Background: I've modeled a project effort prediction as a Google Spreadsheet template. Details of the Model: http://sites.google.com/site/effortprediction/methodology .Google Spreadsheet does not implement beta distribution functions. In PERT a beta distribution is used to estimate the effort of a task in Ideal Person Days (i.e.: one worker 8h a day without distraction).
Problem: What consequences has use of normal distributions instead of beta distributions when the distribution are summed up to a single distribution.
Details A project consists of a list of tasks. Each task is beta distributed and independed, the sum of the tasks therefore should obey to the central limit theorem. The project therefore is normal distributed. How does Expected Value and SD change for the project when I use normal instead of beta distributions for each task? I assume that the tasks distribution is skewed more often to the right than to the left.
- Are the tails fater?
- Is the Expected Value higher or lower with normal distributions?
- Is the dispersion higher or lower?