How an outbreak affects pathway likelihoods
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The document discusses how an outbreak can affect the likelihoods of different pathways for a pathogen's entry. It provides examples of how a previous analysis estimated likelihoods for three pathways (sea, land, air) that totaled to 1. Given an actual outbreak, it demonstrates using Bayesian analysis to update the likelihoods for each pathway based on the original estimates and the fact that an entry must have occurred through one of the pathways.
How an outbreak affects pathway likelihoods
- 1. How an outbreak affects pathway likelihoods An outbreak has occurred. Therefore: • The pathogen must have entered along a pathway; and • The total likelihood of entry for all pathways must equal 1. (The event must (did) occur.)
- 2. How an outbreak affects pathway likelihoods—Example (1) The analysis finds three pathways with these likelihoods (per year): a) By sea (p=10/10 000); b) By land (p=20/10 000); c) By air (p=1/10 000); and d) No entry (p=9 969/10 000).
- 3. How an outbreak affects pathway likelihoods—Example (2) Given the outbreak, what are the likelihoods for each pathway? (Hint: this is a Bayesian analysis) a) Sea likelihood 10/10 000; b) Land likelihood 20/10 000; and c) Air likelihood 1/10 000.
- 4. How an outbreak affects pathway likelihoods—Example (3) Total likelihood is 31/10 000 before the outbreak. Likelihood given an outbreak: a) Sea (10/10 000)/(31/10 000); b) Land (20/10 000)/(31/10 000); and c) Air (1/10 000)/(31/ 10 000).
- 5. How an outbreak affects pathway likelihoods—Example (4) Total likelihood is 31/10 000 before the outbreak. Likelihood given an outbreak: a) Sea 10/31; b) Land 20/31; and c) Air 1/31.