A dual adaptive importance-sampling system and method is provided that can estimate the probability of events by combining dual complementary importance-sampling simulations. The present invention exploits the ability to determine an optimal biased pdf using an iterative procedure that requires relatively little a priori knowledge of how to bias. Hence, the present invention is particularly suited for evaluating the BERs and/or WERs of coded communication and storage systems, and is generally applicable to arbitrarily chosen codes. When applied to coded communication and storage systems, the present invention provides a versatile technique for the fast and accurate estimation of BERs and WERs of FEC codes down to values of 10−20 or lower.