A safety management technique used to measure the performance of a process or an activity in terms of risk reduction which is numerically rated with the values of 1 to 4. SIL 1 presents a high probability of failure while SIL 4 presents a low probability of failure. The SIL is determined using methodologies such as layers of protection analysis (LOPA) and risk matrices. SILs are useful for allowing risk of failure to be appreciated without having to understand fully all of the technical details.