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Sample of a 7 + 7 Risk Assessment Matrix:
Risk Assessment 7 + 7 Risk Matrix
Frequency
In health and Safety today the most popular risk assessment methods' multiple the likelihood by the consequences, examples of this being the 3x3 and 5x5 matrixes. There is, however, another risk rating method that adds these values when calculating risk rating and helps in overcoming some of the inconsistencies and inaccuracies that can be produced using these multiplication methods.
The 7+7 method can be used where data is available or a good degree of judgement can be applied to estimates of the frequency and consequences of each hazardous event. This semi-quantitative risk ranking approach allows for a greater level of accuracy and consistency in the risk estimates that can be obtained.
Note: The size of the matrix and the factor difference in frequency and consequence rankings can be altered to give the best ranges to suit a particular organisation's operation.
|
Description |
Frequency Range |
Mid-point |
Approx. numerical |
Ranking |
|
Remote |
<1 in 175 years |
1 in 500 years |
0.002 |
1 |
|
Rare |
1 in 35 years to 1 in 175 years |
1 in 100 years |
0.01 |
2 |
|
Infrequent |
1 in 7 years to 1 in 35 years |
1 in 20 years |
0.05 |
3 |
|
Occasional |
1 in 1 ¼ years to 1 in 7 years |
1 in 4 years |
0.25 |
4 |
|
Frequent |
1 in 3 months to 1 in 1 ¼ years |
1 in 9 months |
1.25 |
5 |
|
Regular |
1 in 20 days to 1 in 3 months |
1 in 2 months |
6.25 |
6 |
|
Common |
1 in 4 days to 1 in 20 days |
1 in 12 days |
31.25 |
7 |
Consequence
|
Description |
Approx. Numerical value |
Ranking |
|
Minor Injury |
0.005 |
1 |
|
More serious injury / multiple minor injuries |
0.025 |
2 |
|
Major injury |
0.125 |
3 |
|
Multiple Major / single fatality |
0.625 |
4 |
|
Multiple fatalies (2 to 5) |
3.125 |
5 |
|
Multiple fatalies (6 to 25) |
15 |
6 |
|
Multiple fatalies (>25) |
75 |
7 |
Risk Rating = Frequency (Likelihood) ranking + Consequence ranking
It is very important to note however, that adding the frequency and consequence rankings only works if the changes in both the frequency and consequence estimates (as represented by the changes in their corresponding ranking numbers) are separated by the same factor.
This solution works for any factor difference (two, five, ten, one hundred, etc) providing both the frequency and consequence ranking estimates are separated by the same factor.
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11 |
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10 |
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Consequence
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Events with the potential for significantly different outcomes
When assigning frequency and consequence rankings to hazardous events the rankings are based normally on the average frequency of occurrence and the average consequences for the event. For some hazardous events, however, different outcomes can lead to significantly different consequences. For example, a train derailment would typically only lead to minor injuries, due perhaps to passengers falling over inside the train, whereas in extreme cases, derailments can lead to multiple fatalities. It is recommended that in such cases, to get a better understanding of the risk profile, particularly in relation to potential multi-fatality outcomes, two separate rankings should be considered for the hazardous event as follows:
a) the first ranking should relate to the frequency and consequences of the typical (most frequent outcome),
and
b) the second risk ranking should relate to the frequency and consequences of the realistic worst case outcome, if appropriate.