28. The non-transitory computer readable storage medium of claim 26 , wherein the respective subset of the document identification tags associated with each tier monotonically increase with the position of the tier in the ordering; and wherein the respective predetermined range of metric values associated with each tier monotonically increases with the position of the tier in the ordering.
26. The non-transitory computer readable storage medium of claim 25 , wherein each tier in the plurality of tiers is associated with a respective predetermined range of metric values; and wherein instructions for selecting a tier comprise instructions for selecting the tier for which the query-independent information of the new document falls within the respective predetermined range of metric values associated with the selected tier.