13. The apparatus of claim 2 , wherein determining the usage score from the frequency of visit value and the unique visit value associated with the document comprises: determining a frequency of visit score based on the frequency of visit value and a maximum frequency of visit value; determining a unique user score based on the unique visit value and a maximum unique visit value; and determining the usage score from the frequency of visit score and the unique user score.
13. The apparatus of claim 2 , wherein determining the usage score from the frequency of visit value and the unique visit value associated with the document comprises: determining a frequency of visit score based on the frequency of visit value and a maximum frequency of visit value; determining a unique user score based on the unique visit value and a maximum unique visit value; and determining the usage score from the frequency of visit score and the unique user score. 25. The apparatus of claim 13 , wherein: determining the frequency of visit score based on the frequency of visit value and the maximum frequency of visit value comprises determining the frequency of visit score based on a log ratio of the frequency of visit value to the maximum frequency of visit value; and determining the unique user score based on the unique visit value and the maximum unique visit value comprises determining the unique user score based on a log ratio of the unique visit value to the maximum unique visit value.