Triangular distribution is a common formula used when there is insufficient historical data to estimate duration of an activity. It is based on three points that consider estimation uncertainty and risk.
- Most likely (M): estimate based on the duration of the activity given all the other considerations.
- Optimistic (O): estimate based on the best-case scenario.
- Pessimistic (P): estimate based on the worst-case scenario.
- A Guide to the Project Management Body of Knowledge, (PMBOK® Guide) – Sixth Edition, Project Management Institute.
Formula: Expected duration of the activity (E) = (O+M+P)/3
These are then combined to yield either a full probability distribution, for later combination with distributions obtained similarly for other variables, or summary descriptors of the distribution, such as the mean, standard deviation or percentage points of the distribution. The accuracy attributed to the results derived can be no better than the accuracy inherent in the 3 initial points, and there are clear dangers in using an assumed form for an underlying distribution that itself has little basis.
I find it very reliable when asking team members and people with experience in performing the activities they wish to estimate to use this method in estimating the duration of the work they are going to perform