Abstract: A method 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. One or more sets of parameters of a first non-gaussian probability density function are calculated so as to approximate a suitable fit to the amplitude distribution of the received signal, including the effects of deterministic processing in the receiver but excluding the effects of statistical noise fluctuations introduced by the receiver.