In order to assess the safety of monitoring and control systems, statements about the probability and frequency of undesired events are required. Normally, such assessment is carried out qualitatively and can be supplemented by quantitative statements.
VDI/VDE 3542 Blatt 2-2000 history
2000VDI/VDE 3542 Blatt 2-2000 Safety terms for automation systems - Quantitative terms and definitions
1998VDI/VDE 3542 Blatt 2-1998 Safety terms for automation systems - Quantitative terms and definitions