Statistical Considerations on Software-Safety Estimation in Licensing

摘要

During the discussions in preparation of the new versions of the International Electrotechnical Commission (IEC) standards IEC 61508-3 and IEC 61508-7, controversies regarding the proper roles of statistical validation or verification of safety-related software have emerged. These controversies regard changing demand profiles and continuous operation versus on-demand operation. This contribution derives a formula for calculating the failure probability per demand of software that has been tested under a demand profile that is different from the profile of its intended use. It also explains how failure rates can be expressed in terms of failure probabilities per demand, if the operational conditions are known. It further describes how software that is alternately operated continuously and on demand can be characterized in statistical terms and how the two operation modes can be recognized during a statistical evaluation. The notion of "mission" is suggested for sequences of demands or mixtures of demand-driven and continuous operation of software. In order to allow statistical calculations many requirements have to be met strictly. They are listed in the appendix. This article can hopefully facilitate licensing of software in many cases. Remarks are invited.