4. Quantify the bow tie

The aim of bow-tie quantification is to estimate the frequencies of occurrence of hazardous phenomena at the output of the bow-tie (see figure "Schematic representation of a bow-tie"). As a reminder, a frequency is a value with a unit, usually expressed in years-1 . Once we know the frequency of occurrence of hazardous phenomena, we can assess their acceptability (see step 5, "Exploiting the results").

Quantifying a bow tie involves the following three phases:

• Estimating the frequency of occurrence of the dreaded event: This can be done either "directly", i.e. by referring to quantified accidentology databases, in which case the frequency associated with the dreaded event is derived from a statistical analysis of experience feedback; or by "calculation", i.e. based on knowledge of the frequencies of the basic events...