qualitative risk assessment - city university of hong kong
TRANSCRIPT
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
Steps of performing QRA
1. Decide the scope of the risk assessment
2. Form a rigorous risk question(s)
3. Create the risk pathway (scenario tree)
4. Estimate the risk
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?
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
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
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
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.