@PEI_3721
" aircraft system failures use probability (e.g. 10-9), but human performance and training is much more subjective; these aspects include awareness and decision making"
With my math hat I have to say that use of numbers (e.g. 10-9) is often an illusion!
And human performance may be more objective that numbers if you use experience from professional guys like you. At least experience is the only way to validate numbers and system models ! (Descartes said that already)