[trashtester]

I think software projects are best estimated as gamma distributions. To express that as confidence levels, you use the cumulative distribution function of the gamma, which are kind of S shaped, but stretched to the right.

https://en.wikipedia.org/wiki/Gamma_distribution

[trashtester]

For instance, if you have a task that you cannot imagine taking less than 50 days, the actual time can follow the following distribution:

https://www.wolframalpha.com/input?i=gamma+distribution+with...

And the confidence distribution would be:

https://www.wolframalpha.com/input?i=cumulative+gamma+distri...