5. Limits to bow-tie quantification

5.1 Input data inaccuracies

When quantifying a bowtie, it is important to bear in mind that the probabilities of hazardous phenomena estimated by the calculation should be considered as orders of magnitude, not exact values. For this reason, the values obtained by quantifying a bowtie are often rounded off to take into account the error linked to uncertainties in the input data. For example, announcing a probability of occurrence equal to 7.56 × 10 ^–5 would not make much sense in view of the relative uncertainties involved:

• the frequency allocated to the feared event ;

• failure probabilities assigned to mitigation barriers ;

• ...