First of all, it must be understood that the ultimate number is arrived at by manipulating percieved or demonstrated failure rates that may or may not relate to the element being analyzed.
This number is then incorporated into a safety hazards analysis (Fault Tree) and it is further manipulated through Boolean Algebra so that at the top level the ultimate loss of the horizontal stabilizer can be shown to be 1 10-12 or higher. It's all numbers and they do not necessarily reflect what the real failure rate is.
The assumption being that multiple combinations of failures and conditions must occur to have led to this accident.
So what real failure rate was the unexpected archilies heal in this accident ?
and what action should be taken to address it?