Rappel

ES=  earliest start time for a particular activity,

EF=  earliest finish time for a particular activity,

where

EF = ES  (estimated)+ duration of the activity.

PERT probabilistic

In reality, there is considerable uncertainty about how much time actually will be needed for each activity

Thus the duration of each activity is a random variable having some probability distribution.

The original version of PERT took this uncertainty into account by using three different types of estimates of the duration of an activity to obtain basic information about

its probability distribution, as described below.

The PERT Three-Estimate Approach

The three estimates to be obtained for each activity are

Most likely estimate (m) = estimate of the most likely value of the duration,

Optimistic estimate (o)  =estimate of the duration under the most favorable conditions,

Pessimistic estimate (p) = estimate of the duration under the most unfavorable

conditions.

Steps in PERT Analysis

 „ For each activity k

„ Obtain a k, m k (mode)  and b k

„ Compute expected activity duration (mean)

Compute expected activity duration (mean) d k = t e

 „ Compute activity variance v k=s 2

 „ Compute expected project duration D=Te using standard CPM algorithm standard CPM algorithm

„ Compute Project Variance V=S**2 as  as sum of critical path activity variance ( path activity variance (this assumes independence! )

 „ In case of multiple critical paths use the one with the largest variance largest variance

„ Compute probability complete project by time t

„ Assuming project duration normally distributed

Exemple