[quietbritishjim]

> Whatever number you come up with, treat it as the median of a long-tailed distribution (if it matters, the lognormal). To get the mean (expectation), multiply your estimate by about 1.6. To get the 95% confidence bound, multiply by 5. To get the 99% confidence bound, multiply your estimate by 10.

I like the idea of multipliers but the maths here is just meaningless fluff to justify a particular number. If your initial estimate really was a median then it would be an overestimate (i.e. the project ends up taking less time) in about 50% of cases. In practice I find that initial estimates are overestimates in about 0% of cases!

[jbay808]

> If your initial estimate really was a median then it would be an overestimate (i.e. the project ends up taking less time) in about 50% of cases

It is, yes. And it frequently happens that you go to fix something, which seems really difficult, and then you realize that it's actually an easy fix or not a problem at all. But when you fix one thing in half the time you expect, and another in twice the time you expect, this doesn't average out, because the average of 0.5 and 2 is not 1.0.

You might just be discarding the cases where estimates were found to be conservative, either because delays are more impactful and memorable, because the underestimates were close enough to be treated as on-time, or because the slack in the schedule was used to buy time for something else, originally out of scope, that was lumped in.

Anyway, these numbers aren't just pulled out of a hat. It comes from studying vast amounts of high quality (but unfortunately, not publicly available) data collected comparing developer estimates and measured outcomes.

[quietbritishjim]

I was responding to what you said in your original comment:

> Whatever number you come up with, treat it as the median ...

Especially the "whatever number you come up with" bit, which seems aimed at everyone quite generally. When people usually come up with a number, it is likely to be a substantial underestimate. This isn't just a cognative bias where I've forgotten the times the task turned out to be simpler - evidence shows this to be the general trend (a la Thinking Fast and Slow, as I mentioned in another comment). So your rule that it should be treated as a median isn't correct in the majority of cases.

> these numbers ... comes from studying vast amounts of high quality ... data

Well that's a different matter. Of course, whether the result of analysing that data is a median or a mean (or something else) depends on how exactly you analysed it.