There is disclosed a method and computer program product for estimating a measure of the quality of the received signal in a computer simulation of an optical transmission system, wherein the simulation includes additive optical noise generated by components within the transmission system. The method and product are able to account for non-Gaussian statistics of noise fluctuations observed in the receivers of optical communications systems, in order to provide for the more accurate simulation of system performance than may be the case using prior art methods based on Gaussian approximation.