Layers of Protection Analysis (LOPA) is a simplified form of numerical risk assessment. It is an order of magnitude approach and hence precise figures are not used. The technique does have significant limitations compared to more advanced techniques such as Fault Tree Analysis, QRA, etc.

This paper highlights some of the mistakes that are seen in its application and challenges some of the practices that are occurring within LOPA calculations – in particular with the use of conditional modifiers related to exposure times which causes an underestimation of the risk.