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. 19. The system of claim 18 , wherein the predetermined ranges of metric values associated with the plurality of tiers, represented by the data structure, are non-overlapping.