1. A method in a computing device with a processor and a memory for assessing keyword usage, the method comprising: providing frequency of keywords during various periods; calculating by the computing device a frequency impulse score for a keyword, the frequency impulse score indicating amount of recent change in frequency of the keyword according to the following equation: I n = ( f n - ∑ i = 1 n - 1 ⁢ f i * α i ) / f n where I n represents the frequency impulse score for period n, f n represents the frequency of the keyword during the period n divided by the total frequency of all keywords during the period n, and α i represents a decay factor during period i according to the following equation: 
 α i =2 i-n ; calculating by the computing device a frequency weight for the keyword, the frequency weight indicating a recent measure of the frequency of the keyword according to the following equation: W n = log ( ∑ i = 1 n ⁢ freq i * α i ) where W n represents the frequency weight for period n and freq i represents the frequency during period i; and combining the frequency impulse score and frequency weight of the keyword to give the recent usage score for the keyword for a recent period according to the following equation: 
 U n =I n *W n where U n represents the recent usage score for period n.