[ngrilly]

> It also tells us how many team-weeks this fictional, idealised project would require [...] by adding all the estimates together.

I would be wary with just "adding all the estimates together". That's because we tend to estimate the median or the mode of the task duration, and not the average. Means can be added together, but not medians.

[marcosdumay]

> Means can be added together

Is the error distribution of task size estimations normally distributed? Because I do really expect it to have a fat tail, and if it does, you can't add means either.

[jdlshore]

There’s a variety of analyses out there and they very consistently show a log-normal distribution for release predictions. I’ve analyzed Star Citizen’s publicly available data and found the same for their task estimates. It’s very reliable.

You do see truncated log-normals, though, when the estimates are padded.

[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.