8. A computer-readable medium encoded with instructions for controlling a computing device to assess keyword usage, by a method comprising: providing a search log for each of a plurality of periods, a search log of a period indicating search requests submitted to a search engine service during that period, each search request including keywords; calculating a frequency impulse score for a keyword, the frequency impulse score indicating a change in frequency of the keyword occurring in search requests of a search log for a recent period, the frequency impulse score being based on a difference between a ratio of a frequency of the keyword occurring in search requests of the search log for the recent period to a total frequency for multiple keywords occurring in search requests of the search log for the recent period and a ratio of a combination of frequencies of the keyword occurring in search requests of search logs for less recent periods to total frequencies for multiple keywords occurring in search requests of search logs for less recent periods, wherein the ratio of the combination applies a decay factor based on recency of the search log such that the ratio for more recent search logs is weighted more heavily than the ratio for less recent search logs; calculating a frequency weight for the keyword, the frequency weight indicating a measure of the frequency of the keyword occurring in search requests of the search log for the recent period; and multiplying the frequency impulse score and frequency weight of the keyword to give a recent usage score of the keyword.
8. A computer-readable medium encoded with instructions for controlling a computing device to assess keyword usage, by a method comprising: providing a search log for each of a plurality of periods, a search log of a period indicating search requests submitted to a search engine service during that period, each search request including keywords; calculating a frequency impulse score for a keyword, the frequency impulse score indicating a change in frequency of the keyword occurring in search requests of a search log for a recent period, the frequency impulse score being based on a difference between a ratio of a frequency of the keyword occurring in search requests of the search log for the recent period to a total frequency for multiple keywords occurring in search requests of the search log for the recent period and a ratio of a combination of frequencies of the keyword occurring in search requests of search logs for less recent periods to total frequencies for multiple keywords occurring in search requests of search logs for less recent periods, wherein the ratio of the combination applies a decay factor based on recency of the search log such that the ratio for more recent search logs is weighted more heavily than the ratio for less recent search logs; calculating a frequency weight for the keyword, the frequency weight indicating a measure of the frequency of the keyword occurring in search requests of the search log for the recent period; and multiplying the frequency impulse score and frequency weight of the keyword to give a recent usage score of the keyword. 10. The computer-readable medium of claim 8 wherein the frequency weight is based on a logarithm of the frequency of the keyword occurring in search requests of the search log for the recent period and a logarithm of the frequency of the keyword occurring in search requests of search logs for less recent periods by applying a decay factor based on recency.