2022 – November – Youle – Qualitative analysis – the more efficient approach to managing risks SFAIRP

To support the acceptance of safety risk for configuration changes to railways, systems engineering recognises both qualitative and quantitative hazard and risk assessment methods. Quantitative analysis can be perceived as objective and qualitative analysis as subjective. In reality, based on experience of re-signalling and electrification projects that adopt the European Common Safety Method, Qualitative Risk Assessment is preferred over Quantitative Risk Assessment (QRA) to demonstrate that hazard risks will be reduced so far as is reasonably practicable (SFAIRP). This is because QRA otherwise requires:

specialist resources and tools that are relatively costly and time consuming;
difficult to source valid data for the system boundaries and lifetime, geographical boundaries and interfaces, and route operating model;
qualitative estimates and assumptions, or omissions, when there are gaps in the above source data.

This paper explains, using worked examples, why qualitative analysis is a more efficient approach to demonstrating risks are managed SFAIRP for most operational railway hazards.