Recently I was working on a project as a Scrum Master and we had to estimate a project with multiple unknowns.
I recalled the Stochastical Duration Estimation technique that I had learned some year back when training at UCSC-extension in Santa Clara. The course I was in was 'Project Integration and Risk Management'.
With duration being more difficult to estimate than T-shirt or story point sizes, the approach can be applied to T-shirt or Story Point sizes as well.
The technique assumes the 6-point estimation for Task Estimation TE or
O=Most optimistic estimate
M=Most likely estimate
P=Pessimistic estimate
Task Estimate TE = (O + 4*M + P)/6
Variance V = (P-O)^2/36
A Statistical table provides a Z value for Confidence or Probability of Occurrence.
Confidence Level/Probability Z-value
50% 0
75% 0.67
90% 1.28
100% 3.49
Size Estimation S = TE + Z*SQRT(V)
For Project or multiple tasks,
Size Estimation S = SUM(TE) + Z* SQRT(SUM(V))
