qualitative risk assessment - city university of hong kong

Introduction of qualitative risk assessment

Dr. Aaron Yang BVSc, MVS (Hons), PhD (Dist)

20/11/2020

Terminology

• Hazard: something that potentially harms you, it is an event.e.g.1. Attacking by a shark when swimming in the sea 2. Hitting by a motorcycle when crossing the street

• Risk: the probability of an event/hazard, usually a numeric value 0 ≤𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 ≤ 1

• Qualitative risk: probabilities which are NOT expressed in a numeric manner

Example of qualitative risk

Risk category Description

Negligible Extremely rare so that it does not merit to be considered

Very low Very rare but cannot be excluded

Low Uncommon but occurs

Medium Occurs regularly

High Occurs very often

Very high Occurs almost certainly

Advantage of QRA

• A rapid approach

• A proven useful approach to help make policy decisions

• Not requiring specific knowledge to handle complicated probabilities

Steps of performing QRA

1. Decide the scope of the risk assessment

2. Form a rigorous risk question(s)

3. Create the risk pathway

4. Estimate the risk

Scope of the RA

• RA component based on OIE animal health code

I. Entry assessmentII. Exposure assessmentIII. Consequence assessment

Entry

Exposure

Consequence

Entry assessment

• Infected/infectious animals

• Contaminated animal products

• Fomites

• Animal feed

Exposure assessment

• Exposure of susceptible population to the potential hazard

Consequence Assessment• The severity of the consequence after exposure (usually undesirable)e.g. disease, death, welfare concern, economical loss

• The probability of the consequence

• Combine these two to have an overall result (how to combine?)

• Sometimes, just probability alone




3. Create the risk pathway (scenario tree)


Risk question

• informed by hazard, scope and purpose

• need to be rigorous and agreed with all your team members

• Being rigorous means that the question needs to be useful, well described and feasible to answer

Risk question examples

• What is the probability that ≥ 1 live pigs infected with ASFV will enter to Country A by ship in the next 3 month?

• What is the probability that ≥ 1 domestic pigs in certain types of farms will be exposed to ASFV in the next 3 month in Country A as a result of presence of infection in the local wild boar population?

Risk question examples cont.

• What is the probability that ≥ 1 domestic pigs in certain types of farms will be infected by ASFV in the next 3 month in Country A as a result of the presence of infection in the local wild boar population?

• How severe is the economic loss?

Risk pathway

• Risk pathway describes how, for example, ASFV will enter to the destination country from the country of origin.

• It illustrates the epidemiological probabilistic events step by step.

• Forming a risk pathway is NOT just to describe the physical process of exporting events.

Trade of live domestic pigs Step 1: Presence of infection in pigs for exporting

Step 2: Infected pigs returning negative test results at pre-exporting check

Step 3: Survival of infected pigs during transportation

Step 4: Infected pigs returning negative test results

Border

Risk estimation

• What to estimate?

• How to estimate?

• How to get an overall risk estimate by combining the individual risk estimates?

Trade of live domestic pigs Step 1: Presence of infection in pigs for exporting

Step 2: Infected pigs returning negative test results at pre-exporting check

Step 3: Survival of infected pigs during transportation

Step 4: Infected pigs returning negative test results

Border

Probability of a pig infected

Probability of test negative given infection

Probability that an infected pig survives during the transportation

Probability of test negative given infection

Qualitative risk estimates

Risk category Description

Negligible Extremely rare so that it does not merit to be considered

Very low Very rare but cannot be excluded

Low Uncommon but occurs

Medium Occurs regularly

High Occurs very often

Very high Occurs almost certainly

Uncertainty in a risk estimateLow Solid and complete data available;

strong evidence provided in multiple references;

authors report similar conclusions

Medium Some but no complete data available;

evidence provided in small number of references;

authors report conclusions that vary from one another

High Limited or no data available;

evidence not provided in references but rather in unpublished reports or based on observations,

or personal communication;

authors report conclusions that vary considerably between them.

Probability and uncertaintyStep in risk pathway Probability Uncertainty Justification

Presence of infection in pigs for exporting

Medium High The estimate was derived from field data, but only a small number of random samples were tested

Infected pigs returning negative test results at pre-exporting check

Low Medium The diagnostic test accuracy was reported by many different papers, but the variation of the estimates occurred

Survival of infected pigs during transportation

Very high Medium No data sources available, though publications assumed a high probability

Infected pigs returning negative test results at the custom of the destination country

Very low Low High quality publications reported consistent diagnostic accuracy of the test

Step i Step i+1

Negligible Very low Low Medium High Very high

Negligible N N N N N N

Very low N VL VL VL VL

Low VL L L L

Medium M M M

High H H

Very high VH

Note: N=Negligible; VL=Very low; L=Low; M=Moderate; H=High; VH=Very high

Combination of risk estimates

Comments on the matrix

1. The combination of the risk follows the multiplication rule.

2. Combination involves element of opinion, but it cannot violate the mathematics.

3. In principle, the product of two qualitative risk estimates must not be higher than the smaller one of the two.

4. As a result, the lowest risk estimate determines maximum possible overall risk.

What is the overall risk?Step in risk pathway Probability Uncertainty Justification

Presence of infection in pigs for exporting

Medium High The estimate was derived from field data, but only a small number of random samples were tested

Infected pigs returning negative test results at pre-exporting check

Low Medium The diagnostic test accuracy was reported by many different papers, but the variation of the estimates occurred

Survival of infected pigs during transportation

Very high Medium No data sources available, though publications assumed a high probability

Infected pigs returning negative test results at the custom of the destination country

Very low Low High quality publications reported consistent diagnostic accuracy of the test

Severity of consequence (economic loss)Severity category Interpretation (hypothetical example)

Negligible Almost no lossesVery low Death occurs in very small number of pigsLow Small number of pigs dieMedium Herd is culled with compensation, restocking within 3 monthsHigh Herd is culled with compensation, but not be able to restock within 3

monthsVery high Herd is culled, without compensation, cannot be restocked for 6

months

Combination for severity and probability of consequences

Probability Consequence (impact)

Negligible Very low Low Medium High Very high

Negligible N N N N VL L

Very low N N N VL L M

Low N N VL L M H

Medium N VL L M H VH

High N VL L M H VH

Very high N VL L M H VH

Note: N=Negligible; VL=Very low; L=Low; M=Moderate; H=High; VH=Very high

Conclusion

• The scope of the risk assessment needs to be agreed within the team.

• Rigorous risk question is required, usually need to discuss it with more experienced risk analysts.

• Risk pathway needs to reflect epidemiological mechanisms, NOT just physical process.

• Probability/consequence levels need to be clearly defined and justifiable.

• Combination matrices needs to be clearly explained by reasonable logic.

qualitative risk assessment - city university of hong kong

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