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These slides were presented in the plenary session and followed by discussion and may not necessarily represent the final conclusions
Discussion Group 1
Methodologies in pest risk assessment:
qualitative vs. quantitative approaches in the assessment of introduction potential
DG1
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These slides were presented in the plenary session and followed by discussion and may not necessarily represent the final conclusions
Scope of DG1
In the IPPC Glossary: Introduction is “the entry of a pest resulting in its establishment”
Therefore DG1 is limited to the first two of the four stages of Pest Risk Assessment:
Entry
Establishment
Spread
Impacts
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These slides were presented in the plenary session and followed by discussion and may not necessarily represent the final conclusions
A great diversity of approaches for assessing entry and establisment potentials
Qualitative approachesRequire risk assessors to choose from categorical
ratings e.g very low, low, moderate, high, very high.
Quantitative approachesCan be used by risk assessors to obtain numerical
probabilities.
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DG1
risk rating methods (e.g. EPPO scheme);
linking risk ratings to quantities/probabilities;
summarising risk ratings and communicating uncertainty.
Qualitative approaches
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Sub-elements RatingsQuantity imported annually Low, Med., High
1, 2, 3Survive post harvest treatment Low, Med., High
1, 2, 3Survive shipment Low, Med., High
1, 2, 3Not detected at port or entry Low, Med., High
1, 2, 3Moved to suitable habitat Low, Med., High
1, 2, 3Contact with host material Low, Med., High
1, 2, 3
USDA Guidelines for Pest Risk Assessments
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Problems related to qualitative approaches
Ratings not always clearly defined.
No consensus on method for combining ratings.
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Clear definitions of ratings must be provided to risk assessors and stakeholders !
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Quantity of commodity imported annuallyLow (1 point): < 10 containers/yearMedium (2 points): 10 - 100 containers/yearHigh (3points): > 100 containers/year
Examples of definitions of ratings
from USDA Guidelines
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Examples of definitions of ratings
Negligible = 0 (no potential to survive)
Low = 1 (potential to survive on a third or less of the range ofhosts in the PRA area)
Medium = 2 (potential to survive on a third to two thirds of therange of hosts in the PRA area)
High = 3 (potential to survive throughout most or all of the range of hosts in the PRA area)
from Canadian Food Inspection Agency: establishment potential rating guidelines (2002)
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Difficult to make generic definitions
Appropriate definitions may depend on pests and areas
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No consensus on methods for combining scores
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Sub-element RatingsQuantity imported annually Low, Med., High
1, 2, 3Survive post harvest treatment Low, Med., High
1, 2, 3Survive shipment Low, Med., High
1, 2, 3Not detected at port or entry Low, Med., High
1, 2, 3Moved to suitable habitat Low, Med., High
1, 2, 3Contact with host material Low, Med., High
1, 2, 3
USDA Guidelines for Pest Risk Assessments
Cumulative risk rating
(6-18)6-9 Low
10-14 Med.15-18 High
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Biosecurity Australia
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What is the best method for combining scores?
from Hennen (2007)
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DG1
available models;
parameter estimation;
assessing and communicating model accuracy.
Quantitative approaches
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A great diversity of models.
Climate-based systems (NAPPFAST, CLIMEX).
Statistical models (Poisson, binomial, logistic…)
Population ecology model (Leslie matrix…)
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CLIMEX index of establishment suitabilityfor Phytophthora ramorum
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0 1 2 3 4 5 6 7
0.0
0.2
0.4
0.6
0.8
X=Number of successful entry of Tilletia indica in Australia
Pro
babi
lity
Estimation of probability of entryfrom Stansbury et al., 2002
Number of successful entry of Tilletia indica in Australia
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Population ecology model for estimating the probability of establishment of the Asian longhornedbeetle (Anoplophora glabripennis)
from Bartell & Nair (2003).
Eggsn1
Larvaen2
Pupaen3
Adultsn4
s1 s2 s3
f4
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Parameter estimation is a major problem
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Estimation of fecundity rate
θ (eggs/adult/month)
density
14 32 90
from Bartell & Nair (2003).
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How to choose?
DG1
Advantages and disadvantages of each approach for the assessor, decision makers and stakeholders.
How to assess the accuracy of different pest risk assessment methods.
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Qualitative approachesEasy to understand.
A qualitative PRA can be done quickly.
Problems of consistency due to - inaccurate definitions of ratings,- methods used for combining scores.
Explicit definitions needed.
Training workshops could be organized to improve the consistency of the assessments made by experts.
Another option: provide evidences only (no ranking).
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Quantitative approachesTime and resources can be problematic.
Data not sufficient. Expert knowledge often required for estimating parameters.
Uncertainty can be taken into account using probability distributions.
Models can be used to combine probability of entry and probability of establishment.
Models can be used to identify important knowledge gaps.
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Toward a comparison of the accuracies of different approaches?
Frequency
Introduction threshold
InvadersNon-Invaders
SensitivitySpecificity
Score/Prediction