Abstract: A method for determining a significant change of an usage of expressions provided in a network system, including the steps of determining a reference data set including at least an expression frequency of expressions provided in the network system at a predetermined first time; determining a result data set including at least an indication of an expression frequency change based on the reference data set, wherein the expression frequency change indicates the change of the expression frequency of expressions indicated by the reference data set at a predetermined second time; and extracting, from the result data set, one or more expressions according to one or more predetermined filters to determine the change of the usage of expression in the network system.