1. A method comprising: receiving a speech signal produced by a user; forming adapted means for each of a plurality of mixture components by adapting a background model comprising background means for each of the plurality of mixture components based on the received speech signal; receiving a nominal user identification; determining a similarity score between the received speech signal and a training speech signal provided by the nominal user by determining the sum of functions determined for the plurality of mixture components, wherein each function comprises the product of a posterior probability of a mixture component based on the received speech signal and a difference between an adapted mean and a background mean; and using the similarity score to determine if the received speech signal was produced by the nominal user.