q , F q ] = ∑ d ∈ q : r q ⁡ ( d ) > m ⁢ p ⁡ ( d - d q ) ⁢ max f ∈ F q , d ⁢ ⁢  D q , d , f  .
19. The system of claim 16 , wherein the user drill-down model comprises a best facet drill-down model.