A rigorous yet simple quantitative risk assessment method for quarantine pests and non-native organisms

Internationally recognised guidelines for the assessment of risk posed by non‐native organisms generally suggest that the assessment is disaggregated into a series of components each being scored and then the scores added or averaged to give the final result. Assigning odds instead of scores allows a more rigorous probabilistic treatment of the data, which can offer more effective discrimination between organisms. For each component of the assessment, the odds express how likely is the evidence if the organism poses a risk as a quarantine pest. According to Bayesian theory, the odds for all components are multiplied together and the product divided by itself plus one to give the probability that the organism poses a risk as a quarantine pest given the evidence available (assuming that the prior probability is neutral). A general illustration of the different distributions of outcomes obtained from score averaging and probability is provided. The approach is then applied to a set of risk assessments for 256 potential quarantine pests compiled for Tanzania in 1997. The greater discrimination between cases may help improve communication between risk assessment scientists and regulatory decision makers.