18. The system of claim 16 , wherein the data structure further represents an association of each tier in the plurality of tiers with a respective predetermined range of metric values; and wherein the tag assignment module selects the tier for which the query-independent metric of the new document falls within the respective predetermined range of metric values associated with the selected tier.
18. The system of claim 16 , wherein the data structure further represents an association of each tier in the plurality of tiers with a respective predetermined range of metric values; and wherein the tag assignment module selects the tier for which the query-independent metric of the new document falls within the respective predetermined range of metric values associated with the selected tier. 20. The system of claim 18 , wherein the respective subsets of the set of document identification tags associated with each tier monotonically increases 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.