5 × 5 risk matrix
Likelihood × severity scoring
The common risk scoring grid: likelihood from 1 (very unlikely) to 5 (almost certain) multiplied by severity from 1 (minor) to 5 (fatal or multiple), giving a score from 1 to 25, banded low, medium, high and very high.
Most UK construction risk assessments score risk on a five-by-five matrix. Likelihood runs from 1 (very unlikely, would need several failures) to 5 (almost certain to happen if nothing is done); severity from 1 (minor injury, first aid) to 5 (fatality or multiple serious injuries). The product gives a score from 1 to 25, banded into low (1 to 4: acceptable, monitor), medium (5 to 9: act to reduce where reasonably practicable), high (10 to 15: reduce before work proceeds) and very high (16 to 25: stop; do not proceed without senior sign-off and additional controls). The assessment scores each hazard before controls (the inherent risk) and after (the residual risk), and the residual is what you are accepting.
The matrix is a communication tool, not a measurement: two competent people will score the same hazard differently and both be defensible. Its value is in forcing the question "how does this control change the likelihood or the severity?" and in flagging the hazards where the residual score is still high, which are the ones that need a permit, a briefing or a decision from someone senior. Beware of assessments where every residual score is 4 or below; either the controls are superb or the scoring is optimistic.
Where it comes from
- No statute prescribes a matrix; the Management Regulations require a suitable and sufficient assessment and HSE guidance (INDG163) accepts any proportionate method.
Questions people ask
What score means we cannot do the work?
Your own procedure sets it; 16 and above (very high) is the usual stop line, and 10 to 15 (high) usually means additional controls and a permit before proceeding. The number triggers a decision by a named person; it does not make the decision.
Should the residual score always be low?
No. Some work is high risk after every reasonable control (deep excavation, complex lifts, confined space entry). An honest residual of 10 with a permit and a supervisor is better than a dishonest 4.
5 × 5 risk matrix, handled on a phone
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