A method, system and program product accurately model the error characteristics of a communications system, such as a tape storage system. Input parameters are entered which describe defect rates and sizes, Codeword Data Structure bytes, and any interleaving factor. Bit defects from simulated defect sources are generated, defined by the starting and ending bits of each defect within a codeword. Any codewords which are defect-free are filtered out and not processed further, thereby increasing the processing speed of the model. Within the defect streams, overlapping defects are merged, redefining defect regions by starting and ending bits. Because only the definitions are processed, not the entire length of the codewords or defects, processing efficiency is further enhanced. The number of defects that occur in each codeword is determined and the probability of the occurrence of N bytes in error per processed codeword may be computed. If desired, a histogram may be generated which includes the rate at which errors occurred and subsequently used to estimate the probability of an error event. Such information may then be incorporated into the design of an error correction code for the modeled system.