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.

Risk is defined as the probability of an event occurring times the worst possible consequence. The product of these two factors is then plotted on a matrix where one axis (usually the X-axis) represents the probability or the event occurring and the other axis (Y-axis) represents the worst possible consequence should the event occur.

Many organisations use a single matrix for qualitative risk assessment to cover human, environmental and plant risks. In such cases, the consequence axis bears multiple scales or ranges depending on the category being assessed i.e. human, environment or plant. Considerable diversity also exists in the dimensions of the risk matrices used by different organisations. Common risk matrices vary from a simple 3×3 matrix to larger 5×5 or more with various intermediate permutations e.g. 3×4, 4×5 3×5. The consequence and probability scales adopted also demonstrate considerable variation. There is no standard set of definitions in general use. Some risk assessment techniques express probabilities of event occurrence in terms of Low, Medium or High while others use numerical values. Consequence definitions also vary widely. In human terms, the consequences normally range from minor first aid up to permanent disability and fatality while others may extend the consequence range to include a multiple fatality level.

Some risk assessment models (1) are described as three dimensional. These models incorporate the terms exposure, likelihood and consequence. Such a breakdown has the potential to be extremely useful however; exposure and likelihood are normally only related to the probability of the event occurrence. Models incorporating exposure and likelihood are therefore not truly three dimensional. This is not to say that there is not a need for such definitions rather that their combined use does not necessarily render the model 3D. In some instances, exposure is equally applicable to the probability of the consequence. It would be perfectly valid to develop a risk cube where the probabilities of both the event and the consequence were derived using exposure and likelihood in both cases.

A two dimensional matrix has been developed which looks at the probability of harm vs. severity (2) however this model does not take into consideration the probability of the initial event.