Absolute Probability Judgement Procedure
1. Select subject matter experts
The chosen experts must have a good working knowledge of the tasks which require to be assessed. The correct number of experts is dependent upon what seems most practicable, while considering any constraints such as spatial and financial availability. However, the larger the group the more likely problems are to arise.
2. Prepare task statement
Task statements are a necessary component of the method; tasks are specified in detail. The more fuller the explanation of the task within the statement, the less likely it will be that the experts will resort to making individual guesses about the tasks. The statement should also ensure that any assumptions are clearly stated in an interpretable format for all experts to understand. The optimal level of detail will be governed by the nature of the task under consideration and the required use of the final HEP estimation.
3. Prepare response booklet These booklets detail the task statement and design of the scale to use in assessing error probability and by which experts can indicate their judgements. The scale must be one which allows differences to be made apparent. The booklet also includes instructions, assumptions and sample items.
4. Develop instructions for subjects
Instructions are required to specify to the experts the reasons for the session, otherwise they may guess such reasons which may cause bias in the resultant estimates of human reliability.
5. Obtain judgements
Experts are required to reveal their judgements on each of the tasks; this can be done in a group or individually. If done by the former means, a facilitator is often used to prevent any bias and help overcome any problems.
6. Calculate inter-judge consistency
This is a method by which the differences in the HEP estimates of individual experts can be compared; a statistical formulation is used for such purposes.
7. Aggregate individual estimates
Where group consensus methods are not used, it is necessary to compute an aggregate for each of the individual estimates for each HEP.
8. Uncertainty bound estimation Calculated by using statistical approaches involving confidence ranges.
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