1. A method of determining exclusion efficiencies of a plurality of keywords appearing in a search result, the method comprising: for each of the plurality of keywords, using a processor to calculate an exclusion efficiency of the keyword if a search condition is applied to the search result and the keyword is applied as an exclusion keyword to the search result; indicating the exclusion efficiency of each of the plurality of keywords, wherein the processor calculates the exclusion efficiency of a keyword based on a combination of a first parameter and a second parameter, the first parameter is a measure of a correlation between a subset of the search result satisfying the search condition and a subset of the search result excluded when the keyword is used as an exclusion keyword, and the second parameter is a measure of the search result excluded when the keyword is used as an exclusion keyword.