The rationale for the sequence of steps 5 and 6 is to encourage an assessment that spreads the distribution. Reversing the order will tend to yield a narrower distribution: 0.2 followed by 0.1, instead of 0.2 followed by 0.3. The rationale for steps 8 and 9 coming last is clarifying the overall nature of the outcome probability distribution given the issue occurs, prior to estimating the probability that the issue will occur. If the issue involves uncertain outcomes but the source always occurs, both the optimistic and the pessimistic probability scenario will have values of 1 by definition. In the general case the probability that the issue will occur will be uncertain. The rationale for the ordering of steps 8 and 9 is dealing with anchoring effects (explored later). The rationale for intermediate outcome scenarios but no intermediate probability scenarios is a balance between generality and simplicity, assuming that impact uncertainty is greater than probability of occurrence uncertainty, an issue explored further in Chapter 15.
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