# Why software projects take longer than you think: a statistical model (Erik Bernhardsson)

Anyone who built software for a while knows that estimating how long something is going to take is hard. It's hard to come up with an unbiased estimate of how long something will take, when fundamentally the work in itself is about solving something.

(…) If my model is right (a big if) then here's what we can learn:

- People estimate the median completion time well, but not the mean.
- The mean turns out to be substantially worse than the median, due to the distribution being skewed (log-normally).
- When you add up the estimates for n tasks, things get even worse.
- Tasks with the most uncertainty (rather the biggest size) can often dominate the mean time it takes to complete all tasks.
- The mean time to complete a task we know nothing about is actually infinite.

HN discussion https://news.ycombinator.com/item?id=19671673