Conventional two dimensional qualitative risk assessment defines risk as the probability of an event multiplied by the consequence resulting from the event where the consequence is loosely defined as discrete components ranging from no consequence or minor consequence through to a catastrophe. In human terms, this is usually either a fatality or permanent disability.
Typically, this type of risk matrix is split into regions indicating, for example, "unacceptable risk", "acceptable risk" and "investigation required" categories. In the case of events involving risk to human beings, this type of approach is insufficient as the potential outcome is always in the most severe consequence category i.e. fatality or permanent disability. Risk assessment for activities involving human beings is therefore reduced to a one dimensional exercise focusing solely on the probability of the event.
This paper describes a three dimensional approach to qualitative risk assessment which permits acknowledgement of the possibility for a catastrophic outcome and the same time provides a mechanism for addressing measures to reduce such a possibility to a minimum.
By introducing a 3rd axis - "probability of consequence", the risk matrix becomes a risk cube. Each layer of the cube represents a level of consequence with the other two axes representing the probability of the event and the probability of the consequence occurring.