In terms of complexity and rigour LOPA lies between qualitative techniques such as hazard and operability studies (HAZOP) and quantitative techniques such as fault trees and event trees.
The effectiveness of an IPL is quantified by its probability of failure on demand (PFD), in the range 0 to 1.
[2] An IPL must be independent of the other protective layers and its functionality must be capable of validation.
[3] LOPA was developed in the 1990s in the chemical process industry but has found wider application.
When this occurs in the context of the analysis of process plants, LOPA generally leverages the results of a preceding HAZOP.