Relative Safety: Absolute vs. Acceptable and ALARP

How some definitions of acceptability of safety affects the procurement, analysis, development and operation of safety-critical systems

The term "safe system" may be a reference to either absolute safety or to acceptable safety. As soon as a system is determined to be safety-critical, absolute safety may become an impossible target.

Regardless of whether an explicit value is set on human life, acceptable or relative levels of safety generally imply that there is some way of judging how much harm either justifies more effort or demands a different approach.

The principle of As Low As Reasonably Practicable (ALARP) embodies the concept that acceptability can be influenced by the cost of improving safety. It does so by categorising the assessment of a risk into one of three possibilities: those with so great a risk they are never acceptable, those with so little a risk they are always acceptable, and the remaining which are acceptable provided that they have been reduced to as low as reasonably practicable and where the cost of reducing the risk further is not justified by the improvement to safety.

Not all safety standards specify an acceptable level of safety, focussing more on how to ensure and assure that an acceptable level of safety has been achieved.


Acceptable levels of safety can be based on factors including: legal requirements, the regulatory environment, the current level of safety, contractual requirements, and corporate safety targets.


The Ford Pinto fires were a case where different views of the acceptable safety came to light.