[piccolbo]

The central limit theorem is in the limit for the number of variables in the sum approaching infinity. In the finite world, the article explains how it's done. The article is saying, the sum of lognormals is not normal. You are saying: take enough of them and it is normal. The article is still more accurate than your reasoning for 30 stories. From the wikipedia entry for Central limit theorem " As an approximation for a finite number of observations, it provides a reasonable approximation only when close to the peak of the normal distribution; it requires a very large number of observations to stretch into the tails". To prduce a 95% confidence intervals, you have to upper-bound the tails. All methodologies that are based on sum of subtasks estimates are not evidence based. But we knew already sw methodologies are not evidence-based, did we?

[ska]

    You are saying: take enough of them and it is normal.

This doesn't completely undermine your point, but that isn't what they are saying, I think. I read it as saying by CLT that the estimates of the mean of those distributions is normal and centered on [the mean you are actually interested in]. Tails are perhaps somewhat a red herring here, because you don't really care about them unless you are specifically trying to evaluate worst-case-but-really-unlikely.

[mikekchar]

Yes, that is correct. It's been a very long time since I studied statistics, so I'm not sure if the variance of a mean has the same confidence interval as the mean. I suspect not. So you would indeed need to have a very large number of samples to get good error bars. It's a good point which I hadn't really considered. However it will never really get that far anyway because hopefully you'll intervene before the long tail hits you.

I think those really long tails are more of a problem when you are working with "features" that are much longer. If you have 1 day stories and you've been working on the story for a whole week, you know you have a massive problem. It's time to back up and see if there is a way to break it up, or to do it differently.

If you have a feature that is a month, by the time you get to 5 months, you have so much capital invested in the original plan that it's very hard (politically) to say, "Nope... this isn't working out. Let's try something else". Of course, it is very hard to get your organisation to plan to a 1 day level of granularity.