A computer-readable medium including computer-executable instructions that, when executed by a processor, cause the processor to perform acts, via an associated method that includes selecting a fault tree based upon an architecture of a safety instrumented system. The method includes evaluating at least a failure probability due to dangerous detected failures and a failure probability due to dangerous undetected failures as a function of values of factors. A portion of the failure probability due to dangerous undetected failures is based on failures detected during proof testing and a remainder of the failure probability due to dangerous undetected failures is based on failures detected during refurbishment. The method includes generating a probability of failure on demand for the safety instrumented system by combining at least the failure probability due to dangerous detected failures and the failure probability due to dangerous undetected failures according to the fault tree.