There are fundamental epistemic problems with the safety risk assessment of new and high consequence systems. For new systems we generally do not have statistical data on accidents and high consequence events are (usually) quite rare so we end up arguing our case using low base rate data. In the final analysis we usually fall back on some form of subjective (and qualitative) risk assessment.
The risk matrix was developed to guide this type of risk assessments, it’s actually based on decision theory, De’Moivres definition of risk and the principles of the iso-risk contour. The matrix is widely described in safety and risk literature and has become one of the less questioned staples of risk and safety management.
Despite this there are plenty of poorly constructed, ill thought matrices out there in both the literature and standards, this article attempts to establish some basic principles of construction.
