1. Probability does not exists – but does risk exist? Why uncertainty should replace probability in the definition of risk Terje Aven University of Stavanger Risk and uncertainty: ontologies and methods University of Amsterdam, The Netherlands 23-25 January 2013

2. Peter is to jump onto the stone Kjerag bolt 1000 m down Does risk exist? YES !

3. Peter falls down A He is killed C Uncertainty U - the occurrence is unknown Inter-subjective Objective

4. Statfjord A Does risk exist? YES !

5. Government building Oslo 22 July 2011

6. Risk (A,C,U) A: Event, C: Consequences U: Uncertainty (C,U)

7. Risk description (A,C,U) Q: Measure of uncertainty (e.g. P) K: Background knowledge C’: Specific consequences (C,U) C’ Q K

8. Probability- based Historical data Probability- based Historical data Knowledge dimension + + Surprises Risk perspective

9. The need for seeing beyond probability

10. Subjective/knowledge-based probability P(A|K) =0.1 The assessor compares his/her uncertainty (degree og belief) about the occurrence of the event A with drawing a specific ball from an urn that contains 10 balls (Lindley, 2000. Kaplan and Garrick 1981). K: background knowledge

11. Probability is a tool Person a: P(A) = 0.01 Person b: P(A) = 0.1 Probability is subjective - «it does not exist» (de Finetti)

12. Does an objective probability exist ? Not to reflect uncertainty but in some cases to reflect variation x x x x x x x x x Frequentist probability

13. The need for seeing beyond probability

14. P(head) = 0.5 P(attack) = 0.5 Strong knowledge Poor knowledge

15. John offers you a game: throwing a die ”1,2,3,4,5”: 6 ”6”: -24 What is your risk?

16. Risk (C,P): 6 5/6 -24 1/6 Is based on an important assumption – the die is fair

17. Assumption 1: … Assumption 2: … Assumption 3: … Assumption 4: … … Assumption 50: The platform jacket structure will withstand a ship collision energy of 14 MJ Assumption 51: There will be no hot work on the platform Assumption 52: The work permit system is adhered to Assumption 53: The reliability of the blowdown system is p Assumption 54: There will be N crane lifts per year … Assumption 100: … … “Background knowledge” Model: A very crude gas dispersion model is applied

18. Risk concept cont. There is a need for seeing beyond the probabilities – must better cover the knowlede and lack of knowledge dimension

19. A = leakage

20. Mediocristan (Normalistan) Extremistan (black swans) Nassim N. Taleb

21. Threats Known unknowns Unknown unknowns (”black swans”) (A’, C’, Q, K)

22. Risk analysis Cost-benefit analysis, Risk acceptance criteria … Management review and judgment Decision Analysis Management Risk-informed decision making

23. Conclusions «Risk exists but not risk descriptions»