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March/April 2023 issue

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R is for risk matrix

Open-access content Thursday 15th June 2017
From the archive:  Just so you know, this article is more than 3 years old.

leixcon-r-is-for-risk-matrix

Since in most cases it is not possible to quantify either the likelihood or the severity with such accuracy, we make relative judgements -- a broken leg is worse than a cut, and an accident that has never happened seems less likely than one we've seen several times.

A risk matrix puts these judgements into a table, where one dimension represents categories of severity, and the other categories of likelihood. Typically, numbers are then applied to each category, and multiplied together to give a score for the combination of likelihood and severity. The scores are categorised and labelled.

The illustration here shows a typical 3 x 3 risk matrix where a score of 1 is considered trivial or insignificant, a score of 9 as intolerable, and anything in between is tolerable provided it is managed to as low a risk as reasonably practicable. For a 5 x 5 matrix, a score above 12 might be regarded as intolerable, and a score below 4 trivial.

Can anyone in your organisation explain why you use the matrix you use? Is it appropriate for everything you need to risk assess?

The Health and Safety Executive offers a sample risk matrix (bit.ly/2qNgr40) using the categories of likelihood and severity shown in the illustration.

Any organisation that adopts this model should define these categories to match risk profile and risk appetite, and the definitions must be capable of distinguishing between the different hazards an organisation faces -- if they all end up in one or two boxes, the matrix no longer helps with prioritisation of risk control. For example, a small children's nursery might define anything involving a visit to hospital as extreme harm; if the same definition were used for European motor racing, there would be no useful way of distinguishing between a broken leg, a single fatality and a multiple fatality.

One curiosity of the risk matrix is the dependence people place on numbers. Although we can say the score of 4 means something has less risk than the 9 and more than the 1, in reality we know nothing about the relationship between the risks in a diagonal band. It is not possible to prove that a risk of 2 x 2 is higher than a risk of 3 x 1. We don't even know that 3 x 1 is the same as 1 x 3. Think about shoe sizes. A size 7 is larger than a size 3, but if you had a size 7 and a size 3 end to end you can't know whether this would be longer or shorter than a size 6 and a size 4 end to end. You can't know that 7+3 = 6+4 if the numbers represent categories rather than interval numbers.

Any organisation that adopts this model should define these categories to match risk profile and risk appetite

To avoid confusion, ditch the numbers and replace "mostly harmful", "unlikely" and so on with descriptions that match your organisation's risk profile, and simply use the coloured areas to categorise the risk bands.

Then test your risk matrix -- ask colleagues to independently place the same hazards on the matrix and see if they come up with the same answers. Are you all happy with the prioritisation that the matrix gives you? Are you really equally as tolerant of the high likelihood of a harmful event as you are of the medium likelihood of an extremely harmful event? You might extend the red or the green area, and you can create a less symmetrical pattern to match your risk appetite.

Risk matrices can be a useful tool for determining acceptability and priorities for risk management -- but we must test the matrices we are using to make sure they are telling us what we think they are telling us. Poorly understood severity and likelihood categories and arbitrary risk bands will lead us to draw the wrong conclusions.

You may also be interested in...

 Q is for quantitative risk assessment

Tuesday 16th May 2017
But this is not a quantitative risk assessment (QRA). It is not even “semi-quantitative”. It is a qualitative risk assessment which uses numbers to prioritise and aid decision-making. The same result could be achieved with a coloured grid (refer to the risk assessment matrix in this article).
Open-access content

 S is for Safety II

Wednesday 19th July 2017
If Safety I is defined to be “as few things as possible go wrong”, Safety II’s definition can be “as many things as possible go right (under varying conditions).” OSH expert Professor Erik Hollnagel says that, by focusing on understanding accidents, Safety I ignores the many opportunities there are to learn from what goes right most of the time, and how people respond to abnormal conditions to prevent problems escalating (bit.ly/2ttlb3S).
Open-access content

 T is for triviality

Thursday 10th August 2017
In defining “significant” the Health and Safety Executive (HSE) uses the word “trivial” to define its opposite: “Significant risks are those that are not trivial in nature” (www.hse.gov.uk/risk/faq.htm).The word trivial does not appear in the legislation. However, it does occur in court judgements.
Open-access content

 P is for perception of risk

Wednesday 5th April 2017
Much of the research in perception of risk is on how the general public perceives threats such as nuclear power, vaccinations, transportation modes or smoking. However, awareness of these studies’ findings is useful when we consider how to communicate risk messages to workers, and where organisations’ activities affect the public.
Open-access content

 U is for unsafe acts and conditions

Monday 18th September 2017
Heinrich’s precursors of unsafe acts and conditions – social environment and ancestry, and fault of the person – have been challenged by some as being overly class-conscious, and even racist. Similarly, ideas about causality have become more sophisticated, making the domino theory seem too simplistic. In Heinrich’s own words, “each accident was assigned either to the unsafe act of a person or to an unsafe mechanical condition, but in no case were both personal and mechanical causes charged”.
Open-access content

 O is for occupational safety & health management standards

Thursday 9th March 2017
Adam Smith fuelled the industrial revolution with his suggestion that, by standardising tasks in industry and giving each worker just one simple job to do, cheaper labour could replace skilled workers on a production line. Known as the division of labour, the idea was reinforced in the 1920s with Frederick Taylor’s scientific management, and was applied by Henry Ford to his car production lines. However, the same ideas led to overexposure to noise, vibration, hazardous substances, poor ergonomics and to repetitive use injuries.
Open-access content

Latest from Lexicon

Lexicon: Z

 Z is for Zeigarnik

Friday 24th April 2020
We round off the alphabet by considering how to use the Russian psychologist's work in OSH
Open-access content
Y is for Yerkes-Dodson

 Y is for Yerkes-Dodson

Thursday 2nd April 2020
The strength of the stimulus as the experimental variable, with performance as the result.
Open-access content
x

 X is for gens X, Y and Z

Wednesday 4th March 2020
While it is no longer acceptable to assume that all men are stronger than all women, or that people of one colour have different personalities to those of another colour it is, it appears, entirely acceptable to declare that anyone born since 1980 is addicted to social media and will ‘challenge traditional hierarchical HSE systems’, while anyone born before that date is a luddite with no understanding of the modern age, but will be quite happy to toe the line.
Open-access content
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