We will take the Speaker to lunch at Thai@Home before his talk with folks from the Algebra seminar, leaving at 1:00PM from the 2nd floor of Carslaw. NOTE CHANGE OF LUNCH TIME. Please note that for the remainder of semester colloquia at the University of Sydney will be held at 2:30 PM for compatibility with the algebra seminar, and on weeks where the algebra seminar is running we will have a joint lunch. Abstract: Extreme risks must be evaluated in such contexts as quarantine, terrorism and banking. Unfortunately, an extreme risk is one that hasn’t happened yet, so directly relevant data is non-existent. The talk surveys what is done in the Basel II compliance regime in banking and in Australian quarantine risk analysis, where there are formal processes for using small data sets to keep expert opinion honest. The usefulness of Extreme Value Theory is considered. Extreme risks raise in acute form the "reference class problem", of how to decide what is the right class in which to take statistics to bear on an individual case. The views of philosophers and legal theorists on the reference class problem are canvassed.