By just focussing on probability you miss something....
If something has a low probabilty and a very high impact, you probably want to still do the analysis and look at mitigation.
In Large scale projects I would not only look at the risks - the impact and the probabilty, but assess the cost of mitigation. If mitigation is inexpensive (time and/or money) then build it into your project plan.
You can analyse the cost of mitigation versus the potential cost of ignoring the risk...
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