Estimation Tools - E & SD Values
These values are used to calculate an E value for the estimate and a standard deviation (SD) as L-estimators, where:
E = (a + 4m + b) / 6
SD = (b − a) / 6
E is a weighted average which takes into account both the most optimistic and most pessimistic estimates provided. SD measures the variability or uncertainty in the estimate.
To produce a project estimate we do the following:
- Decompose the project into a list of estimable tasks, i.e. a work breakdown structure
- Estimates the E value and SD for each task.
- Calculate the E value for the total project work as E (Project Work) = Σ E (Task)
- Calculate the SD value for the total project work as SD (Project Work) = √(Σ SD (Task) 2)
The E and SD values are then used to convert the project estimates to confidence levels. Information Systems typically use the 95% confidence level, i.e. E Value + 2 × SD, for all project and task estimates.