As for Waterfall, a combination of decomposition (WBS), PERT / 3 point estimation (weights sum up 6) and iterative reevaluation could be used.
What is exactly the 3 point estimation and PERT? Google says:
- www.techrepublic.com/blog/it-consultant/use-pert-technique-for-more-accurate-estimates/
- http://www.pmdocuments.com/pert-three-point-estimation-technique/
- http://www.pmbypm.com/3-point-estimate-is-different-from-pert/
- https://en.wikipedia.org/wiki/Three-point_estimation
"PERT is an estimating technique that uses a weighted average of three numbers (see below) to come up with a final estimate.
- The most pessimistic (P) case when everything goes wrong [also called "Worst Case Scenario"]
- The most optimistic (O) case where everything goes right [also called "Best Case Scenario"]
- The most likely (M) case given normal problems and opportunities [also called "Expected Case Scenario"]
The resulting PERT estimate is calculated as (O + 4M + P)/6. This is called a "weighted average" since the most likely estimate is weighted four times as much as the other two values. You'll notice that the final PERT estimate is moved slightly toward either the optimistic or pessimistic value - depending on which one is furthest from the most likely. Generally this ends up moving the final estimate toward the worst case, since the worst case value tends to be further out from the most likely that the optimistic number."
Some tools (like MS Project) will allow you to do some What If scenarios, by defining different weights (above the weights are 1, 4, 1, and all weights should sum up to 6). What if it is most probable that mostly everything goes wrong (could be tested with weights like 1, 1, 4). What if mostly everything goes right (4, 1, 1)?
