31. The computer program product of claim 30 , 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 further includes instructions for selecting 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.
32. The computer program product of claim 31 , wherein the predetermined ranges of metric values associated with the plurality of tiers, represented by the data structure, are non-overlapping.