System safety requirements for the two most recent USAF aircraft systems have consisted of an accident probability, a confidence level, and the test period for demonstrating the requirement. For each set of values for these parameters, there is a maximum number of accidents that can occur during the demonstration period. It is this maximum number of accidents that is of importance in the demonstration period. The task of determining the maximum number of accidents is not as simple and straightforward as some have tried to make it. The significance of the three parameters is explained and the procedure is given for using statistical sampling techniques to arrive at the equivalent accident rate.