Probabilistic calculations that are done to verify the integrity of a Safety Instrumented Function design require failure rate and failure mode data of all equipment including the mechanical devices. For many devices, such data is only available in industry databases where only failure rates are presented. The failure mode information is rare, if available at all. Many give up and just say 50% safe and 50% dangerous thinking this is conservative. In some cases this is not a conservative assumption. In other cases it can be an over-kill.