Basic Assumptions:
The estimated time of an activity is calculated with certain assumptions, i.e.
- The activity durations fit a Beta distribution, illustrated in Figure. In the assumption beta distribution is used to justify the approximation of mean μ and the standard deviation σ. In traditional PERT method, mean and standard deviation from a, b and m are calculated using following formula
Figure 4.10: The Beta-distribution curve
μ = ( a + 4 m + b )/6 -------- (1)
σ = (b × a)/6 m --------- (2)
- The variance is calculated by squaring the value of sigma, that means variance V = σ2.
- The range from a to b in the three-estimate approach covers 6 standard deviations. In view of this property, it is supposed that there are six standard deviation from a to b.
- The activity durations are independent statistically.
- Now the critical path means the path that has the longest expected value of total project time. The central limit theorem is utilized to justify this assumption.
- The overall project duration contain a normal distribution.
At last, using estimates of variability for the activity on the critical path, the probability of completing the project by particular times are estimated.