Institute of Work Health and Safety / Risk

The matrix is a conversation tool, not a calculator

Multiplying two guesses does not produce a measurement.

The likelihood and consequence grid is the most widely used artefact in the field, and it is regularly asked to do something it cannot. Assigning a rating to likelihood and another to consequence, then reading a colour from a grid, produces a number that looks like a measurement and is a record of a judgement. That is not useless, but it is a different thing, and treating it as arithmetic leads to some odd outcomes.

The most common is precision theatre. A team spends forty minutes arguing whether something is a three or a four, when the difference between those two ratings is smaller than the uncertainty in either. The time would be better spent on the question the matrix exists to prompt, which is what could actually happen here and what would stop it.

The second is the disappearing catastrophic risk. Something with a very severe consequence and a genuinely low likelihood can score lower than a frequent minor injury, and then be deprioritised on that basis. Multiplication treats those as equivalent. Nobody actually believes they are, which is why most serious frameworks handle low-likelihood catastrophic hazards through a separate route rather than the general grid.

Used well, the matrix is a structure for a conversation among people who know the work, and its value is the conversation and the record of who was in it. Used badly, it is a way of converting a discussion into a colour so that the discussion does not have to happen again.

There is a further reason to be careful with the matrix, which concerns how its output is subsequently used. A rating recorded in a register is read later by people who were not present, who have no access to the reasoning, and who will treat the colour as a finding. The judgement that produced it, including the disagreement and the assumptions, is the part with information in it and the part that is discarded. A two-line note on what the group actually worried about, kept alongside the rating, is worth more than any refinement of the scale.