Maybe I'm missing your point. It seems you're attempting to answer the wrong question: is this task accurate, given all the changes that have happened? This is

Maybe I'm missing your point. It seems you're attempting to answer the wrong question: is this task accurate, given all the changes that have happened? This is irrelevant for large scale estimation. The question for scheduling prediction is: what distribution of time will it take to mark any task in this queue as FIXED/INVALID/WONTFIX/OBSOLETE/etc? The queue can have any amount of vagueness you want in it. Regardless of the embedded work, regardless of whether or not it changes, becomes invalid, doesn't exist, etc - these are all probability weights for any given task/project.