[ngrilly]

I think most of us in software engineering assume the probability distribution has a fat tail. I've seen some authors name this the "blowup factor". For instance, your most likely estimation is 10 days, best case is 5 days, and worst case is 30 days. I think adding means is still meaningful (see central limit theorem and law of large numbers).

[marcosdumay]

It's exactly the central limit theorem that breaks for fat tailed distribution.

Some fat tail distributions also break the law of large numbers, but I don't think task size estimation is this flawed.