A simple way for estimating tasks is to use the PERT (Program Evaluation review technique) weighted average method. This method uses a weighted average duration estimate to calculate task duration, it gives the opportunity to take into account information based on different types of estimates values (such as poorly defined areas, probabilistics, and ranges for the schedule). This method is based on the Beta distribution model because it can model events which are constrained to take place within an interval defined by a minimum and maximum value. (For this reason, the Beta distribution is used extensively in PERT, CPM and other project planning/control systems to describe the time to completion of a task).
The term weighted average means that the equation uses weighted factors to calculate the expected task duration.
The equation and process modelling a task for PERT is the following:
E= Expected Value
O= Optimistic Value (this is equivalent to a minimum value)
M= Most Likely Value
P= Pessimistic Value (this is equivalent to a maximum value)
(i) Fist, acquire estimates for the pessimistic value, most likely value and optimistic value time to completion. For example, let's suppose that: P=20 programmer days M=12.5 programmer days O=10 programmer days
(ii) Then, using the "PERT Weighted Average" equation: E=[(10+4(12.5)+20]/6 = 13.333 programmer days = expected task duration = mean value