The method of calculating the average probability of failure. To calculate and check – does Safety Instrumented System (SIS) meets the requirements of the SIL level. SIF – it is a set of equipment (safety loop) intended to reduce the risk of failure or hazard. Each SIF have to be checked separately. In the functional safety standards based on the IEC 61508 standard, four SILs are defined, with SIL 4 the most dependable and SIL 1 the least.