What term defines the measure of the probability and severity of harm?

The risk matrix is the standard risk assessment tool in aviation SMS and other risk management industries. It’s a standard tool because it’s simple and accomplishes all of the needs of a risk assessment, namely:

• Rank safety concerns;
• Understand if an issue’s risk is “tolerable”;
• Ensure assessments are consistent; and
• Ensure assessments are easy and intuitive to perform.

In this article, we will look review:

• The purpose of defining criteria for risk matrices;
• What is severity on a risk matrix and how to define criteria for it; and
• What is likelihood on a risk matrix and how to define criteria for it.

Why Define Criteria for Risk Matrix

Defining criteria for each level of likelihood and severity is what makes your risk matrix so valuable. Doing this ensures that:

• You can easily provide guidance for assessing issues;
• You can justify risk assessments to auditors;
• Risk assessments are performed consistently; and
• Acceptable Level of Safety is well defined and has an identifiable visual cue.

Furthermore, defining criteria allows you to “make your risk matrix your own,” so that you are assessing issues with terms that are unique to your organization’s size and needs.

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What Is Severity on Risk Matrix

Severity on the risk matrix represents the severity of the most likely consequence of a particular hazard occurrence. In other words, if a hazard occurs and is not mitigated, what is the severity of the most likely problem that will occur. As ICAO says of severity, “the severity... of a hazard’s projected consequence.”

For example, in the event of a hazard “Runway incursion,” the most likely problem that will occur is probably either an Accident or Near Miss. If you determine that an Accident is the most likely probable concern, the severity would be “Catastrophic”. If you determine that Near Miss is the most probable concern, then “Serious” would likely be your severity.

So, when you are defining criteria for severity in your risk matrix, keep in mind that you are defining criteria for most likely safety incident following a hazard occurrence.

What Is Likelihood on Risk Matrix

Likelihood on a risk matrix represents the likelihood of the most likely consequence occurring in the event of a hazard occurrence. To put it another way, if a hazard occurs, what are the chances the most likely safety mishap will occur.

To use the example from earlier, in the event of a hazard “Runway incursion,” the most likely problem that will occur is probably an Accident. The likelihood of this occurrence would be fairly low, as there are many control measures mitigating this problem.

So, when you are defining likelihood in your risk matrix, you should create criteria that define the likelihood of the risk occurrence (not the likelihood of hazard occurrence).

Ways to Define Criteria for Severity on Risk Matrix

It’s extremely important that you use multiple different criteria to define severity on your matrix. These criteria will give you multiple modes of justification for each risk assessment’s severity.

Each level of severity should use the same criteria but have in increase in “damages” for each rising level of severity. Here are some ways to define criteria for severity, with some examples below each type of criteria:

1. How much financial damage is associated with each level;
   • Level 1: < $1,000
   • Level 2: $1,000 - $50,000
   • Level 3: $50,000 - $200,000
   • Level 4: $200,000 - $1m
   • Level 5: < $1m
2. How much environmental damage is associated with each level;
   • Level 1: Slight/no effect
   • Level 2: Minor effect
   • Level 3: Localized effect
   • Level 4: Major localized effect
   • Level 5: Multiple environments affected
3. Personnel injury;
   • Level 1: Slight/no injury
   • Level 2: Minor Injury, no hospitalization
   • Level 3: Major injury, hospitalization
   • Level 4: Single fatality
   • Level 5: Multiple fatalities
4. Mission damages.
   • Level 1: No effect
   • Level 2: Minor degradation
   • Level 3: Single significant mission failure element
   • Level 4: Total single mission failure
   • Level 5: Multiple missions affected

You may have additional qualifying element you wish to define for Severity that are relevant to your organization. Also, your criteria for each level of severity should certainly be different than the examples above – for example, $1m may be “Catastrophic” for small and medium sized service providers, but may only be “Moderate” severity for a large operator.

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Ways to Define Criteria for Likelihood on Risk Matrix

When defining likelihood, aviation service providers usually define criteria in one of two ways. These two ways are by either:

1. Quantitative approach: Number of expected occurrences/operations number of occurrences/time period
2. Qualitative approach: Relative chances.

In terms of the quantitative approach, for number of expected occurrences you might define it as:

• A: (high)
  • 1/100 operations
  • 5+ occurrence/year
• B: (medium-high)
  • 1/500 operations
  • 2-4 occurrences/year
• C: (medium)
  • 1/1,000 operations
  • 1 occurrence/year
• D: (medium-low)
  • 1/5,000 operations
  • 1 occurrence/3 years
• E: (low)
  • 1/20,000 operations
  • 1 occurrence/10 years

This can be a good method if you have significant historical data to analyze occurrences per number of operations for specific hazards.

We recommend that most providers use qualitative criteria for defining likelihood, as it’s fairly straightforward and convenient for SMS of all ages and sizes. You might define these criteria as either expected frequency or expected chances of occurrences.

• A: (high)
• B: (medium-high)
• C: (medium)
• D: (medium-low)
• E: (low)

These are just loose examples, and you should define your chances based on the size, complexity, and number of operations in your company.

NOTE: you can set level A your high likelihood or low likelihood, as there is no set direction your matrix needs to go.

Last updated March 2022.

• Occurrence of Risk
• Tolerable Risk
• What is Risk Assessment?

What are accurate definitions of risk and safety? ISO/IEC Guide 51, which is an international standard, gives the following definitions.

What is risk? Risk is defined as the “combination of the probability of occurrence of harm and the severity of that harm.” In other words,
Risk = Severity of harm × the probability of the occurrence of harm

What is safety? Safety is defined as “freedom from which is not tolerable" In other words, tolerable risk is still present even when considered “safe”.

Occurrence of Risk

What does it mean for risk to be present? Examine the following figure.

In the picture on the left, there is a lion, which is a hazard, but this alone is not enough to cause harm to occur. In the picture on the right, a person is present. This introduces the possibility of harm occurring should the lion attack the person. The picture on the right indicates a situation in which “risk” exists. It is necessary to understand that this situation is distinct from the situation in which a “hazard” exists.

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Tolerable Risk

The definition of “safety” is “freedom from risk which is not tolerable” according to ISO/IEC Guide 51. In other words, safety can be achieved by risk reduction to a tolerable level.

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What is Risk Assessment?

Even if harm does not occur, there may exist potential for risk and harm. If these are left alone, it can be said that the situation possesses a high possibility of industrial accidents occurring. As technology has progressed, a great variety of machines and chemical substances have come to be used, which has led to diversification in the potential for risk and harm. Also, to further reduce industrial accidents, it is necessary to implement safety measures such as safety light curtains and safety laser scanners, in a preemptive, not reactive, manner.

What is risk assessment? Risk assessment is a method for confirming safety in order to ensure the safety of workers and other individuals and to reduce to the absolute minimum the possibility of harm. It indicates the following process.

1. (1) Make clear the intended use and usage conditions of the machine and estimate incorrect usage such as operation mistakes.
2. (2) Identify the hazards present in the machine.
3. (3) Estimate the degree of risk and the frequency of situations in which risk is present.
4. (4) Judge whether the degree of risk has been reduced to an acceptable level.

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